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Stress on the Streets
(Trees Suffer from Road Rage, Too.) Reprinted from the Minnesota Shade Tree Advocate VOL. 2, NO. 3 Summer 1999
“Hey, we didn’t hit the tree.” Street trees are frequently put at risk, though the results may not become evident for some time. Minnesota, like the rest of the United States, is becoming more urbanized. More than 75% of us live in urban areas. The threat to rural forests that are being developed in third-, fourth- and fifth-ring subdivisions (peri-urban forests) is real and of concern. But the danger to these areas is geographically and resource limited. There are only so many forests and when those areas are developed, they’re gone.
PHOTOS COURTESY GARY JOHNSON
A growing concern that has no limits involves construction damage to publicly-owned, urban trees—those trees that were either survivors or were planted after development. These trees now line our streets, parking lots, schools, businesses and parks. In some communities, they may be 50 to 70 years old or more. And even though the development and construction of our communities may have taken place decades or a century ago, construction damage continues on a regular basis and with alarming frequency. Street trees (boulevard trees, parkway trees, tree lawn trees) are most at risk. Streets and curbs don’t last forever. They require periodic resurfacing, re-grading and re-engineering of curb shapes and heights. Sidewalks crack, heave and become dangerous for pedestrians. Many streets were designed and installed decades ago, long before they became popular as arterial streets. They have become too narrow for modern public safety vehicles or street maintenance trucks, or are designated for parking on both sides.
Construction crews may not realize the effects of compaction, soil changes and root damage.
This tree suffered root damage on three sides . . . including topsoil scraping. The outlook is grim.
Utilities fail or become outdated and must be replaced. New utilities or services are developed and installed below ground—fiber optics systems, landscape irrigation systems and invisible fences. In most boulevards, the top eight feet of soil is laced with an incredible network of buried utilities. When streets, curbs, sidewalks, driveways and utilities are installed, improved or expanded, trees suffer construction damage. Most people don’t recognize it, however, because the damage is usually underground. And most people don’t realize how vulnerable street trees are to the stresses imposed on them from root loss or soil changes. Rarely are street trees badly wounded or scarred above ground during these activities, and rarely are street trees exceptionally healthy before the damage.
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Minnesota Tree Care Advisors Boulevards were originally designed as tree lawns—literally, lawns for public trees to grow in. Except for water and sewer lines, driveways and sidewalks, there were no conflicts of use.
Stressed to begin with Boulevards are far from ideal growing sites for trees. Soils range from rocky and sandy to hard-as-concrete clay, bone-dry to bog-soggy. Occasionally you may find beautiful sandy loam soil in a boulevard, especially in some of the older neighborhoods and towns. (Occasionally you may find a $20 bill lying on the ground, too!) Soil pH is variable. In some of our boulevard research areas, the soil is close to the native soil pH range, 5.5-6.5. A few blocks away, it may reach 8.5 or higher! Unfortunately, the majority of trees planted in boulevards perform best in soils that are well drained, organic and with a pH range of 6.0-7.0. Under normal circumstances, tree roots spread in an area two to four times their height. However, if a curb is three feet away from a tree trunk, that is effectively the spread of the roots on that side. Major roots have a tendency to grow to the curb, turn and then run linear with it. Some roots grow under curbs, streets, sidewalks and driveways, but it’s unpredictable. The windthrows from the storms of 1997-98 illustrated this perfectly and made us wonder how some of those towering trees stayed vertical for as long as they did. Street tree roots don’t penetrate very deeply, rarely more than three feet. In some areas with compacted clay, the majority of the roots are in the top 18 inches of the soil profile. Now, add deicing salts, turf Chapter 3 Page 63 competition, pesticides and animal waste to the soil matrix, acts of unintentional “vandalism” to the trees (stapling trunks, breaking branches, lawn mower wounds and tree topping), and you’ll develop a renewed respect for any tree that even survives these conditions, much less thrives in them.
What they can tolerate, what they can’t Although “construction damage” is an ambiguous term, most of the damage to boulevard trees involves root loss. Trenching, excavating and re-grading the surfaces of the tree lawns removes tremendously high percentages of supportive branch roots and conductive (water and nutrients) finer roots. A relatively healthy boulevard tree can usually tolerate one-sided root loss from trenching or excavation. It will definitely be less stable and more vulnerable to drought, insect and disease stresses for a few years, but it can recover if given adequate care. Trees that suffer two-sided root loss are unstable and less likely to recover. Unhealthy Letters to the Advocate trees are more likely to die from two-sided root loss. Even healthy trees are likely to suffer some branch death and require deadwood removal pruning two to four years after construction. Depending on the tree species, age and relative health, however, many of these trees can recover if they receive immediate and proper care after the construction. Trees that suffer three-, four- and five-sided root loss (five-sided root loss includes four sides cut vertically and the top re-graded) make good firewood or woodchips, depending on the age of the tree. It’s probably not wise to save these trees if the root loss is unavoidable. Remove them, and replace them after the construction project is completed.
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Other variables The width of the boulevard plays an important role in the survival and health of the trees after construction, provided that the trees are centered between the curb and sidewalk. Trees growing in boulevards 10 feet or wider have a much better chance of tolerating the root losses compared to trees growing in three- to five- foot wide lawns. There’s simply more room for a higher percentage of the trees’ root systems to grow in and avoid damage. Age or size of the tree. Younger or smaller trees recover faster from root loss than older and bigger trees. There are less wood and leaf Chapter 3 Page 64 surface areas for their roots to supply water and nutrients to, and there are more actively growing tissues in a younger tree. Relatively healthy trees survive root loss better than highly stressed trees. Severely stressed trees are living on the edge, so to speak. The added stress of root loss, soil compaction or pH changes can effectively push them over the edge. Tree species vary in their tolerance to root and soil changes. Some species—silver maple, hackberry, green ash and bur oak—are more tolerant of root loss and soil changes. Other more sensitive species— ironwood, white oak, black walnut and blue beech—quake and shake at the sight of a bulldozer.
Some logical steps to reduce the damage and loss
Most of the damage to boulevard trees involves root loss.
1. Seek variances. Not all streets need to be widened. If your community values the boulevard trees more than acres of concrete, let your city know. You pay the bills and they work for you. Create reasonable compromises that protect the best trees by avoiding root loss. Keep streets the same width. Allow curves in the street to avoid damaging trees. Compromise by restricting parking to one side or no sides of the street rather than both sides. This will negate the necessity for landing-strip street widths. 2. Combine utility trenches whenever possible. Not every utility needs its own separate trench. Simply combining just two utilities to a common trench eliminates one trench, and may reduce the damage from two-sided to one-sided. 3. Save the best; chip the rest. Don’t save trees that are unhealthy, too tall for the root space they’ll be confined to, hazardous or of a very sensitive species – if the damage is unavoidable. 4. Insist that foresters and arborists make the evaluations and decisions regarding trees. These significant investments deserve the attention of knowledgeable professionals. 5. Do not allow construction activities on boulevards that compact or pollute the soil. Allow no parking of vehicles or equipment, no storage of construction materials or excess soil, no cleaning of concrete truck chutes or equipment.
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Keeping trees well watered at all times is the single most effective treatment for helping them recover from damage.
6. Whenever possible, insist that tunnels be drilled under the roots for utilities near trees rather than trenches being dug through the root systems. 7. If new curbs will replace old ones, hand-form the curbs near the roots, using forms that “slip” in between the existing curb and roots. Avoid curb excavation that cuts roots. 8. Assume some ownership and responsibility for the health of the trees. Don’t let them become stressed before, during and after the construction process. Keeping the trees well watered at all times before, during and after construction is the single most effective treatment you can do to help them recover from damage. 9. Nurture the trees’ remaining roots forever. Think beyond the box! Boulevards don’t absolutely need to be covered with mown turf. Mulch the root systems with wood chips. 10. Plant a “blooming boulevard” in the mulched area with perennials, shrubs and/or ornamental grasses, but not—for maintenance efficiency— annual flowers like petunias. 11. Replant with logic. If the remaining boulevard is only three feet wide, don’t replant with a tree, especially a big tree. Consider creating “green easements” within your community - replanting the trees on the property owner’s side of the sidewalk. Select trees that are adapted to the unique soil and space characteristics of your boulevard. For information on trees adapted to specific sites, refer to these publications: “The Right Tree” brochure, available from Minnesota Power Association at 1-800-228-4966 or the University of Minnesota Extension Service Distribution Center at 612/625- 8173.
Soil compaction from trucks and other vehicles is often worse than bulldozer compaction, especially when the soil is a wet clay.
Depending on the tree species, age and relative health, many trees can recover if they receive immediate and proper care after the construction.
Root damage and soil pollution —a deadly prescription if not remedied.
That’s not black dirt - waste asphalt at the base of a tree...
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Minnesota Tree Care Advisors Also available from the Distribution Center are these University of Minnesota Extension Service Publications: Recommended Trees for Southeastern Minnesota Recommended Trees for Southwestern Minnesota Recommended Trees for Tallgrass Prairie Recommended Trees for North Central Minnesota These steps will initially cost the residents of a community more than carte blanche removal of trees and roots, and uninterrupted trenching. But long-term costs will be reduced through fewer tree replacements, increased property values and less damage to the gray infrastructure (sidewalks, curbs, streets). It’s worth the investment.
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It’s More Than Wrapping Snow Fence Around Tree Trunks Reprinted from the Minnesota Shade Tree Advocate VOL. 2, NO. 3 Summer 1999 Trees and forests are biological systems—systems that operate beautifully and for a long time when all their parts are present and working. Take away one part and the system skips a beat, maybe faltering just slightly. Remove another part or two and the system falters more noticeably.
Accepting 30- 50% forest loss during development ignores the biological principle that the system has been disrupted and will falter.
With continued faltering, remaining parts begin to suffer as the burden of the whole weighs heavier on them. In time this oncehealthy system becomes vulnerable. What may once have been relatively tolerable stresses might now mean a forest at risk. One summer of drought, one defoliation by insects or diseases or one unusually generous winter serving of deicing salt can mean disaster. Some parts of a forest’s system are obvious— vegetation that includes trees, shrubs and groundcovers. Just as important are the less obvious parts—soil, water, soil microorganisms, ambient temperatures and humidity. Success or failure of the system depends on the health of the sum of all the parts. Individual trees are systems, too. The major parts – canopies, stems and roots – are in turn linked to soil, water, air and soil microorganism quality. Damage to one part of a tree’s system affects everything else. If a significant amount of the root system is lost or damaged during construction, the whole system is affected and sometimes fails. As root systems decline, photosynthesis is reduced as well as water and nutrient transport to the stem, branches and leaves. Branches “shut down” and fail (die). Energy reserves that contain (compartmentalize) the decaying, dead branches are not as plentiful and the containment weakens as decay becomes more aggressive. A summer-long drought comes along. The tree becomes more stressed; opportunistic insects like borers move in and finish it off. And it probably all started with someone thinking, “What’s the worst that could happen if I cut off these roots?”
The character of Minnesota is changing, courtesy of urbanization, development and
Construction damage at the landscape level is insidious, and the cause and effect relationship often escapes even the trained eye. Relatively small pockets cut into a forest for a home-site seem almost inconsequential. But then roads and driveways are cut for access to the site, and utilities are trenched in for 20th Century conveniences. It’s likely that more than one building pad will be cut in so several sites can be served by the roads and utility trenches. Wherever two or more people settle, it seems a convenience store soon follows. Gas pumps, liquor stores and pizza parlors spring up. As
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construction damage to forest and tree systems.
the area develops, it attracts more commuters seeking the solitude and freedom of living in the country. Roads are expanded and improved to accommodate the increased traffic, and voila!—urban sprawl inches forward once more. What’s happening to the forest system? The formerly solid, continuous forest is becoming “fragmented,” – cut up into smaller, separate or loosely connected units. Even if individual trees are labeled “significant” and carefully protected, more edges of the forest are exposed to drying sun and wind, more chainsaws collect dead (or live) wood for the fireplace and deicing salts drift off those new and improved roads. Forest edges get “tidied up,” ridding the visual park of unsightly leaf and branch litter. Lawns are established, and petrochemical products (pesticides, fertilizers) are used to support the newly planted and the declining remnant forest trees. “New and improved, fast-growing” plants like tallhedge buckthorn, honeysuckle and Amur maple are planted. The biological character of the original forest slowly changes to the character of the 20th Century residential landscape. Urbanization is characteristically a slow, deliberate degradation of the natural system. Soil temperatures and moisture contents fluctuate more than before. Soil chemistry (usually pH) becomes altered, and native soil microorganisms shift in character. Individual trees along the new edges begin failing; other interior trees become the new edge trees. Natural, forward, forest succession (the natural replacement of one group of plants by another) often screeches to a halt. The character of Minnesota is changing, courtesy of urbanization, development and construction damage to forest and tree systems. In our lifetime, we’ll never see century-old oak savannas, sugar maple and basswood forests or red oak and hornbeam forests regenerate if they are chopped up for subdivisions. And our grandchildren will never see them either if the forest floor is stripped, compacted, sodded and paved. Now is the moment to intervene. Act to protect connected forest systems from urbanization. Encourage your community and developers to subdivide in open spaces or at the edge of forests, rather than within them. Cluster new homes, rather than spreading them out. Build up rather than out. Reconnect fragmented forests such as the Greening the Great River Project is attempting to do. Contact Jean Moulle at Metro DNR Forestry and get information on the drafted “Conserving Wooded Areas in Developing Communities: Best Management Practices in Minnesota.” (Web reference) Protecting individual trees during construction is good. But accepting 30- 50% forest loss during development ignores the biological principle that the system has been disrupted and will falter. Believing that protecting half of the forest system will ensure a healthy remnant forest is about as (bio) logical as believing that wrapping snow fence around a tree trunk will protect it from construction damage.
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Tree Preservation During Construction: A Guide to Estimating Costs by Gary R. Johnson
Contractors took some fairly simple measures to protect this 165year-old bur oak during construction of a large concrete sculpture and seating area. The original plan to cut and fill the grade within the root zone was altered to preserve the tree roots, and 11 concrete piers were sunk into the ground to eliminate the need for grade changes. The build-able area was fenced off to allow only foot traffic. Mechanized equipment was restricted from the critical root zone; concrete was pumped in from a truck parked a safe distance away. A strategically placed sign explained the tree preservation effort.
Hoffman Homes, Inc., took extraordinary steps to preserve this bur oak in an Eagan, Minnesota, development. The original plan for the foundation was altered to preserve more roots, a retaining wall was constructed to avoid filling soil over much of the root system, and the tree was professionally braced, cabled, and pruned. This photo, taken in mid-spring one year after construction, illustrates the importance of the tree to the home site.
Introduction Builders and developers are increasingly being called upon by ordinance or consumer demand to protect selected trees during construction. This guide helps you estimate the costs of doing so. Tree preservation requirements vary considerably from one community to the next. So do actual costs, due to regional variations in labor (and to a lesser extent, materials) costs. Therefore, "costs" are listed in this guide as labor hours, equipment hours, and materials for a range of tree preservation activities. You can customize these tables by selecting the activities that apply to your situation and multiplying the figures given by dollar values appropriate for your area. In most instances, hours and supplies are listed as ranges. These allow for variables such as experience level of labor, weather, and soil conditions. If you have an inexperienced labor force working on a severe slope in the muddy season of the year, it would be logical to use the high-end estimate. As the name implies, this is a guide, not an absolute. You'll find it particularly valuable if you are performing tree preservation tasks for the first time. As you gain experience, it may simply become a reminder of the steps involved. If you record
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Minnesota Tree Care Advisors your actual labor hours, quantities, and volumes in the 'your notes' columns, you'll have a handy, customized reference for future jobs. Sample Ordinance Requirements and Language This section reviews and clarifies common requirements and defines some terms often used in tree preservation ordinances. aeration A process in which holes are drilled into the soil within the critical root zone to provide relief from compaction. Holes typically are 8 to 10 inches deep and 1.5 feet to 4 feet on center (o.c.). balled-and-burlapped nursery stock Trees that have been either hand-dug or tree-spade-dug with a soil ball enclosing the roots. The soil ball is wrapped in burlap and may or may not be enclosed in a wire basket. bare-root nursery stock Trees that are dug and shipped during dormant (leafless) seasons and have no soil harvested with the roots. caliper inch (stem caliper) The diameter of a tree trunk measured 6 inches above the ground for trees less than or equal to 4 inches and at 12 inches for stems greater than 4 inches. This measure is used for nursery stock. certified nursery stock Nursery-grown trees that have been inspected by a state's department of agriculture and certified free of pests and diseases. condition The general health and structural integrity of a tree, as evaluated by a qualified professional. container nursery stock Trees that have been either grown their entire lives in containers or fieldpotted into containers. critical root zone A way to define the protection zone for an individual tree. It is commonly calculated as the roots and soil within 1) the dripline, or 2) an area defined by a circle with a diameter 24 to 36 times the d.b.h. of the tree (1 to 1.5 feet of radius for each inch of d.b.h.). crown The section of a tree containing most of the branches and leaves. d.b.h. Literally, diameter at breast height. The d.b.h. is measured 4.5 feet from the ground on the uphill side of a tree. deadwood pruning The removal of only deadwood from a tree's crown by a tree care professional. deciduous tree A tree that normally drops its leaves in winter. dripline The land area within a circle defined by the extent of the farthest growing branches of a tree. evergreen tree
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Minnesota Tree Care Advisors A tree that normally has green foliage during all seasons. forest restoration plan (reforestation plan, landscape plan) A plan prepared by a qualified professional for the ordinance authority that specifies replacement tree locations and restoration of other vegetation and disturbed land. inventory At minimum, a list of all significant trees and their d.b.h. sizes. Some ordinances also require that inventories indicate tree condition. Inventory requirements usually can be satisfied by locating significant trees on a site map and coding them with a number or acronym that corresponds to the map legend. Many ordinances require that significant trees be marked with a metal or plastic tag with the corresponding legend code until construction and inspection are completed. Most ordinances require that the inventory be prepared by a qualified professional. land disturbance permit A permit granted by the ordinance authority for removal/loss of significant vegetation and/or changes in site grade and soil. protection zone (preservation area) The part of a parcel outside the build-able area. qualified professional A tree specialist qualified to prepare inventories, tree removal plans, tree protection plans, and reforestation/ landscape plans. Qualified professionals include (but are not necessarily limited to): certified registered land surveyors, landscape architects, foresters, arborists (tree care professionals), nursery and landscape professionals, and International Society of Arboriculture certified arborists. radial trenching The process of improving tree condition by digging trenches in a radial fashion, starting approximately 3 feet from a tree's trunk and extending to the perimeter of the critical root zone, then backfilling with amended soil, organic matter, and/or fertilizer. There are no standards for depth and width, but trenches normally are approximately 12 inches wide and 12 inches deep. There are no standards for number of trenches per tree since they may only be placed where the trenching operation will not damage primary tree roots. removal/loss Definition varies substantially among ordinances. For individual trees, removal/ loss may include soil changes within the critical root zone, root loss from trenching within the critical root area, and mechanical wounds on tree trunks. For wooded land parcels, it may include excavating, grading, filling, or other earth exchanges, as well as actual clearing. Limits may be defined by volume (soil), land area (grade changes), or percentages of total vegetation cover (tree/vegetation loss). removal plan A site plan with proposed build-able areas and significant trees to be removed that is submitted to the ordinance authority. replacement trees Trees planted as compensation for significant trees lost or removed
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Minnesota Tree Care Advisors during construction. Many ordinances specify species, minimum and maximum sizes, and type of nursery stock (e.g., bare-root, container stock, or balledand- burlapped). Some ordinances allow the builder/developer to plant replacement trees on other sites within the same development (e.g., boulevards). root collar The point of attachment of major woody roots to the tree trunk, usually at or near the ground and associated with a marked swelling of the tree trunk. significant trees The units the builder or developer is charged with protecting and/or replacing. Significant trees are the focus of most tree preservation ordinances. Not all trees are significant and the definition of a significant tree varies among ordinances, so examine the language carefully. tree Any self-supporting woody plant, usually having a single, woody trunk and a potential d.b.h. of at least 2 inches. Trees may be deciduous or evergreen. tree density The total d.b.h. of significant trees per specified land area. Ordinances often will simply distinguish between high and low density. tree preservation plan An outline prepared by a qualified professional and submitted to the ordinance authority that describes proposed preservation areas, buildable areas, and preservation tactics. tree replacement table A table that specifies the number of caliper inches of replacement trees needed to replace the lost or removed trees. The table considers 1) percent of significant tree d.b.h. inches removed, 2) tree density, and 3) size of approved replacement trees. For example, an ordinance may require 0.5 caliper inch in-replacement trees for each d.b.h. inch lost from a low treedensity land parcel where 20 to 29.9 percent of the significant tree d.b.h. is removed, and may require that replacement trees have caliper measurements of 2 inches. In this case, if 100 d.b.h. inches were lost, 50 caliper inches would need to be replaced, which means 25 trees would need to be planted. trunk The part of a tree between the root collar and the first branch of the crown. trunk cambium The layer of trunk tissue that gives rise to the water, nutrient, and chemical energy-conducting tissues. This layer is usually found directly under the outer bark. vertical mulching The addition of organic matter and/or fertilizer into holes drilled within the critical root zone to improve tree condition. vibratory plowing A process that severs roots to a depth of up to 5 feet. It is sometimes required in areas where diseases may be transmitted through root grafts. wildings
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Minnesota Tree Care Advisors Trees that have been transplanted from natural forest habitat and have been deemed suitable for replacement trees by the ordinance authority. Normally, wilding trees must have a dug root system approximately 30 percent larger than a similar nursery-grown tree.
Estimation Tables The following tables will help you estimate the costs of various tree protection measures. Figures given are common ranges for time and materials. Where equipment is used, estimates are for both labor and equipment hours. Notes specifying the conditions on which ranges are based and a list of variables that affect where in the range you will fall are given after each table. You are encouraged to keep track of your cost data for future reference by filling in the "your notes" row in each table. Obtain Permit(s) (per site) inventory parcel
2.0-6.0 hours
(your notes)
create preservation and restoration plan
1.5-4.0 hours
(your notes)
apply for permit(s)
2.0-8.0 hours
(your notes)
Comments: ranges are for a wooded, 0.2- to 0.5-acre parcel Variables: tree density, lot size, number and types of permits required, ordinance authority review process, revision of plans and applications (can skew hours completely out of ranges)
Fell, Buck, Load Trees (per tree) Tree Size (d.b.h.)
fell/buck
6"
12"
18"
24"
30"
0.2-0.5 hours
0.4-1.2 hours
0.751.75 hours
1.0-2.25 hours
1.25-2.75 hours
(your notes)
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load
0.3-0.5 hours
0.5-0.75 hours
0.5-0.75 hours
0.75-1.2 hours
1.0-1.75 hours
(your notes)
Comments: includes dropping the tree, cutting it to length, and loading it into a trailer; 18-inch d.b.h. uses front-end loader; 24- and 30-inch d.b.h. use boom Variables: species, tree density, access, topography, tree size
Remove Stumps (per tree) Tree Size (d.b.h.)
remove with stump grinder
6"
12"
18"
24"
30"
0.15-0.2 hours
0.2-0.25 hours
0.25-0.3 hours
0.3-0.33 hours
0.3-0.4 hours
0.1-0.2 hours
0.3-0.45 hours
0.5-0.7 hours
0.5-1.0 hours
0.75-1.5 hours
(your notes)
remove with backhoe (your notes)
Variables: vegetation density, access, topography, stump size
Mulch (per 1,000 square feet)
mulch
already placed
including hauling
12.5 cubic yards
12.5 cubic yards
2.0-3.5 hours
4.0-6.25 hours
(your notes)
spread mulch by hand
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spread mulch by tractor
1.75-2.5 hours
not applicable
(your notes)
Comments: includes coarse wood ships spread 4 inches thick; tractor has one-cubic-yard bucket; "already placed" indicates mulch has been distributed in piles in strategic locations Variables: topography, vegetation, density, access
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Buried Root Systems and Tree Health by Gary R. Johnson
Reprinted from the Minnesota Shade Tree Advocate, Volume 3 Number 4 Stem girdling roots (SGRS) are those roots that grow either partially or completely against and compress (girdle) stem tissues of trees. Xylem and phloem (conducting) tissues in the stems become much smaller in diameter at the point/s of compression, compromising the transport of water, nutrients and photosynthates ("food"). Trees become stressed and more vulnerable to secondary problems (drought, insect attacks). Often, the compressed areas of the stems are weak points and far too often are the points of failure during windstorms. For instance, in the catastrophic windstorms of 1998 in Minnesota, 73.3%* of the lindens that were lost actually broke at compression points from SGRS, and most broke below ground.
Tree snapped at SGR compression point below ground.
Above-ground stem girdling roots
Dysfunctional root system - Apple tree failed in a wind storm
Poor stem condition related to stem girdling roots and excess soil over root system.
SGRs can and do form above ground, especially with maples and poplars. However, they can develop on most species below ground and out of sight. How can this happen? If a tree's root system has been buried too deep, the stem is subsequently buried. When root systems are buried too deep - with some trees, that's one inch of soil over the first, main order (first branch) roots - secondary woody roots grow upward, closer to the soil surface. Often, some of these roots end up growing against the stem tissues, either partially or completely encircling the stems. Since 1997, the University of Minnesota Forest Resources Department
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Early fall color is a sign of potential root problems
has randomly sampled 303 trees (ash, maple, linden). Depths of soil over the first roots ranged from 0 to 13 inches. Analysis of the data later revealed a statistically significant relationship between depth of soil over the roots, condition of the trees and the frequency of stem girdling roots. As more soil was added over the root systems of those trees - for whatever reason - stem conditions declined and the frequency of stem girdling roots increased. So, deeper (planting) is not better. In the long run, it's worse for the long-term health and stability of the trees. *Based on the storm damage research conducted by the Department Forest Resources, University of Minnesota, 1995-present.
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Rooting Around: Tree Roots By Dave Hanson
Take a good hard look at your favorite tree. What do you see? Nice form, leaves, branches, bark, but, how many would respond – “Oh baby! There must be some fine roots holding up this tree.” Unfortunately for trees, not too many people respond in this fashion, the world underground does remain a mystery. What is happening down there? What are tree roots and for that matter where are the roots? Why and how do roots get into sewers, cause foundations to crack and sidewalks to lift? The reason that these mysteries remain: we cannot easily see and touch tree roots, we cannot readily access them and typically we do not encounter them without getting out a shovel. This brief article will skim the surface, so to speak, and attempt to provide some insight and answers to these questions.
Bur oak - Support roots
So, let’s start at the beginning and address two basic questions. Question number one, what are tree roots? There are three basic types of tree roots: Structural support, Fluid transport, and nutrient / moisture absorbers. Structural support comes in the form of largerdiameter perennial woody roots that anchor the tree to earth. Near the base of the tree are found the sinker or striker roots that help stabilize the tree and help it exploit deeper soils, but they seldom extend below 1 meter deep (Harris et al.). A second group of perennial support roots grow horizontally radiating away from the tree and also provide transport of nutrients, water and oxygen to the canopy. In return, the canopy delivers energy back to the roots for their use and for storage. The last of the basic root structures to discuss here are referred to as absorbing roots (fine root hairs and the root tips roughly 1/16th inch diameter) that provide nutrient and moisture absorption for the tree. With that basic understanding, let’s move on to question number two. Where are the roots of a maturing or mature tree? Many consider the drip-line of the tree to be the limit of the root spread; therefore, the assumption is made that this is the area to fertilize and water for the
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benefit of the tree. A better mental picture to form is a wine glass positioned on a dinner plate. In support of this mental picture six species, including green ash, maple, and oak, were studied by Dr. Edward Gilman of the University of Florida. Dr. Gilman and associates performed root growth trials in Florida and New Jersey. The findings clearly debunked the “drip-line” notion of tree roots. On average they found that tree roots extended 3 times the spread of the branches and that more than 50% of the absorbing roots were beyond the dripline. Several numbers can be found in the literature concerning where the bulk of fine, absorbing roots are found in the soil profile. The stated depths range from the top 24 inches to the top 18 inches with 50% of the fine roots being in the top 6 inches of the soil profile. Dr. Gilman’s research tightens these numbers up considerably. From the same root growth trials in Florida and New Jersey Gilman states: “fine roots are concentrated in the top 12 inches of soil with many in the top 2 inches.” Okay, so some trees have roots near the soil surface to collect nutrients and moisture. “But, I’ve never watered my tree because its taproot gets all the water it needs from the Saint Peter sandstone aquifer.” There are species that as seedlings produce and rely on tap roots, but quite honestly, as “taprooted” species mature, tap roots become insignificant in the overall root structure. The bulk of nutrient and moisture uptake is taken over by the absorbing roots near the surface. Tap roots and other “sinker” roots can penetrate into the soil layers, but research indicates that depth of rooting is dependant on soiloxygen. Dr. Kim Coder from the University of Georgia states that good root growth requires a soil atmosphere of 15 percent oxygen. He continues by saying that below 5 percent soil-oxygen, root growth will stop and below 2% roots begin to decline and die. Dr. Coder describes the advancing root tips existence as quite precarious and more of a “good news / bad news” scenario. If the root progresses too deeply, oxygen deprivation will be an issue and on the other extreme if a root progresses too shallowly, a dry spell will likely cause its demise. As a matter of fact, this precarious situation translates into a short life span for an absorbing root with the root tip being replaced many times per growing season. The bottom line, unlike horses, roots do not smell water. Along the same line, roots do not seek water. Instead, roots tend to grow where the growing is good and the growing tends to be good in the top 12 inches of soil where temperature, moisture, nutrients and more importantly soil-oxygen are usually adequate. To underline the fact that roots are opportunistic “absorbers” – there has been a push lately to discourage calling fine absorbing roots, “feeder roots” simply because the term “feeder” implies an aggressive, hunter-gatherer approach to seeking life’s necessities. Roots simply follow the moisture gradient of the surrounding soil and continue to grow where the growing is good. Meaning that roots tend to grow where there is a good supply of moisture, nutrients and of course soil-oxygen.
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Excavated Lindens, three years into a nine year planting depth study. These specimens had been planted 10 inches too deep, note the roots returning to the surface. Supposition: roots were returning to zones with better soil-oxygen concentrations.
Now is the time to step back and put this information in perspective. Tree roots require oxygen to remain viable and soil-oxygen decreases the deeper in the soil profile that a root penetrates. Add in soil compaction to your thought process. By compacting a soil the pore space is reduced which in turn reduces the amount of soil-oxygen that can be present. Okay, one more thing – re-landscape the lawn with a bobcat and add 4 inches of soil to bury the roots a little deeper. Hold on, the lawn typically means turf and that means roots from grass and competition for moisture, nutrients, and you guessed it soil-oxygen. The picture should be coming clearer, tree roots, all too often overlooked, spread as far and as wide as they can in compacted soils and at depths below 12 inches trying to survive. Keep in mind that this is biology, and the ultimate root structure depends on many factors below ground. Site conditions, compaction, excess fill, flooding and infrastructure have direct and indirect impacts on soil moisture, soil oxygen, soil texture and structure. Moving on into the second round of questions: It has happened again! Another basement floods because a sewer line has been clogged by tree roots. Maybe this time it is another foundation or sidewalk being moved or lifted by tree roots. Are trees “perpetrators” of these acts or is it simply guilt by association. One side of the argument states that the tree and its roots are not at fault, but rather there is fault with the design or construction of the infrastructure. Honesty is probably the best policy; trees are not as innocent in the above scenarios as many of us would like to believe. However, why has the tree become the guilty party? Design error is often the only answer to be arrived at whether it is the landscape design or the engineering design. Let’s face it, trees require space for rooting and resource exploitation, if a tree root happens upon a nice moist,
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Minnesota Tree Care Advisors oxygenated resource-rich environment (a sewer line for example) where it can thrive and perform its duties – it will. In commenting on tree roots in sewers Richard Harris et al. states “the conditions of aeration, moisture, and nutrients are so favorable that it inevitably grows until it clogs the sewer.” The question in many minds still remains as to how the root happens upon the sewer line. Martin Mackenzie of the US Forest Service recently spoke to a conference audience describing the “battering-ram” root that breaks through even the toughest sewer lines and concrete blocks. Of course this statement was “tongue-in-cheek.” Roots grow quite slowly and don’t have much chance to hurl forward with any great speed. In the case of finding a sewer line there is both speculation and research that explains the encounter. Speculation from Harris et al. is that in some instances sewer line trenches are compacted to a lesser degree than the surrounding urban soil. Thus, homeowners may unwittingly take advantage of this and plant trees in the trench or perhaps the tree roots will encounter this less compacted environment and continue to grow into it unimpeded. Dr. Kim Coder researched the thermal gradients that exist between sewer lines and the surrounding soils which allow two things to happen. One is a moisture condensation layer on the sewer line itself. Secondly, a moisture condensation column develops in the soil layers above the sewer line. Once a tree root encounters this moisture condensation column it grows downward (providing adequate soil-oxygen exists) along the increasing moisture gradient in the soil toward the sewer line. Upon encountering the sewer line, the root tips continue growing towards more favorable conditions, eventually a root tip may find and exploit a crack or fissure in the sewer line. Hence, back to the argument proclaiming faulty infrastructure – “the root tip exploits a crack or fissure in the sewer line.” Once the root tip has entered the line, the real damage to the line can begin. The tree roots tend to form a mass of root tips that slow the flow enough to allow sediments to be trapped, thus clogging the line. Or the root may begin to develop as a woody, perennial root and exert pressures that may crush or burst the sewer line. Larger, woody, perennial roots exert substantial radial pressure and over time can displace man-made structures. The perennial root increases in diameter by adding a growth layer every year. Commonly, sidewalks are lifted by this type of radial growth. Displacement of structures can be caused by horizontal roots growing under a slab concrete floor or sidewalk or by a vertical “striker or sinker” root growing downward next to a foundation. In any case the root growth is slow and forces can be substantial enough to displace or otherwise damage structures. Another mechanism by which trees cause structural damage involves the presence of expansive clays. Expansive clays respond to soil moisture changes by expanding and contracting. Man-made structures often rely on the surrounding soils for additional support. If a tree’s roots encounter this “support soil” of expansive clay the moisture regime can be dramatically altered by the evapo-transpiration function of the tree. The clay soil type can be dried excessively causing it to contract, thus shrinking away from the structure it is supporting. The literature identifies several tree species as “culprits” in the “aggressive” root department. The following species are on a “Not Recommended for Planting” list maintained by the Michigan Department of Natural Resources as those having invasive rooting
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Minnesota Tree Care Advisors habits: silver maple (Acer saccharinum), white mulberry (Morus alba), white poplar (Populus alba), Eastern cottonwood (Populus deltoides), weeping willow (Salix alba), black willow (Salix nigra), and American elm (Ulmus americana). Is it necessary to “label” these trees as culprits and black list them? A better approach is to pay attention to putting “the right tree in the right place.” More and more attention is being paid to tree selection since it is the “key” to a trees long life. Site conditions truly need to be considered prior to starting the species selection process. It isn’t always successful to fall in love with a species and then try to fit it in our landscape. In wrapping all of this together, a number of additional questions come to mind? For instance: What does all of this say for our notion of watering and fertilizing within the drip-line? Yes, the water and fertilizer helps, but in light of root plates reaching well beyond the crown - consider expanding the applications to a broader area. With a tree’s absorbing roots concentrating near the soil surface, how well do the trees and turf co-exist? Trees and turf are in a competitive battle for resources and let’s face it – turf is a tough competitor. From a trees perspective: The more lawn that can be replaced with mulch the better. What does this portray for trees when parking lots, sidewalks, driveways, buildings, roads and other infrastructure are placed in close proximity to trees? These structures almost always require a compacted base layer to provide additional support for the impervious surface being applied. The needs of the tree and the reach of the root plate are too often ignored leaving trees with reduced stability and with reduced capacity to uptake nutrients and moisture. There is some good news out there… Recent methods of constructing sewer lines have dramatically reduced the potential for root intrusions. Longer sections of less porous pipe are being used for constructing sewer and water lines. This helps cut down on the conflict between tree roots and sewer lines that often results in a large, mature tree being removed. Researchers and trials in California are looking at composite sidewalk surfaces that flex, thus giving trees room to grow and more soil to exploit for resources. Researchers at Cornell University have been testing and working with CU Structural Soils® for some time. These soils are compactable, yet allow tree roots to penetrate. Placing these structural soils in the rooting zone under sidewalks, pavers, parking lots, roads and other structures allows trees to expand root plates farther. So, the but the
next time you look at your favorite tree – give some thought to root structure. You may not have the, “Oh Baby!” type of reaction, after reading this article the roots should at least be creeping into outer recesses of your mind.
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For More Information Bassuk, Nina. Cornell Structural Soil. Internet. Available 9/25/2001. (view cornell website) Coder, Kim D. Tree Roots and Infrastructure Damage. Internet. Available 1/27/2007. (view tree roots website) Coder, Kim D. “Don’t Stumble Over Surface Tree Roots” Grounds Maintenance. August 1 1998 Gilman, Edward F. Where are Tree Roots? Extension Service Bulletin ENH 137, Florida Cooperative Extension Service, University of Florida, Institute of Food and Agricultural Sciences. Internet. Available (view extension website) 9/25/2001. Harris, Richard W., Clark, James R., Matheny, Nelda P., Arboriculture: Integrated Management of Landscape Trees, Shrubs, and Vines. Third Edition. Prentice-Hall, Inc. 1999. Graphic, ISU Forestry Extension. Tree Roots October 2001. Internet. Available 9/25/2001. Graphic, The Morton Arboretum, Internet. Available 9/25/2001. Pool, Bob.“With Rubber Sidewalks, Trees Are on the Rebound,” LA Times. 14, July 2001. Internet. Available 9/25/2001. (view rubber sidewalks website) Randrup, Thomas B, McPherson, E. Gregory, Costello, Laurence R. “Tree Root Intrusion in Sewer Systems: Review of Extent and Costs.” Journal of Infrastructure Systems, Vol 7, No. 1, pages 26-31. March 2001. Soil Compaction: Causes, Effects, and Control. 2001. Internet. Available 9/25/2001. (view compaction website)
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Protecting Trees and Shrubs Against Winter Damage Bert T. Swanson and Richard Rideout
Copyright Š 2003 Regents of the University of Minnesota. All rights reserved. Minnesota's harsh climate is often responsible for severe damage to landscape plants. Winter sun, wind, and cold temperatures can bleach and desiccate evergreen foliage, damage bark, and injure or kill branches, flowerbuds, and roots. Snow and ice can break branches and topple entire trees. Salt used for deicing streets, sidewalks, and parking lots is harmful to landscape plantings. Winter food shortages force rodents and deer to feed on bark, twigs, flowerbuds, and foliage, injuring and sometimes killing trees and shrubs. All is not bleak, however, as landscape plants can be protected to minimize some of this injury.
Cold Damage Cold temperatures can damage plants in several ways. Plants that are not hardy in Minnesota will be killed or injured during the winter unless protected in a microclimate. Plants that normally grow in hardiness zone 3 (northern Minnesota) and hardiness zone 4 (southern Minnesota) may also be injured if winter conditions are abnormally severe or if plants have been stressed by the environment. Injury is more prevalent and more severe when low temperatures occur in early fall or late spring, when there is little or no snow cover during the winter or when low temperatures are of prolonged duration. Pronounced fluctuations in temperature can be extremely detrimental to plants throughout the fall, winter, or spring.
Figure 1. Repairing sun scald damage.
Sun Scald Sun scald is characterized by elongated, sunken, dried, or cracked areas of dead bark, usually on the south or southwest side of a tree. On cold winter days, the sun can heat up bark to the point where cambial activity is stimulated. When the sun is blocked by a cloud, hill, or building, bark temperature drops rapidly, killing the active tissue. Young trees, newly planted trees, and thin-barked trees (cherry, crabapple, honey locust, linden, maple, mountain ash, plum) are most susceptible to sun scald. Trees that have been pruned to raise the lower branches, or transplanted from a shady to a sunny location are also sensitive because the lower trunk is no longer shaded. Older trees are less subject to sun scald because the thicker bark can insulate dormant tissue from the sun's heat ensuring the tissue will remain dormant and cold hardy. Sun scald can be prevented by wrapping the trunk with a commercial tree wrap, plastic tree guards, or any other light-colored material. The wrap will reflect the sun and keep the bark at a more constant temperature. Put the wrap on in the fall and remove it in the spring after the last frost. Newly planted trees should be wrapped for at least two winters and thin-barked species up to five winters or more. To repair sun scald
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Minnesota Tree Care Advisors damage, cut the dead bark back to live tissue with a sharp knife, following the general shape of the wound, rounding off any sharp corners to facilitate healing (Figure 1). Wrap the trunk in subsequent winters to prevent further damage. Do not use a wound dressing. Spraying the area with a fungicide may help prevent fungal infection of the wound.
Winter Discoloration of Evergreens Browning or bleaching of evergreen foliage during winter occurs for four reasons: 1. Winter sun and wind cause excessive transpiration (foliage water loss) while the roots are in frozen soil and unable to replace lost water. This results in desiccation and browning of the plant tissue. 2. Bright sunny days during the winter also cause warming of the tissue above ambient temperature which in turn initiates cellular activity. Then, when the sun is quickly shaded, foliage temperature drops to injurious levels and the foliage is injured or killed. 3. During bright, cold winter days, chlorophyll in the foliage is destroyed (photo-oxidized) and is not resynthesized when temperatures are below 28째 F. This results in a bleaching of the foliage. 4. Cold temperatures early in the fall before plants have hardened off completely or late spring after new growth has occurred can result in injury or death of this nonacclimated tissue.
Foliar damage normally occurs on the south, southwest, and windward sides of the plant, but in severe cases the whole plant may be affected. Yew, arborvitae, and hemlock are most susceptible, but winter browning can affect all evergreens. New transplants or plants with succulent, late season growth are particularly sensitive. There are several ways to minimize winter injury to evergreens. The first is proper placement of evergreens in the landscape. Yew, hemlock, and arborvitae should not be planted on south or southwest sides of buildings or in highly exposed (windy, sunny) places. A second way to reduce damage is to prop pine boughs or Christmas tree greens against or over evergreens to protect them from wind and sun and to catch more snow for natural protection. Winter injury can often be prevented by constructing a barrier of burlap or similar material on the south, southwest, and windward sides of evergreens (Figure 2). If a plant has exhibited injury on all sides, surround it with a barrier, but leave the top open to allow for some air and light penetration. Figure 2. Protecting evergreens from winter burn with a burlap screen.
Keeping evergreens properly watered throughout the growing season and into the fall is another way to reduce winter injury. Never stress plants by under- or overwatering. Decrease watering slightly in September to encourage hardening off, then water thoroughly in October until freeze-up. Watering only in late fall does not help reduce injury. Anti-desiccant and anti-transpirant sprays are often recommended to prevent winter burn. Most studies, however, have shown them to be
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Minnesota Tree Care Advisors ineffective. If an evergreen has suffered winter injury, wait until midspring before pruning out injured foliage. Brown foliage is most likely dead and will not green up, but the buds, which are more cold hardy than foliage, will often grow and fill in areas where brown foliage was removed. If the buds have not survived, prune dead branches back to living tissue. Fertilize injured plants in early spring and water them well throughout the season. Provide appropriate protection the following winter.
Dieback Deciduous trees and shrubs can incur shoot dieback and bud death during the winter. Flower buds are more susceptible to injury than vegetative buds. A good example of this is forsythia, where plant stems and leaf buds are hardy, but flower buds are very susceptible to cold-temperature injury. Little can be done to protect trees and shrubs from winter dieback. Plants that are marginally hardy should be planted in sheltered locations (microclimates). Plants in a vigorous growing condition late in the fall are most likely to suffer winter dieback, so avoid late summer pruning, fertilizing, and overwatering. Fertilize in the spring on sandy soil or in the fall on heavy soil after the leaves have dropped. Root Injury Roots do not become dormant in the winter as quickly as stems, branches and buds, and roots are less hardy than stems. Roots of most trees and shrubs that grow in Minnesota are killed at temperatures at or below 0 to +10째F. These plants survive in Minnesota because soil temperatures normally are much higher than air temperatures and because soil cools down much more slowly than air temperature. Many factors influence soil temperature. Moist soil holds more heat than dry soil, so frost penetration will be deeper and soil temperatures colder for sandy or dry (drought) soils. Snow cover and mulch act as insulators and keep soil temperatures higher. With newly planted trees, cracks in the planting hole backfill will allow cold air to penetrate into the root zone, reducing fall root growth or killing newly formed roots. To encourage fall root growth and to reduce root injury, mulch new trees and shrubs with 6 to 8 inches of wood chips or straw. If the fall has been dry, water heavily before the ground freezes to reduce frost penetration. Check new plantings for cracks in the soil and fill them with soil.
Frost Heaving Repeated freezing and thawing of soil in fall or spring causes soil to expand and contract, which can damage roots and heave shrubs and new plantings out of the ground. A 4- to 6-inch layer of mulch will prevent heaving by maintaining more constant soil temperatures.
Snow and Ice Damage Heavy snow and ice storms cause damage by bending and breaking branches. Multiple leader, upright evergreens, such as arborvitae and juniper, and multiple leader or clump trees, such as birch, are most subject to snow and ice damage. Relatively small trees can be wrapped together or the leaders tied with strips of carpet, strong cloth or nylon stockings two-thirds of the way above the weak crotches (Figure 3). These wrappings must be removed in spring to prevent
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Figure 3. Protecting trees from snow or ice damage.
girdling, and to allow free movement of the stem. Proper pruning, to eliminate multiple leaders and weak branch attachments, will reduce snow and ice damage. For trees with large wide-spreading leaders or large multi-stemmed trees, the main branches should be cabled together by a professional arborist.
Salt Damage Salt used for deicing walks and roads in winter can cause or aggravate winter injury and dieback. Salt runoff can injure roots and be absorbed by the plant, ultimately damaging the foliage. Salt spray from passing autos can also cause severe foliar or stem injury. To prevent salt damage, do not plant trees and shrubs in highly salted areas. Avoid areas where salty runoff collects or where salt spray is prevalent, or use salt-tolerant species in these areas. Burlap barriers (Figure 2) may provide protection to some plants from salt spray.
Animal Damage Mice, rabbits (rodents), and deer can all cause severe damage to plants in the winter. These animals feed on the tender twigs, bark, and foliage of landscape plants during the winter. They can girdle trees and shrubs and eat shrubs to the ground line. Deer can cause significant injury and breakage by rubbing their antlers on trees during the fall. Rodents Trees can be protected from rodent damage by placing a cylinder of 1/4-inch mesh hardware cloth around the trunk. The cylinder should extend 2 to 3 inches below the ground line for mice and 18 to 24 inches above the anticipated snow line for rabbit protection (Figure 4). Hardware cloth can be left on year-round, but it must be larger than the trunk to allow for growth. For small trees, plastic tree guards are also effective. You can protect shrub beds from rabbits by fencing the beds with chicken wire; however, check such fenced areas frequently to ensure a rabbit has not gained entrance and is trapped inside.
Figure 4. Protecting trees from rodents.
If you have many trees or shrubs to protect, using screens and wraps may be too expensive and time consuming. In such situations, repellents may be the best solution. Remember that a repellent is not a poison; it simply renders plants undesirable through taste or smell. The most effective repellents for rodents are those containing thiram, a common fungicide. You can either spray or paint repellents on trees and shrubs. Repeat applications are necessary particularly after heavy precipitation. If these methods are ineffective, commercial baits containing poisoned grain are available. However, baits may be hazardous to humans, pets, and beneficial wildlife. Injury or death can result for animals that eat the bait directly and for animals that consume bait-killed rodents. Shelter or containerize baits so they stay dry and are accessible only to targeted rodents. Beverage cans laid on
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Minnesota Tree Care Advisors their sides work well for this purpose. Trapping and shooting, where legal, will also control rodents. Deer Deer feed on and damage terminal and side branches of small trees and shrubs. Repellents containing thiram provide some control if feeding pressure is not extremely heavy. Plants can be sprayed or painted with the repellent; however, the most effective procedure is to hang heavy rags near the plants to be protected that have been dipped in concentrated repellant. Repeated plant applications or dipping of rags is necessary. Deer can also be successfully excluded with fencing. To be effective, fences must be high and constructed properly. If deer are starving, there is little that will prevent feeding. Providing a more palatable forage may help, but it may also attract more deer.
Conclusion Although plant cold hardiness and winter injury are common concerns associated with Minnesota winters, appropriate plant selection, selecting the proper site, proper cultural practices, and preventive maintenance will significantly reduce or prevent severe injury or loss of landscape plants. Even though plants respond differently to winter stress and each winter provides a different set of stressful conditions, plants possess a remarkable ability to withstand extremely severe winter conditions. Minnesota winters should not discourage planting of traditional or new plant species. Learn to take advantage of microclimates to enable interesting or different plants to be grown. Minnesota's list of landscape plant species needs to be expanded, not reduced.
Bert T. Swanson Professor Department of Horticultural Science University of Minnesota Richard Rideout City Forester City of Milwaukee, WI Reviewed by Jeffrey H. Gillman Nursery Management Specialist Department of Horticultural Science
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