18 minute read
UNR Cooperative Extension
Nevada’s Priority Agricultural Weeds: Perennial Pepperweed
By: Brad Schultz, Extension Educator, University of Nevada Cooperative Extension, Winnemucca, NV
Introduction
Perennial pepperweed (Lepidium latifolium L.), also commonly called tall whitetop, is a long-lived perennial weed originally from Eurasia. Following importation as an ornamental plant, perennial pepperweed subsequently spread throughout the western states, commonly inhabiting irrigated or sub-irrigated meadows and pastures, stream banks and associated riparian areas, irrigation ditches, marshes, floodplains, shorelines, exposed lakebeds, irrigated cropland, and areas that receive abundant run-on moisture (e.g., roadsides and seasonal streams). There are many adverse effects from perennial pepperweed. A solitary plant can rapidly develop a large root system that contains many buds, with each bud capable of developing into a new shoot or plant. A single plant can expand into a large dense stand of perennial pepperweed that displaces more desired vegetation, ultimately decreasing the land’s value for livestock grazing, field crops, wildlife and fisheries, and resale.
Perennial pepperweed’s root system is large but not dense; thus, it does not hold the soil together well. Sites that endure regular high-flow flood events and also have extensive stands of perennial pepperweed have an increased high risk for severe erosion. Furthermore, bank erosion from flooding typically breaks large perennial pepperweed root systems into many pieces. Root segments as short as one-inch long and one-tenth of an inch in diameter often have a bud capable of developing into a new plant. New infestations are common immediately after a flood because root segments are deposited downstream and many reside on or just below the surface of newly deposited sediment that remains moist for weeks after the flood. Moist sediment deposits are an optimal growing site for perennial pepperweed, from both root fragments and seed. Established perennial pepperweed plants also have roots that can grow deeper than most of our native plants. These deep roots can extract many salts from these depths and deposit them on the soil’s surface during leaf fall. The accumulated salts can reduce the germination and growth of better forage plants.
1A
Figures 1a and 1b. Perennial pepperweed seedlings at 4-6 weeks of age (1a) and a one-year-old plant (1b). The seedlings have developed a tap root but have not become perennial. The yearling plant has developed a lateral root from which a new shoot has developed. Additional buds are present throughout the root system of the yearling plant
1B
Plant Biology
Reproduction occurs from two sources: 1) the aforementioned buds on the roots, with as many as one bud per inch of root; and 2) from seed. The initial tap root grows downward deep into the soil (Figures 1a and 1b), and may reach a depth of almost 3 feet in less than 90 days. Lateral roots extend horizontally, often reaching tens of feet after several years. The majority of the root system is found in the top two feet of the soil, but in deep soils perennial pepperweed roots can penetrate to over 10 feet deep, often tapping into a shallow water table. Almost always, a substantial amount of the root system resides below the tillage zone of most equipment.
Root biomass often exceeds shoot biomass. This results in the roots storing a large amount of energy or carbohydrate reserves. The stored energy reserves keep the buds; hence, the plant alive during long dormant periods. They also provide the carbon to initiate the first leaves after dormancy breaks, or to facilitate regrowth after a disturbance removes the leaves and stems. Once the first few leaves develop, perennial pepperweed plants have enough leaf area for photosynthesis to meet the energy (carbohydrates) needs for additional growth and development. New plants can establish on small patches of bare ground, in otherwise well-vegetated meadows and pastures. The tap root subsequently develops lateral roots that extend far into the areas well-vegetated with desired perennial grasses. Some of the buds on these lateral roots develop shoots that emerge above-ground. These shoots enable the perennial pepperweed plant to compete with the desired vegetation for all of the resources needed for plant growth (e.g., sunlight, water and nutrients). Once seedlings have developed six to eight leaves on their root crown the root system starts to develop buds. These buds allow perennial pepperweed to regrow each spring, or following a disturbance. Once root buds have developed, elimination of perennial pepperweed (or any weed with root buds) requires killing both the buds and eliminating the seedbank. For perennial pepperweed plants at least oneyear old, the plant develops new leaves and shoots from buds on the previous year’s root crown and the established root system. These shoots become a rosette of leaves, which produce many carbohydrates that the plant reinvests into further growth, largely in the form of stems (tillers) that grow upward above the rosette and eventually flower. The tip of each elongating stem has a terminal growing point called the apical meristem, which becomes the plant’s flowers. When flowering starts, numerous additional lateral stems rapidly develop, each with many flowers. At peak flowering, most of the leaves on a perennial pepperweed plant reside in the upper one-third of the plant canopy, but just below the dense flowers. The carbohydrates the produced in these “upper leaves” largely move upward to produce developing seed, not downward as stored energy reserves in the root system. The carbohydrates used to sustain the buds on the roots largely come from the leaves in the lower half of the plant. Carbohydrate movement to the root system begins to increase at the bud growth stage, increases further at flowering, and peaks from flowering to seed production. Systemic herbicides intended to kill the root buds, piggyback on these carbohydrates and move from the lower leaves to the roots and accumulate in the buds, where they inhibit growth and eventually cause plant death. Understanding when carbohydrates are most likely to move from the leaves – and from which leaves – to the roots is important for timing herbicide applications. Unfortunately, the ability of perennial pepperweed to produce and translocate (move) carbohydrates declines substantially when irrigation or flooding saturates the soil. After only three days of saturation, plants may decrease their carbohydrate output by 62 percent. This dramatically reduces the translocation of carbohydrates to the roots, which probably reduces herbicide transport to the buds on those roots. Mature plants with leaves above the water tolerate and survive flooding quite well, but grow poorly during the high-water period. Systemic herbicides applied under these conditions are not likely to provide substantial control of the weed because there is poor movement of the active ingredient to the root buds. As the plant matures, the dense flowers and leaves just below them create a physical barrier for placement of an herbicide on the lower leaves. But, placement of an herbicide on the lower leaves typically provides better movement of the chemical to the growing points (buds) in the roots, which must be killed for effective weed control to occur. The most effective herbicide applications will be those that place the chemical on the lower leaves
(Figures 2 and 3).
Perennial pepperweed has exceptionally high seed production, reaching values of 3,000 seeds per inflorescence and 16 billion seed per acre. Seed production is greatest when perennial pepperweed grows in moist, non-saline soil. Seed production declines substantially when soils remain saturated and/or have high salinity. Up to 95 percent of the seed crop is viable and non-dormant, and can germinate immediately if it falls onto moist soil and temperature conditions are adequate for germination. Seed can remain submerged for at least 12 to 18 months and still remain viable. Up to 14 percent of the seed may remain in the seedheads into December. The retention of seed in the seedheads reduces the amount of seed lost to insects, soil pathogens, deep burial and other processes that typically reduce the number of viable seeds on a site. High seed head retention; however, may improve the efficacy of using fire to destroy seeds, provided the site can be safely burned before seed dispersal. The best germination occurs when the depth of buried seed is less than four-tenths of an inch and the soil remains moist (75 percent water holding capacity). Almost no seedlings emerge from seed buried deeper than one inch. This suggests that recently flooded areas with abundant bare-ground and slow drying soil are an optimal germination and establishment site. They should be included for periodic scouting as part of an early detection rapid response program intended to prevent widespread establishment of perennial pepperweed.
NON-CHEMICAL CONTROL
There are no known biological controls for perennial pepperweed. The success of mechanical (tillage, cultivation, etc.) or physical techniques depends greatly upon plant age. These approaches are most successful on seedlings before they become perennial (the 6-8 leaf stage of development) and unsuccessful on mature plants with established root systems. Mature plants have a large number of buds on their extensive root system and have large amount of stored energy. This facilitates rapid regrowth following tillage, handpulling and other types of physical disturbance. Tillage and cultivation treatments typically increase the weed problem because they create many root fragments (with buds) and often move them beyond established infestations. Mowing never works as a one-time stand along treatment, but can play an important role in managing perennial pepperweed when used in combination with other tools. Mowing perennial pepperweed shoots typically stimulates regrowth of existing shoots, but also additional shoots from buds on the roots. The abundant new growth decreases energy reserves, while also improving access to green leaves on subsequent regrowth. Literally, all of an herbicide applied toward this regrowth will fall on photosynthetically active leaves because dead stems and leaves are absent. Increasing herbicide contact with green leaves, especially those toward the base of the plant usually increases herbicide effectiveness. In California, mowing perennial pepperweed plants at the bud growth stage and applying an herbicide to the regrowth at the flower-bud stage dramatically improved overall control, particularly with glyphosate. On a research plot in Nevada, mowing of green growth, followed by herbicide application to the regrowth provided some enhanced control (but often not much) for some, but not all herbicides. A single application of chlorsulfuron (Telar), with or without any mowing treatment, resulted in complete absence of perennial pepperweed in midJuly of the following growing season. The different result between the two studies may be due to the long growing season at the California site (almost year-long). There was adequate soil moisture for both regrowth of mowed plants, and continued growth after the herbicide treatments. Many sites in Nevada do not have sufficient soil moisture in many years to support growth for that long a period. Fire is unlikely to be a successful stand-alone treatment for mature perennial pepperweed.. Mature, green perennial pepperweed can be difficult to burn, and fire does not adversely affect the viability
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Figures 2a-2c. Figure 2a shows a perennial pepperweed infestation treated with Imazapic in late July 2005 (a wet year) at to shortly after peak flowering. Treatment occurred with an ATV, with the boom located below the flowers. When the boom passed through the infestation it pulled the perennial pepperweed plants over and exposed the lower leaves to the herbicide. Figure 2c shows the same location as Figure 2a one year after treatment. Only a couple of perennial pepperweed plants were present one year after treatment. Figures 3a and 3b. A stand of mature perennial pepperweed treated with an aerial application of metsulfuron, dicamba, and 2,4-D (Cimarron® Max) in early June 2007 (Figure 3a) and one year later (Figure 3b). The application occurred near peak flowering and during a very dry year. The result was very little control the following year. Physical protection of the lower leaves from an herbicide application located well above the flowers and poor growing conditions contributed to an unsuccessful treatment. Subsequent aerial treatments of this site when growing conditions were better resulted in much better success. *
2A 3A
2B 3B
2C
Table 1. The list below identifies the active ingredients and many representative products known to control perennial pepperweed in rangeland, pasture and non-crop settings. Product selection should occur only after applicators have read all current product labels and identified the appropriate products for their specific situation, including potential effects to nontarget species. Many of the active ingredients listed in this table are available in premixed formulations with other active ingredients. These premixed packages (products) are not listed in this table. A complete list of all active ingredients and products labeled to control perennial pepperweed can be searched for at the Crop Data Management Systems (CDMS) website www.cdms.net
Active Ingredient Representative Proucts Selective Soil Residual Growth Stage
Metsulfuron Escort, Cimarron Yes
Imazapic Plateau and others Yes Some Seedlings, or flower bud to flowering stage on mature plants Yes Seedlings, or bud to later flowering stage on mature plants
2,4-D Many Yes No
Best at flower bud to flowering stage Glyphosate Roundup/many others No No Seedlings, or flower bud to flowering stage on mature plants Imazapyr Habitat, Arsenal only at low rates Yes Seedlings, or bud to later flowering on mature plants
of the buds on the root system. Regrowth will occur either the year of the fire treatment, provided soil moisture is adequate, or the following year. Targeted flaming, however, can effectively control young seedlings before they become perennial. Fire may be an appropriate tool when combined with other treatments, particularly an herbicide. Similar to mowing, fire can remove old, decadent, dead and/or taller vegetation that would reduce the amount of an herbicide reaching the green leaves of perennial pepperweed. Also, a spring burn when leaves are only a few inches tall and interspersed with substantial dead litter is likely to kill those leaves and stimulate buds on the roots to produce additional stems and leaves, potentially increasing leaf surface area while reducing carbohydrate levels in the roots. Increasing the leaf area and applying a follow up herbicide when energy reserves are less and carbohydrate movement the roots is greatest best, increases the probability of a successful herbicide treatment. Also, later summer, fall or early winter burn may kill many of the seeds that still reside in the dead flowers, and possibly some located in deep litter just above the soil surface. Fire typically does not kill seeds in the mineral soil. Grazing may suppress perennial pepperweed but does not reduce its spatial extent unless it is very intense, all of the growing season, for several or more years. Cattle, sheep and goats will graze perennial pepperweed, particularly at the rosette growth stage. As the plant matures coarse stems elongate and the leaves often develop waxy coatings. At this growth stage, sheep and goats are more likely than cattle to select the plant for feed, and goats generally will select a greater portion of their total diet as perennial pepperweed, than will sheep. Perennial pepperweed can store more energy in its roots than most, if not all, perennial grasses; therefore, it can withstand heavy prolonged use better than the desired perennial grasses. Once grazing animals leave the grazed pasture perennial pepperweed tends to regrow rather quickly. When using a grazing animal to control weeds, management must consider how the timing, duration, and intensity of defoliation directed toward the weed may affect the desired residual herbaceous species needed to inhabit the site posttreatment. Treatment of the perennial pepperweed should not result in a permanent adverse effect on the desired species. It’s only a matter of time until some other weed occupies the ground, or the pepperwed returns. Flooding can be used in areas where water depth can be controlled for long periods. This approach works best when the entire plant is submerged for at least several months and perhaps as long as six months. Partial submergence is ineffective. Flooding has little effect on the seedbank, and could enhance seed germination and seedling density as the water recedes and soils dry slowly. When this happens, a follow up treatment of the seedlings can be very effective: seedlings are the easiest growth stage to control. The sequential combination of mowing, tillage and tarping can be effective for small infestations not suitable for an herbicide treatment. The mowing and tillage components effectively kill the aboveground biomass and break the upper roots into small pieces. Each root segment can produce a new plant but the small size of most root segments limits the amount of stored energy available to support the new plant. The tarp prevents sunlight from reaching any regrowth and the new shoots eventually deplete their stored energy and die. It will likely require a longer period to deplete the energy reserves of the large intact roots that reside below the tillage zone. The desired residual vegetation on the site also would die; thus, the site would have to be revegetated with desired species to reduce the risk of reinfestation by either perennial pepperweed or another weed. This is an example of an integrated weed management approach molded to the small scale of specific infestations. The best management approach to minimize the risk of perennial pepperweed rapidly invading (or reinvading) a site is to promote a dense stand of vigorous perennial grasses, with large and deep root systems, and very little bare ground. When bare ground is largely absent, there are very few sites upon which a viable seed can eventually settle, germinate, and if it germinates, live long enough to establish a root system that facilitates long-term survival and reproduction.
HERBICIDE CONTROL
There are numerous herbicides used to control of perennial pepperweed (Table 1). Most have a lengthy soil residual which helps control seedlings the following growing season. A number of research studies have shown 2,4-D and glyphosate provide less long-term control than most of the other herbicides. These two herbicides, however, may have a very appropriate role in some situations. For example, when a soil residual is not desired. Every infestation is unique, as are all management operations, and all tools need to be evaluated accordingly. The active ingredients, chlorsulfuron, and metsulfuron have proven very effective. The key to achieving the best potential success with any herbicide treatment is placing the herbicide on the plant’s lower leaves from the bud-flower to peak flowering growth stages. This improves translocation to the buds (sites of action) on the roots (Figures 2 and 3). Treatment should occur when the plant will be actively growing will have high photosynthetic rates for several weeks after application. The long post-application growing period improves the potential for the herbicide to move deep into the root system and potentially kill more buds. If the treatment area has desired vegetation that needs to increase following the herbicide application it is important to use a chemical that will not harm those plants. Herbicide selection should always consider the effectiveness of the chemical on the weed and its effects on non-target species. Read the product label thoroughly to fully understand what that production can accomplish, the best conditions for its application, and potential adverse effects to non-target species.
INTEGRATED MANAGEMENT
Effective control of perennial pepperweed is a long-term management issue that must operate within the weed’s complex biology, the land owner’s operational constraints, and the production potential of the area infested. No single tool will provide effective control long-term. Integrated management combines two or more different weed control methods to perpetually harm the weed and benefit desired species. Once treatment begins the nature of the infestation changes: sometimes in desired and expected ways, and sometimes not. As the nature of the infestation changes, the tools used to contain and manage the infestation should change. The specific tools used in year one of a treatment program, or the order and intensity for which they occur, may be entirely different three or four years later. The applied treatment methods may occur concurrently, sequentially, or in some cycle. Each infestation is unique; thus, no standard prescription is available. The best integrated approaches have prevention of establishment as an over-arching approach. This article is based on a UNR Cooperative Extension Special Publication SP-21-01, published in early 2021. The Special Publication includes more detail for plant biology, treatment approaches and integrated weed management than could be printed on these pages. It is available at: https:// naes.agnt.unr.edu/PMS/Pubs/2021-4108.pdf Similar information for horary cress (short white top) and Russian knapweed can be found at: https:// extension.unr.edu/profile.aspx?ID=877#hPubs
* Listing a commercial herbicide does not imply an endorsement by the authors, University of Nevada Cooperative Extension or its personnel. Product names were used only for ease of reading, not endorsement. Herbicides should be selected for use based upon the active ingredient and the specific bioenvironmental situation. Listing a commercial herbicide does not imply an endorsement by the authors, University of Nevada Cooperative Extension or its personnel. Product names were used only for ease of reading, not endorsement. Herbicides should be selected for use based upon the active ingredient and the specific bioenvironmental situation.
