Features

Appreciating Winter Hardiness

by Wayne Mezitt, MCH

Our clients often express confusion why some plants thrive in their gardens and others disappoint. Why can crape myrtle (Lagerstroemia) and camellia do well on Cape Cod but fail in our mainland gardens? What causes winter-burned leaves or disappointing flowers on some rhododendrons, while others in my yard look just fine?

As professionals, reasons for these situations may seem obvious to us. But most homeowners are less savvy, and that’s why applying our professionalism for them is imperative.

Researchers tell us that plants develop their optimum tolerance for cold temperatures with gradual acclimation each winter. “Cold hardiness requires that plants have acclimated properly through the natural process of lengthening nights and cool temperatures to become dormant. This is an active metabolic process requiring adequate moisture and proper nutritional balance.”

Flower buds are generally less coldhardy than growth buds on the same plant. The actual flower bud hardiness of each individual plant is determined by “the relationship between the degree of cold hardiness (super-cooling ability of florets) and the acclimation intensity in flower buds.” Buds are at their hardiest condition when full dormancy conditions have been reached.

Every tree, shrub, and perennial has its own unique genetic composition that determines its winter hardiness. If a plant cannot tolerate the minimum winter temperatures in a region, it won’t thrive. A plant’s hardiness rating is a fundamental element we New Englanders consider when designing a garden, and it’s frequently the only major factor in our decision.

Note: Be sure your clients understand that “hardiness” differs from “heartiness” (which is the vigor and robust qualities shown by the plant). Many plants are hearty in appearance even though they are not sufficiently hardy to thrive in certain climates.

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Horticulturists have measured the minimum (and maximum) range of temperature tolerances for virtually every species on the market. The USDA Temperature Zone ratings (which map the nationwide historical minimum temperatures) are the most widely accepted criterion for plant adaptability.

For Massachusetts: • Worcester has historically been in USDA Zone 5, experiencing average minimum temperatures ranging from –20° to –10° F. • Boston is in USDA Zone 6, typically expecting minimum winter temperatures from –10° to 0°F. • Cape Cod is mostly USDA Zone 7, 0° to +10°F. average minimum.

Garden plants such as firethorn (Pyracantha), dove tree (Davidia), cherrylaurel (Prunus laurocerasis), and Leyland cypress (Cupressus × leylandii) generally succeed on Cape Cod and may perform well in Boston, but are disappointingly less consistently winter-hardy most years in the Worcester area, northward, and westward. USDA Zones rate the above-ground hardiness of the plant in the ground, not necessarily its root hardiness. Potted plants tend to be less winter-cold tolerant than their in-ground sisters because their root system is exposed and can be damaged when matching the ambient air temperatures. But it’s important not to use the historic USDA Zone ratings as the sole determinant of a plant’s suitability for a location! As our climate changes, average minimum winter temperature zone ratings continue to evolve. Many additional considerations (some are called microclimate factors) should be taken into account in choosing the appropriate plants for your client’s needs. • Annual temperature extremes and fluctuations; humidity • Expected snow cover, first/last frost dates, length of the seasons

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Two convenient locations: A&B Insurance Group, LLC 235 Littleton Rd, Unit 3 Westford, MA 01886 Wendy M. Ashe, AFIS Richard Bourgault, AFIS 978-399-0025

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• Soil characteristics including moisture, drainage, acidity, and organic content • Shade conditions and intensity of sunlight • Exposure to wind, salt spray, and traffic • Proximity to bodies of water or structures that tend to moderate temperatures • Competition from other plants for water, nutrients, and light • Disturbance by animals and pests

Some cultivars have been bred/selected to suit challenging conditions, and a few species of lesshardy genera tolerate colder temperatures. For example, the blue hollies (Ilex x meserveae) show little of the winter damage that often affects the appearance of other types of evergreen hollies. Winter protection with bags Rhododendron PJM differs from most rhododendrons, proving tolerant to low temperatures, full sun, as well as open, windy locations that can damage the familiar large-leaf rhododendrons. The species Abelia mosanensis (fragrant abelia) is far more winter-hardy than other abelia species which suffer winter damage in northern climates. Many new cultivars of plants now on the market, like dogwood, forsythia, and viburnum, have been bred and selected for improved winter Partially blasted rhododendron bud hardiness.

Local plant societies such as the Massachusetts Horticultural Society, Grow Native Massachusetts, the Massachusetts Chapter of the American Rhododendron Society, and Native Plant Trust provide helpful information for plant performance in regional gardens. Most plant catalogs publish zone ratings for the plants they list, but these can often be “generous” in their claims; if a plant hardiness rating looks too good to be true, be wary.

Working with your clients so they understand why so many factors, including winter hardiness, affect their garden plant choices will enhance your credibility with them and also result in better outcomes for their yards and your professional reputation.