Appreciating Winter Hardiness
by Wayne Mezitt, MCH
O
ur 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 www.mnla.com
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