The USDA’s plant hardiness zone maps are drawn from the average extreme minimum temperatures over the previous 30-year period. They don’t use the lowest ever temperatures that have occurred, however, and gardeners need to remember this, especially when they’re choosing plants that are marginal or completely out of their zone.
While the PHZM is drawn in great detail – the 2016 version is the most detailed yet – it doesn’t take into account microclimates that are too localised to show up.
Microclimates are small areas where temperature and other weather conditions differ from the surroundings. There may be a heat island caused by concrete and blacktop, or colder spots caused by hills. Individual gardens also have microclimates – a garden may be sheltered from the winds by a high wall, or there may be a southern-facing wall that traps sun and radiates heat later in the day. Local gardening experts like thetreecenter.com can offer advice on microclimates.
Sometimes plants acquire cold hardiness in autumn, in response to cooler temperatures and shorter days. In late winter and spring this hardiness is lost as temperatures and daylight hours increase. However, a spell of unusually cold weather in early fall might damage plants even though the temperature hasn’t reached its lowest yet. A bout of unseasonably warm weather, followed by a return to normal, can also hurt plants. The PHZM doesn’t account for this, either.
PHZMs are simply guides, based on average lowest temperatures, not the lowest. Plants at the edge of their growing zone could be suddenly blighted by an exceptionally cold winter, even though they’ve been happy for several years. Gardeners need to use common sense when selecting marginal plants and if an unusually cold spell develops, they should insulate vulnerable plants.
Factors besides temperature and hardiness
There are other environmental factors that can mean success or failure for plants – the wind, the soil type and moisture levels, snow, pollution and winter sunshine can affect plants a lot. Additionally, the placing, size and general health of a plant can have an influence on its fate.
Plants need to go where they’ll receive their optimal amount of light – plants needing partial shade and that are marginal might find a lot of winter sunshine problematic as it will cause too many fluctuations in its temperature.
Every plant needs a different amount of moisture and this requirement sometimes changes throughout the seasons. Plants that can handle a bit of cold might be pushed over the edge if the soil’s a bit cold and dry.
Each plant also has an optimum temperature range; for some this range is wide and for others narrow. For narrow range plants, it’s best to keep them well within their hardiness zone.
For some plants, a short snap of cold isn’t the end of the world, but a prolonged chilly period is.
High air humidity can prevent a lot of cold damage because it reduces the loss of water from leaves, buds and bark. Low humidity can intensify cold injuries, and this is especially so for evergreens.