Today’s gardeners have easy access to amazing plants from all over the world: agapanthus from South Africa, ferns from New Zealand, grasses from Japan and orchids from Brazil. But how do you know which of these plants will grow well in your garden?
Where Do They Come From?
Knowing a plant’s native habitat can give you valuable clues about where it will be happy to grow. Plants native to the tropics won't tolerate temperature extremes because they evolved in environments where temperatures usually vary no more than 10 to 20°F.
On the other hand, cold climate plants are accustomed to coping with large temperature swings. They have developed special strategies to survive, such as dropping their leaves in fall (to conserve moisture) or modifying the structure of their cell walls (to protect them from freezing). Plants from hot, dry climates have their own adaptations, such as waxy or hairy leaf surfaces (cycads and lavender) or succulent stems and leaves (cactus and aloe).
Hardiness Ratings and the USDA Zone Map
In the U.S., most garden-worthy trees, shrubs, perennials, bulbs and other plants have been tested for their cold tolerance or “hardiness” and are rated using a numbering system developed by the U.S. Department of Agriculture. The USDA Plant Hardiness Zone Map divides the country into 13 climatic zones. These range from zone 1, where the lowest average winter temperature is -50°F, to zone 13, where winter temperatures rarely dip below 65°F.
If you garden in zone 6, for example, and purchase a plant that is rated as hardy in zone 6, you can be quite confident the plant will survive winter in your yard without any special attention. For a state-by-state USDA Hardiness Zone Map, click HERE.
How Plants Prepare for Cold Temperatures
Plants that can adapt to cold winter temperatures rely on environmental cues to tell them when the seasons are changing. Shorter days and cooler nights signal that winter is approaching, and this activates the physiological changes required for survival. Plants pull moisture down into their root systems. Leaves drop. Growth slows. Similar changes occur in spring. As days get longer and temperatures rise, plants wake from winter dormancy and resume active growth.
It takes time for plants to make these subtle physiological changes, which is why an unexpected cold snap can be damaging. Plants that would normally survive 10° during the depths of winter, can be killed by a 20° night in early December, because they haven’t had time to adjust. The same thing can happen in spring. A week of unusually warm days can lure plants out of dormancy and leave them vulnerable to damage from late spring frost.
The AHS Heat Zone Map
Summer heat can be as challenging to plants as winter cold. The American Horticultural Society has supplemented the USDA Hardiness Zone Map with a Heat Zone Map. It uses a similar numbering system with 12 climate zones, ranging from zone 1 (less than 1 day per year over 86°F) to zone 12 (more than 210 days per year over 86F).
Thousands of ornamental plants have been evaluated for their heat tolerance, and many plant labels now display both rating systems. A daffodil, for example, may be labeled 3-8, 6-1 (winter hardy in zones 3-8 and heat tolerant in zones 6-1). The AHS Plant Heat Zone Map is available HERE.
Hardy vs. Suitable
Many of the most desirable flowering plants can’t survive cold winter temperatures. The list includes garden favorites such as geraniums, fuchsias, dahlias, petunias, tuberous begonias and cannas. But during the summer months, these non-hardy plants are perfectly “suitable” for almost every growing zone. When fall arrives and the weather turns cold, some of these cold-sensitive plants can be brought indoors for the winter. Others can simply be treated as annuals. When purchasing plants, it's a good idea to always check the hardiness rating so you know what to expect.