Understanding Cold Hardiness and Heat Tolerance
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. How do you know which of these plants will grow well in your garden?
Learn About the Plant’s Native Habitat
Knowing where a plant originally comes from can provide valuable clues about the growing conditions it will prefer.
Elephant ears, caladiums and bromeliads flourish in tropical conditions with high humidity and temperatures that don’t vary more than 20°F. On the other hand, cold climate plants can handle (and sometime require) large temperature swings. They have developed special strategies to survive the cold, 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. These may include waxy or hairy leaf surfaces (cycads and lavender) and succulent stems and leaves (cactus and aloe).
It can take a lifetime to learn about the provenance, preferences and idiosyncrasies of various plants. Fortunately, there’s a useful tool that can help you know in advance, which plants are likely to survive in your garden.
Cold Hardiness Ratings and the USDA Zone Map
The U.S. Department of Agriculture issued the first USDA Plant Hardiness Zone Map back in 1960. It divides the country into 13 climatic zones, based on the average annual extreme minimum temperature. Cold zones range from 1, where the lowest average winter temperature is -50°F, to 13, where winter temperatures rarely fall below 65°F. Adding the letters “a” and “b” further divides the zones into 5°F increments.
Before plants reach the market, most garden-worthy trees, shrubs, perennials, bulbs and annuals are tested to see how well they will survive cold temperatures. Such tests are typically conducted by commercial growers, universities or arboretums who then correlate their data with the hardiness zones.
How Plants Prepare for Cold Temperatures
Plants rely on environmental cues to tell them when the seasons are changing. As winter approaches, shorter days and cooler nights activate important physiological changes that must occur before a plant can survive the coming cold. The approach of spring sets other changes in motion. 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, so unexpected cold snaps can be damaging. Plants that would normally survive 10° temperatures during the depths of winter, can be killed when they are unprepared for a 10° night in early December. 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 a late spring cold snap.
The USDA Hardiness Zone Map is a valuable reference tool, but it’s good to remember that other factors also contribute to a plant’s ability to survive cold winters. These include the reliability of snow cover, the duration of winter cold, temperature fluctuations, precipitation amounts and soil conditions.
Heat Tolerance and the AHS Heat Zone Map
Summer heat can be as challenging for plants as winter cold. At temperatures hotter than 86°F, many types of plants begin suffering physiological damage. They may stop blooming and putting on new growth. The leaves may become limp and the edges may curl or become discolored. Heat stress also makes plants more susceptible to pest and disease problems.
To help gardeners determine which plants are able to handle hot summers, the American Horticultural Society developed a Heat Zone Map. It uses a numbering system that’s similar to the hardiness zone map. There are 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).
Some commercial growers, universities and arboretums are now evaluating plants for their heat tolerance as well as cold tolerance. 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.
RHS Hardiness Scale for the UK and Europe
In 2012, the Royal Horticultural Society (RHS) introduced a plant hardiness scale for the UK and Europe. Like the USDA system, it is based on minimum winter temperatures. But there are two big differences. The zones are in 5°C increments and they run in the opposite direction: from H1 (very tender) to H7 (very cold hardy). Unfortunately, the RHS hardiness scale has not been mapped to the UK and Europe.
How to Create and Leverage Microclimates
Gardeners hate playing by the rules. Knowing a plant is unlikely to survive the winter doesn’t stop us from wanting to give it a try. If you don’t mind living dangerously, you can experiment by using microclimates to stretch your hardiness zone.
To create slightly warmer growing conditions, choose a planting location that faces south. This will ensure the plant gets maximum exposure to sun and heat. A protected area that’s sheltered from wind also gives heat-loving plants an advantage. A south facing wall will absorb heat during the day and release it at night, raising the nearby air temperature by several degrees.
Frost pockets are areas in the landscape where cold air has a tendency to collect. They typically occur at the bottom of an exposed slope, in a valley, or in places where the air is relatively stagnant. Areas that are exposed to winter winds can also be colder than normal and cause plants to dry out. Avoid these locations when planting marginally-hardy plants.
If you garden in hardiness zones 8-11, you may be looking for microclimates with cooler-than-average growing conditions. Consider an east facing position that only gets morning sun. Overhead foliage or nearby plants can provide shade during the hottest part of the day. Depending on the orientation, shadows cast by walls, fences or hedges will keep the soil cooler than normal.
The Difference Between Hardy and Suitable
There are many plants that can’t survive very cold winters. But during the summer months, these plants are “suitable” for growing almost everywhere in the country. These summer-only plants include many garden favorites such as geraniums, impatiens, fuchsias, dahlias, elephant ears, petunias, tuberous begonias and cannas.
At the end of the growing season, when the weather turns cold, you have a couple options. You can treat any non-hardy plants as annuals and plan to purchase new ones in the spring. Or, you can bring these plants indoors for the winter and bring them out again in the spring. Learn more here: Winter Storage for Tender Bulbs and Tubers.
Gardeners in the U.S. are very fortunate to have the USDA Hardiness Zone Map. It’s an invaluable reference for navigating the plant world.