Plant Hardiness, Heat Tolerance and Climate Zones

A plant’s performance is governed by many climate-related factors, including the growing season’s length, rainfall amount, winter lows, summer highs, wind, or humidity.

Various initiatives have taken place to determine how well plants would withstand the cold winter temperatures (Plant Hardiness Zones) or when plants would start suffering from the heat (Plant Heat Zones).

A more sophisticated approach has been followed by Sunset, which defines Climate Zone Maps based not only on hardiness or heat tolerance but also on precipitations, wind, humidity, and various other climate-related factors.

Selecting plants suited to your climate will be a key step to success. If you know your hardiness zone, heat zone, or Sunset climate zone, find the best plants or exciting garden ideas for your geographic area.

Alternatively, review our detailed plant selection guides for your region and find the best bulbs, perennials, shrubs, ornamental grasses, or trees for your garden.

USDA Hardiness Zones

Based on the minimum ten-year average winter temperatures, plant hardiness zone maps have been progressively developed by the Department of Agriculture (USDA) for the United States and then more or less applied to the rest of the planet. The purpose of these hardiness zones is to identify how well plants will withstand the cold winter temperatures of these zones.

The USDA Hardiness Zone Map divides North America into 13 zones of 10°F each, ranging from -60°F (-51°C) to 70°F (21°C). If you plan to buy a shrub, perennial, or tree, you need to ensure that this new plant will tolerate year-round conditions in your area. Therefore, to ensure your new acquisition will survive and grow year after year, you will need to compare your area’s hardiness zone with your plant’s hardiness zone (included in most American reference books, nursery catalogs, and gardening magazines describing plants).

However, remember that this USDA map is mostly a guideline. While the USDA map reflects the garden climates of the eastern half of North America, it does not factor in any elevation or precipitation, which impacts western climates. It does not include microclimates, humidity, or summer heat tolerance either.

AHS Heat Zones

To partly resolve some of the above issues, the American Horticultural Society developed a Heat Zone Map based on the National Weather Service (NWS) daily high temperatures recorded between 1974 and 1995. While the effect of heat is not as immediate and radical as severe cold, it may slowly damage and kill your plants. The most obvious heat symptoms include withering flower buds, drooping leaves, leaves discoloration (leaves may turn white or brown), or non-growing roots.

The AHS Plant Heat Zone Map includes 12 zones, each indicating the average yearly number of days with temperatures over 86°F (30°C). This threshold represents the point when the plants start suffering from the heat. These Heat zones range from less than one heat day (Zone 1) to more than 210 heat days (Zone 12). Similarly to the hardiness zones, most garden plants provide heat tolerance information. Therefore, you will find 4 numbers on each plant: Maximum Hardiness Zone, Minimum Hardiness Zone, Maximum Heat Zone, and Minimum Heat Zone. For example, a tulip may be 3-8, 8-1. If you live in USDA Zone 7 and AHS Zone 7, you will know that you can leave tulips outdoors in your garden year-round. An English wallflower may be 5-8, 6-1. It is relatively cold hardy, but can’t tolerate extreme summer heat.

Again, this Heat Zone map should be used as a guideline, and gardeners may find that many plants will survive outside their respective heat zone. The reason is that other factors could have an impact on the life of your plant, such as a lack of water (resulting from the heat), light (cloud cover, dappled shade), day length (the longer the summer day, the more impact on plant survival), air circulation (fast-moving air on hot days may quickly dehydrate the plants), surrounding elements (hard structures of stone, concrete) emit heat, raise the air temperature and soil pH.

Sunset Climate Zones

A plant’s performance is not only governed by its hardiness or heat tolerance but also by many other factors, such as the length of the growing season, timing and amount of rainfall, winter lows, summer highs, wind, and humidity. Sunset Magazine has developed a Climate Zones map, which considers all these factors and has long been the standard among gardeners living in the 13 western states. Unlike the U.S.D.A. map, which only tells where a plant may survive the winter, the Climate Zones map shows where that plant will thrive year-round.