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USDA Hardiness Zones


What Are The USDA Hardiness Zones?

The USDA Hardiness Zone map is a climate survey completed in 2012 compiling the average minimum extreme temperature in more than 60 cities throughout the country including outlier states Alaska and Puerto Rico. The average extreme minimums and assigned zones come from more than three decades of data and climate research to best help growers weather the harsh winter months. Understanding hardiness zones is crucial to knowing which crops are able to overwinter successfully in any region across the country. Although the USDA Hardiness Zone map gives specific temperature ranges for winter, the map does not address other important factors such as humidity, precipitation, isolation, city heat mapping, or maximum average summer temperatures. Similar to an almanac, the USDA Hardiness Zone map can be useful to help predict the oncoming winter.

What Is My Growing Zone?

The USDA Hardiness Zone is a user friendly color-coordinated map intended to provide immediate overwintering temperatures and is as simple to use as just matching the color to the key. While the USDA provides subcategories to each zone (6a, 6b, 7a, 7b) the temperatures only vary by about 5 °F in the dead of winter and are not commonly used by many growers, as the temperature difference is not enough to affect any annual summer crops. Despite finding your specific hardiness zone, it is important to know that small regional micro conditions such as shade, valleys, canyons, isolation, and bodies of water will affect both summer and winter gardens. Although the USDA Hardiness Zone map may inform you about the average minimum extreme temperature for your grow zone, there are many other factors that will determine the success of a crop.

Perennial Winter Gardening

The USDA Hardiness Zone map is most applicable and useful for overwintering gardens during the coldest months of the year. For example, cruciferous vegetables such as broccoli, cabbage, collard, and kale are well-known for their tolerance to frost and can thrive outdoors down to a freezing 25 °F. Knowing the minimum temperature in which broccoli will grow, we can use the color coordinated key to learn that broccoli will overwinter in zones 9 and up. Growers in slightly colder hardiness zones may try broccoli in their garden, but will have to create some type of microclimate (protective wall, burlap sack, row cover) to keep plants warmer than the surrounding outside temperature. Snow is also a naturally warming incubator and, albeit counterintuitive, actually helps protect many crops through to spring.

Annual Summer Gardening

As mentioned, the USDA Hardiness Zone map does not provide any summer data including humidity, precipitation, urban heating mapping, or maximum temperature averages, leaving some trial and error to the home gardener. However, understanding which summer crops prefer cooler conditions helps growers plan their sowing dates around the final frost of the spring and the first hard autumn frost. For example, frost hardy vegetables such as broccoli, cabbage, kale, and leaf lettuce will bolt to seed in the middle of summer if not grown in cool, temperate gardens. Growers can adjust their sowing dates so that the mature vegetable greens will experience the chill of an early spring or the light frost at the beginning of fall. Microclimates such as shade, valleys, and bodies of water will artificially cool an environment and allow a variety of plants to thrive in regions that may otherwise be too warm.

Plant Hardiness Zones United States

The continental United States is home to a wide range of climates ranging from hardiness zones 3-10 which feature the warm zone 10 tropics of Florida and southern California all the way up to the frigid zone 3 expanses of North Dakota, Minnesota, and Maine. Even within small regional boundaries, hardiness zones can vary greatly based on differences in altitude. For example, many parts of Colorado, Utah, and Appalachia feature a variety of hardiness zones due to local mountain ranges that greatly affect climate. Alaska, as can be expected, is home to the Arctic Circle and the coldest of hardiness zones 1-3 while Hawaii and Puerto Rico boast the perennially warm tropics of zones 10-13. Understanding hardiness zones has helped decide which regions in the United States are optimal for commercial agriculture. The 35 °F winter conditions in Yuma, Arizona have proven to be ideal for commercial lettuce farming while the 25 °F minimum throughout the south has made collard greens a fall and winter staple to about half of the country.

Plant Hardiness Zones Canada

Although the USDA Hardiness Zone map pertains only to the United States, much of the data collected over the 30-year sample period was used by Natural Resources Canada (NRC) to draft their own national model of a hardiness map based on the survival rate of various flowers, shrubs, and trees. The first Canadian Hardiness Zone map was developed in the 1960’s, about the same time as the revised American model was released in 1967. Unlike the American model which only features the extreme annual minimum temperature, the NRC model is adjusted to factor in wind, snowfall, maximum temperatures, and rainfall for both summer and winter. The NRC model also limits the grow zones to 9 while the USDA model has designated 13 zones to account for southern humid tropics all the way up to the vast freezing stretches of the Midwest. American gardeners in Canada are probably best to think of Canada’s grow zones as being similar to Alaska, with the warmest most temperate zones immediately on the coast where the ocean naturally warms and regulates extreme climates.

Microclimates

A microclimate is a limited and restricted grow space that has a different temperature and set of growing conditions than that of the larger surrounding region. Microclimates can be natural such as valleys, canyons, and bodies of water or man-made such as row covers, greenhouses, and shaded awnings. Providing shade is the simplest and most popular means to creating a microclimate. For example, lettuce and similar cold hardy favorites do not perform well in midsummer heat and are often provided additional shade and protection to keep the greens from bolting to seed. Walls of any kind are a substantial provider of microclimates, able to protect crops from regional heat, wind, rain, and critters that may otherwise challenge a home garden grown out of zone.

Alternative Hardiness Zones

While the USDA Hardiness Zone map has been the agricultural standard in the United States for the past decade, there have been many challengers to it arguing for a system that addresses the more nuanced facets of regional horticulture. In the United States, the USDA classification system has arguably favored the east coast because of its lack of dynamic terrain whereas the western United States features vastly different biomes. Even the popular west coast magazine Sunset has created its own alternative Climate Zones for western gardeners that takes into account precipitation, wind, and elevation. Canadian Forest Service devised a national method to determine plant hardiness that incorporates additional environmental factors such as wind, snowfall, maximum temperatures, and rainfall for both summer and winter. The Köppen climate classification is more than a century old and divides regions into five biomes Tropical, Dry, Temperate, Continental, and Polar, each of which have about a dozen subcategories.

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