USDA Cold Hardiness Zones
If you look at most plant labels, you’ll see a zone designation, such as “Hardy to Zone 7.” These labels refer to the USDA Plant Hardiness Zones, which divide North America into zones based on estimates of the minimum annual temperature. A plant is considered “hardy” if it will survive the winter in that particular zone.
Each zone represents a 10 F temperature difference and is then further subdivided into “A” and “B” according to 5 differences. Zone 1 is the coldest and is subject to frost year-round while Zone 11 is the warmest and completely frost-free. If a plant is “Hardy to Zone 7,” that means it should survive the winter in Zones 7 and warmer.
Once you know the hardiness zone you live in, you can choose plants that will survive the winter in your area. Learn more at USDA.
Find Your USDA Hardiness Zone
USDA Zones in Greater Detail
Great for the East
The USDA map does a fine job of delineating the garden climates of the eastern half of North America. That area is comparatively flat, so mapping is mostly a matter of drawing lines approximately parallel to the Gulf Coast every 120 miles or so as you move north. The lines tilt northeast as they approach the Eastern Seaboard. They also demarcate the special climates formed by the Great Lakes and by the Appalachian mountain ranges. (Citation: Website – The National Gardening Association)
Zone Map Drawbacks
But this map has shortcomings. In the eastern half of the country, the USDA map doesn’t account for the beneficial effect of a snow cover over perennial plants, the regularity or absence of freeze-thaw cycles, or soil drainage during cold periods. And in the rest of the country (west of the 100th meridian, which runs roughly through the middle of North and South Dakota and down through Texas west of Laredo), the USDA map fails. (Citation: Website – The National Gardening Association)
Problems in the West
Many factors beside winter lows, such as elevation and precipitation, determine western growing climates in the West. Weather comes in from the Pacific Ocean and gradually becomes less marine (humid) and more continental (drier) as it moves over and around mountain range after mountain range. While cities in similar zones in the East can have similar climates and grow similar plants, in the West it varies greatly. For example, the weather and plants in low elevation, coastal Seattle are much different than in high elevation, inland Tucson, Arizona, even though they’re in the same zone USDA zone 8. (Citation: Website – The National Gardening Association)
Microclimates are small pockets of climate variations that differ from the surrounding climate.
By identifying and using microclimates you can grow fruit not normally recommended for your climate zone. Pay attention to the way the sun travels across your property throughout the season. Look for cold spots and hot spots. A maximum/minimum thermometer placed at different locations will tell you a lot about your microclimates. Here are a few other things to look for:
- Wind – Strong winds can decrease temperatures and damage trees. You may want to plan for a windbreak in a specific location to provide protection.
- Slope – Cold air can be like an invisible flow of cold water, which is why a valley floor is typically colder than higher up on a slope. But you can use slope to your advantage. If you’re in a warmer area and your trees need more chill hours, plant in the cooler valleys. If you have a colder climate and want to ward off frost damage, make use of available slopes.
- Thermal mass – Rocks and water absorb heat during the day and share it with nearby plants during the night. So a stone or brick wall can lend warmth to a late-fall producing fruit, just as jugs of water help warm a greenhouse. A pond can act as a heat sink during summer and a cold sink in winter.
- Sun – There’s good reason the winter outerwear company is named “The North Face” – you need those down- filled coats when you’re in a cold, north-facing area. And that idea can help guide your plant location. A Northern aspect would be a good spot if your fruit trees need more chill hours. Conversely, a Southern aspect provides more warmth. And an East-facing location enjoys softer morning sun, while a Western- facing spot basks in hotter and more intense afternoon rays.
Sunset Gardening has developed a different system of climate zones that is more informative, taking into account microclimate factors such as frost-free days, humidity, prevailing winds, effects of ocean, elevation and regional weather patterns. Learn more at Sunset.
These are the number of hours between 32°F and 45°F in your climate zone from November to mid-February. Temperate fruits need go through anywhere from 100 to 1,400 chilling hours to bear fruit the next season. Gauging cumulative chill and matching varieties for your area is more of an educated guess than an exact science, as low temperatures vary considerably within a climate zone and from year to year.
Chilling requirement is a concern for USDA zones 9B and 10, predominately in Southern and coastal regions where chilling hours average 100 to 600 per year. If you are within this area, take note of the chilling requirements listed for fruits and choose accordingly.
Most of Northern California receives between 800 and 1,500 chilling hours per year, which is sufficient for most fruits. Persimmons, almonds, olives, berries and pomegranates all have low chilling requirements. Low chill varieties are available for apples, pears, apricots, nectarines, peaches and plums. Use the map to see if you are in a low chill area and estimate your amount of winter chill.