Understanding The USDA Hardiness Zones
Copyright Robin Follette, 2006
Reprints by permission
The USDA zones are too often misunderstood. As a result of the misunderstanding, they’re often misused. Let’s clear up the confusion.
The USDA zones break the United States into 11 individual zones. There is a 10° difference in zones. Each zone tells us the annual average coldest temperature in that area. Since the coldest temperature happens in the winter, zones have nothing to do with the crops you grow only in spring, summer or fall.
Zones don’t tell us when to expect our last spring frost or first fall frost. Those dates vary by several weeks throughout the zones. A colder zone than the one you’re in might have an earlier last frost date, allowing you to plant tender annuals earlier. The other zone has a lower average temperature in the winter but it warms up sooner in the spring.
Zones don’t tell us the length of daylight, the average high temperature, the number of days above a certain temperature, soil temperature or what can be planted at any given time. We’ve given the USDA zones credit for information they don’t supply.
So when do we need to use zones? If you farm in warmer climates you need to know how much cold a plant must have. You won’t find citrus trees growing here in Maine because it’s too cold in winter. Even if they could survive our cold winters, frost would kill the blossoms in spring. Some fruit trees need a period of dormancy over the winter before they’ll blossom and produce again. If you’re in a zone that doesn’t get cold enough to provide this rest period you’ll have a harder time growing those fruits. The tree will most likely survive but not produce. We’re having this problem with apples in parts of New England this year. Our winter was so mild that many of the trees didn’t blossom this spring. It felt good to have 50° days in January and February but we’re paying the price for it now. Apple production on my farm this year will be disappointing. About half of my trees didn’t blossom at all. My cherry trees didn’t blossom well so there are few cherries on the trees. My cherries are biennials so I won’t have another chance at a decent cherry harvest again until 2008.
If you’re growing perennials in a cold climate you need to know what zone those plants are hardy in, and in which zone you’re located. Perennial vegetables such as asparagus, artichokes and rhubarb are hardy enough to survive winter in some zones but not in others. I grow artichokes as an annual because they won’t survive at -25°F. We need to know what zones perennial herbs are hardy in. Perennial herbs in warmer climates are annuals in the colder climates. When you’re farming in the of warm and cold you can mulch some herbs heavily in the fall and get them through winter. Flower bulbs will survive very cold winters and break dormancy as soon as the soil starts to warm, sometimes before all of the snow is gone. If the same bulbs are planted in a warm zone the plant is likely to have a lot of foliage but poor flowering ability. Flower farmers pay close attention to zones for this reason.
The Zones
There are 11 numbered zones that break down into lettered zones. They start at zone 1 and go to zone 11. Zones 2 through 10 are divided into a and b zones with 5° differences.
- 1 is Below -50° F
- 2a is -50° to -45° F
- 2b is -45° to -40° F
- 3a is -40° to -35° F
- 3b is -35° to -30° F
- 4a is -30° to -25° F
- 4b is -25° to -20° F
- 5a is -20° to -15° F
- 5b is -15° to -10° F
- 6a is -10° to -5° F
- 6b is -5° to 0° F
- 7a is 0° to 5° F
- 7b is 5° to 10° F
- 8a is 10° to 15° F
- 8b is 15° to 20° F
- 9a is 20° to 25° F
- 9b is 25° to 30° F
- 10a is 30° to 35° F
- 10b is 35° to 40° F
- 11 is Above 40°F
You can find out what zone you’re in by using the Plant Hardiness Zone Map.
Microclimates
The USDA zone map doesn’t guarantee a definite average minimum temperature. Microclimates are small areas inside a zone that are a little warmer or cooler than the surrounding area. There are factors to take into consideration. Hills, valleys and windbreaks change the flow of air. A change in air flow can cause warmer or cooler air to be trapped in an area, or move out around that area. Buildings will absorb heat during the day and release it into the evening and night (radiant heat), keeping that small area a little warmer. If you’re unfamiliar with the microclimates in your area you should ask other local growers to share their information.
Wanted to know the recommended date for planting in Zone 5. St Louis, Mo.
Please re-read this article carefully. Zones have nothing to do with planting dates.
Thank you for your article. It all makes sense when its laid out like this but sometimes it seems the only information we have regarding a particular plant is its “Zone Hardiness.” Yes, we are responsible for understanding our own little micro-climate but even then, its often a matter of trial and error.
Case in point: I like to plant “exotic” species on my 1-acre plot of land here in New Jersey’s Pine Barrens (Zone 6b, perhaps 7a). To date my experiments have been limited to palms and bamboos. The palm tree didn’t last the first hard frost but the bamboo (Moso) is into its 4th year and appears healthy.
My next experiment will include Eucalyptus trees. I’ve recently purchased two trees from a nursery in Tennessee that claims to be in Zone 7 and grow hardy Eucalyptus trees. Coming, as my wife and I do, from California I’m hoping to have the air redolent with the menthol smell of the trees in a few years.
I know there are probably some of the readers who will balk at my planting non-native species in an ecologically “sensitive” region, but my philosophy is that variety is better, especially as the Pine Barrens I live in are akin to a virtual monoculture of pitch pines, blueberries, and scrub oak.
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