What Exactly Are USDA Hardiness Zones?

When you grow fruit trees, it’s important to consider their cold tolerance. Tropical and subtropical trees, for example, may not survive cold winters in more temperate regions.

The good news is that the research has already been done for you. The United States Department of Agriculture (USDA) developed a system of hardiness zones and published maps to describe their geographic distribution.

Conveniently, when you buy a fruit tree, its label will usually show up to what zone it is hardy.

Overview of USDA Hardiness Zones

Each USDA hardiness zone is associated with a low temperature range. This indicates the lowest temperature you can expect to see in that zone on average.

Originally, when the system was first devised in 1960, the zones were divided into 10-degree increments. In later years, as the USDA was able to gather more detailed data, the system was further refined and subdivided into 5-degree increments as shown in the table below.

Zone	Average Annual Extreme Minimum Temperature
1a	-60 to -55 °F/-51.1 to -48.3 °C
1b	-55 to -50 °F/-48.3 to -45.6 °C
2a	-50 to -45 °F/-45.6 to -42.8 °C
2b	-45 to -40 °F/-42.8 to -40 °C
3a	-40 to -35 °F/-40 to -37.2 °C
3b	-35 to -30 °F/-37.2 to -34.4 °C
4a	-30 to -25 °F/-34.4 to -31.7 °C
4b	-25 to -20 °F/-31.7 to -28.9 °C
5a	-20 to -15 °F/-28.9 to -26.1 °C
5b	-15 to -10 °F/-26.1 to -23.3 °C
6a	-10 to -5 °F/-23.3 to -20.6 °C
6b	-5 to 0 °F/-20.6 to -17.8 °C
7a	0 to 5 °F/-17.8 to -15 °C
7b	5 to 10 °F/-15 to -12.2 °C
8a	10 to 15 °F/-12.2 to -9.4 °C
8b	15 to 20 °F/-9.4 to -6.7 °C
9a	20 to 25 °F/-6.7 to -3.9 °C
9b	25 to 30 °F/-3.9 to -1.1 °C
10a	30 to 35 °F/-1.1 to 1.7 °C
10b	35 to 40 °F/1.7 to 4.4 °C
11a	40 to 45 °F/4.4 to 7.2 °C
11b	45 to 50 °F/7.2 to 10 °C
12a	50 to 55 °F/10 to 12.8 °C
12b	55 to 60 °F/12.8 to 15.6 °C
13a	60 to 65 °F/15.6 to 18.3 °C
13b	65 to 70 °F/18.3 to 21.1 °C

And here is the corresponding map for the United States.

As expected, the map shows that the higher zones are mostly in the Western and Southern States and the lower zones are mainly in the Midwest and Northeast. The coldest zone in the U.S. can be found in Alaska and the warmest zone is in Puerto Rico.

The USDA has also published more detailed for different regions of the United States as shown below.

More Detailed Hardiness Maps

More recently, the USDA has also made it easy to look up plant hardiness zones directly on their website. Their database is very detailed, even at the neighborhood level.

The USDA system of plant hardiness zones has been widely adopted over the last few decades and a number of other countries such as Canada and the United Kingdom have published similar maps for their own regions.

Shortcomings of the USDA Hardiness Zones

At the same time, it is important to note that USDA hardiness zones do not tell you everything you need to know about local conditions and how suitable they are for the plants you want to grow.

Notably, while they address hardiness to cold temperatures, they don’t cover heat tolerance. Nor do they tell you about local soil characteristics such as pH.

Perhaps the most widely misunderstood aspect of the USDA hardiness zones, however, is in reference to chill hours. The USDA zones can tell you if your tree is cold tolerant where you live but they do not tell you if there are enough chill hours in your area.

Many types of fruit trees, in particular deciduous fruit trees, require exposure to a certain amount of chill. There are actually a number of different models to measure the chill hour requirement but to keep things simple, it can be roughly defined as the number of hours between 32°F and 45°F. Without sufficient chill hours, your tree may not bear fruit the following season.

The number of required chill hours is something that is usually printed on fruit tree labels. Unfortunately, there is no USDA map for chill hours although various research institutions have published data for different states and regions. It is your responsibility to check if your local area will have enough chill hours for your tree.