Understanding USA Hardiness Zone Maps
Growing a home garden and keeping houseplants are extremely fun hobbies. There is perhaps no better way to get in tune with the rhythms of nature than growing plants at home. Whether you are cultivating food crops or ornamental plants, there is always more to learn about horticulture.
One of the most important factors for a successful home garden is your choice of plants. More specifically, you must choose plant species that are genetically suited to thrive in your allocated cultivation environment. Whether indoor, outdoor, or in a greenhouse, this process is critical.
The United States Department of Agriculture (USDA) developed the “Plant Hardiness Zone Map” to help people better understand what types of crops grow well in different climates. This map is now used by home growers, commercial farmers, and landscape companies alike to help choose plants for a variety of purposes, from food production to tree nurseries.
What are Hardiness Zones?
Hardiness zones are different regions in the United States that dictate where different plant species can grow outdoors. The USDA developed these hardiness zones based on temperature gradients of 10°F, with each zone beginning with “the average annual minimum winter temperature.”
A good way to conceptualize the hardiness zones is with cold or warm temperature thresholds beyond which certain plant species cannot survive. For example, tropical species like pineapples only grow in warmer zones, while heartier species like maple trees only grow in colder climates.
In developing the Plant Hardiness Zone Map, the USDA created 13 different planting zones. Across this scale of climates, Zone 1 comprises the coldest regions, and Zone 13 encompasses the hottest.
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What Types of Hardiness Zones are in the USA?
There are multiple geographic factors that influence the formation of different hardiness zones.
Latitude — Latitude is the most common factor that influences shifts in hardiness zones. Looking at the USDA Plant Hardiness Zone Map, you will largely see a pattern where hardiness zones shift as latitudes changes either to the north or south. Latitude is by far the most obvious influence of climate zones.
Elevation — Elevation is another important factor influencing hardiness zones. In fact, some of the most dramatic divergences from latitudinal shifts in climate come from mountain ranges that cut south in places like Colorado. High elevation mountains have much colder climates than other areas of the USA in similar latitudes.
Ocean — Another powerful influence over the development of hardiness zones is the ocean. Looking at a map of the U.S., you will notice that warmer planting zones extend much further north in coastal regions. Largely because oceans create warmer climates during the cold of winter than geographies located further inland.
USA Hardiness Zones 1-5 
Hardiness zones 1-5 encompass the coldest climates and planting zones in the USA.
On the coldest end is Zone 1, which has minimum temperatures of -50 to -60°F. Due to how far north it is located, Alaska is the only U.S. state that includes hardiness Zone 1. Because of the extreme cold, very few plant species live in the tundra-like conditions of such climates. As such, people living in Zone 1 grow hearty food crops like broccoli in the controlled environments of greenhouses.
In hardiness Zone 4, the minimum average temperature is -20 to -30°F. This zone spans a good way across the northern reaches of the United States, as well as in the higher elevations of mountains in places like Utah. In total, there are 22 states that have Zone 4 growing climates. Notable plant life that does well in Zone 4 includes apple and nut trees.
USA Hardiness Zones 6-10 
USA hardiness zones 6-10 include the most planting climates in the nation.
The coldest planting zone in this middle spectrum is Zone 6, which experiences minimum average temperatures of -10 to 0°F. Importantly, planting Zone 6 runs east and west across the middle of the nation and is present in as many as 38 U.S. states. Gardeners in Zone 6 enjoy a relatively long growing season of March through November.
Closer to the warmer end of these different climates is hardiness Zone 9. In this planting zone, the minimum average temperature is 20 to 30°F. As the temperatures in Zone 9 are relatively high when compared to other regions of the northern hemisphere, only 15 U.S. states include this climate. You can find Zone 9 conditions in southern locales like California and Florida, as well as further north in coastal Oregon.
USA Hardiness Zones 11-13 
Hardiness zones 11-13 feature the hottest climates in the United States.
Even Zone 11, which is at the coldest spectrum of these climates, is only featured in a few select regions of the U.S. With minimum average temperatures of 40 to 45°F, Zone 11 never experiences freezes. While these climates are rare in the U.S., they boast abundant plant life.
Finally, the hottest of all the planting zones is Zone 13. Amazingly, these climates almost never reach temperatures lower than 60 to 65°F. Zone 13 is tropical and can only be found in small corners of Hawaii and Puerto Rico. Tropical fruits and plants like mango, papaya, and pineapples do well in Zone 13.
Understanding USA hardiness zones is a great first step in planning a successful outdoor garden. By doing a bit of research on your local hardiness zone, you can choose plants best suited for the climate. Especially for beginners, getting as many factors on your side regarding the correct species and the local environment is key to cultivating strong plants.
Please bear in mind, with concerns about global warming on the rise, many scientists feel hardiness zones are beginning to shift. As warmer temperatures creep further north and into higher elevations, it will change what types of plants naturally thrive in such locales. If you live towards the edge of a hardiness zone, you are well-advised to conduct additional research about potentially changing dynamics in your local climate.
