R W Williams
The most important thing to keep in mind when choosing which plants to grow in your yard is this: will they grow well in your climate? In particular, will they survive the winter?
The United States Department of Agriculture (USDA) and plant nurseries, seed suppliers and experts on flowers and edibles across the country answer this question in two ways. Together, since the 1960s, they’ve helped gardeners and garden designers, commercial nurseries and fruit and vegetable growers - anyone who grows plants - by identifying which plants are most likely to thrive and grow well in your local climate.
Knowledge of the US hardiness zones is invaluable to any gardener when they are planning their garden. Particularly, it can help them to identify the best plants to include as part of their flower bed ideas that are suited to growing in their region, or crops that are most likely to produce a healthy harvest for vegetable garden ideas.
Why are US hardiness zones important
Climates in the US differ widely, even within states, therefore it is essential to know and understand which plants will grow well and thrive in your local region to ensure your gardening success and avoid expensive planting mistakes.
What are the US hardiness zones?
The US hardiness zones are identified on the USDA’s Hardiness Zone Map. The map is available as an interactive online map, but you can also download a map of your region to keep on your phone or tablet, or print to have in your potting shed or seed journal for easy reference.
In an ongoing program of research, the USDA has divided the country into thirteen areas - or zones - according to their coldest average winter temperatures. These are presented on a map that splits the country into numbered bands whose coldest winter temperatures are 10F apart.
Each one of these bands is known as a hardiness zone (sometimes referred to as a planting zone or growing zone) and each zone is again divided into two 5F zones, a and b, to provide even greater accuracy. Factors that go into creating these zones also include elevation, nearness to oceans or other large bodies of water, terrain and landscape differences, such as valleys or mountains.
For example: Los Angeles, CA is in USDA Zone 10b (with average winter temperatures down to 35F), while Concord, NH, NY is in Z5b (with average winter temperatures down to -15F).
The usual shorthand is Z10 or Z5 for zone 5. As our climate changes, the USDA updates the map. The hardiness zone map was last revised in 2023.
The USDA hardiness zone map is available to everyone online, simply type in your zip code and you’ll find your hardiness zone in an instant. Whenever you see mention of a plant hardiness zone, growing zone, or planting zone, the information provided is based on the USDA’s hardiness zone map.
Finding your zone
It’s important to realize that this is not a state-based system and, also, to zoom in and check the detail in your area.
For instance, the state of Vermont is shown as one zone on most versions of the map. However, by clicking on the state, more detail is revealed and it quickly becomes clear that Vermont actually includes several hardiness zones, ranging from 3b to 5b.
The differing hardiness zones within a US state can be even more extreme. In Arizona, the region that includes the Sonora Desert may have cold temperatures averaging from 20-40F, in Zones 9b to 10b. However, isolated parts of central and northeastern Arizona are located in Zone 4b, and may see low temperatures of -25 to -20F.
Never assume you are in a particular zone. Instead, click on the map to find the zone and information that is specific to your area. You may discover that you are able to grow plants you never thought possible.
You may also live in an area experiencing a microclimate, and this could affect your growing success. Water, wind, and landmasses, such as mountains or even buildings, are factors in creating a microclimate. A city can be a hardiness zone higher than the rural farmland that surrounds it.
What are hardiness ratings?
To go with the USDA Hardiness Zone map The American Horticultural Society, along with regional and state agencies and experts, as well as plant nurseries and seed suppliers, have rated all the edible and ornamental plants we grow according to the coldest winter they will survive – their hardiness rating.
Plant tags, online descriptions, seed packets and anyone who provides gardening guidance should provide a hardiness rating for each plant they discuss.
It’s important to remember that while plants will not usually survive in zones colder than the one for which they’re rated, they will usually grow in at least some warmer zones. For example, most daylilies are rated as zone 3 or zone 4, but most will also grow in warmer zones, up to Z9.
Choosing plants for your hardiness zone
Once you know the hardiness zone of your yard you can check that against the rating of the plants you would like to grow and see instantly whether or not they’re suitable.
Tags on plants in nurseries and plant centers will almost always give the plant’s hardiness rating. If not, ask the staff or look it up online on your phone. Websites hosted by plant nurseries and seed suppliers will also rate each plant they offer. Be aware that different varieties of the same plant may vary in their hardiness ratings, so check carefully.
Another consideration is that the USDA hardiness zone map does not take into account summer temperatures, periods of drought, humidity, snow fall or other factors – all of which can affect a plant's ability to thrive in a specific location.
Where a plant is sited in a garden can also have an impact. Plants growing against a sunny wall, in well-drained soil or against the house in foundation planting will often survive in one zone colder than their rating. At the same time, it’s wise to think of plants growing permanently in containers as being in one zone colder than the rest of your yard.
Other approaches to rating plants according to the climates in which they grow include the Heat Zone system, devised by the American Horticultural Society to rate plants according to the summer temperatures in which they will thrive, and the Sunset system, a more detailed approach develop by Sunset magazine, mainly for the west. Neither of these systems have been universally adopted.
You can find out more about each specific US hardiness zone by reading our in-depth guides to each one here:
FAQs
Do US hardiness zones determine pruning and harvest times?
If you're considering when to plant vegetables, the planting times and resulting harvest will depend on the US hardiness zones, as well as the length of time from germination to fruiting.
Pruning, however, is something that depends on the specific plant. Pruning times are generally after a tree or shrub blooms, but different plants have different needs. For example, an azalea should be pruned in late spring in the mid-Atlantic, before buds set for next year. Pruning late in fall will disrupt the blooming times.
USDA hardiness zones provide an invaluable guide to which plants will survive the winter in your garden. But remember that factors other than coldest winter temperatures can play a part. The advice of local nurseries, extension services and nearby gardeners can provide insight and guidance that no map can provide - however good.
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