Understanding Plant Hardiness Zones in the USA

My Garden Life
November 3, 2021
Table of Contents
Hardiness zones, sometimes called planting zones or growing zones, are a guideline developed by the U.S. Department of Agriculture (USDA). Knowing your zone helps to determine which plants will survive year-round in different regions of the United States.

Explanation of US Hardiness Zones

close up of a thermometer on a post on a snowy day

There are 13 zones total, based on a 10-year average of winter temperatures and divided into 10°F zones numbered 1 through 13. Within each zone are two 5°F sub-zones, labeled “a” and “b”. The sub-zones show changing elevation. Hardiness zones are useful for deciding which plants to buy for your garden. But keep in mind that zones don’t account for local microclimates. A microclimate is a pocket within a zone where elevation, precipitation, summer heat or wind direction may have a drastic effect on temperatures.
Find your hardiness zone with our Plant Hardiness Zone Finder.

Applying Hardiness Zones to Your Garden

What does this mean for gardeners? Garden centers generally note the hardiness zone based on the lowest temperature a plant can survive. If the actual temperature where your garden is located is lower than your hardiness zone, some species may not survive.
Hardiness zones provide you with a general guide of what plants will do well in your garden. You can ensure survival by taking note of conditions that might put your garden in a microclimate. A microclimate can cause it to be colder or hotter than your regional zone.

Accounting for Microclimates in Your Area

small urban front yard filled with a variety of small trees and perennial flowering plants

What qualifies as a microclimate? Factors as simple as which direction your garden faces with respect to the sun and wind direction can influence the temperature. If your garden is on the north side of your house, for example, plants are unlikely to get enough sun in the winter to survive through the season. While some plants rated for your hardiness zone will tolerate the shade, others may fail to thrive or perish.
a mountain home on a snowy day

Elevation can also complicate matters. Gardens at high elevation may experience more frost in the span of a year than gardens in the same hardiness zone but at a lower elevation. Conversely, gardens at a low elevation can experience weather phenomena, like cold sinks, that cause unexpected frost or prolonged cold in a zone that is normally temperate. Note: A cold sink is when cold air from a high elevation slides down and becomes trapped at a low elevation.
You can keep a weather log to track temperatures, rainfall and frost dates in your garden. If you’re starting out, a simple internet search for climate records in your zip code can yield at least a basic understanding. You can also compare your temperature readings to that of your local airports. If there is a vast difference, your garden is most likely in a microclimate.

Grow Plants Not Rated for Your Zone as Annuals

flowering dianthus plant dusted with snow

You can take a risk and choose plants that are not rated for your hardiness zone. While they may do well in the warmer months, they will suffer cold damage, poor growth or reduced flowering in the winter. Especially tender species will not survive the first frost. The water in the plant’s cells will freeze, resulting in dehydration and permanent damage to cell walls.
If you decide to grow plants that are outside your hardiness zone, treat them as annuals (replacing them each spring). Bring potted plants indoors before the first frost or grow them in a protected greenhouse through the winter.

Use Your Hardiness Zone to Plan a Vegetable Garden

wood-framed raised garden bed with early spring vegetables; onions and leaf lettuce

Hardiness zones are not as critical to annual plants, including many vegetables, but your zone will determine:
  • If you need to start seeds indoors and transplant them.
  • If you have enough time for vegetables that take several months to mature.
  • When you can expect your final harvest for the year.

Your Growing Zone is a Tool for Choosing the Right Plant

beautiful Southwestern home with a drought tolerant landscape

Understanding your hardiness zone adds another tool to the gardener’s toolbox. The best weapon against plants that fail to thrive is knowing the appropriate climate conditions, ideal soil, sun and water needs. Then, choose plants that tolerate those conditions. While it can sometimes mean accepting hard truths about what won’t grow in your garden, you can be empowered to choose plants that prefer your garden’s conditions and will grow more bountiful with each passing year.
If you love tropical plants but they’re outside of the range of your hardiness zone, don’t despair. You can still enjoy the exotic feel of the tropics with Flowering Tropical Plants for Your Patio.
Mandevilla plant in a patio pot with red blooms

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