Green Thumb League

What’s Your Zone? Understanding Hardiness Zones

Plant hardiness zones–sometimes called growing zones–will help you figure out what plants will thrive in your garden.

Hardiness Zone Basics

The plant hardiness zones range from 1-13, based on the average annual extreme minimum winter temperatures each region experiences. Each zone is color-coded, ranging from pale gray in Zone 1a (northern Alaska) to dark maroon (Puerto Rico).

The easiest way for gardeners in the United States to find your zone is to enter your Zip Code into the USDA Plant Zone Hardiness database. You can also browse individual state maps.

Most states–even tiny ones like Vermont–contain multiple zones. Continuing with our Vermont example, you’ll find 5 zones: 3b (-35 to -30 degrees) all the way to 5b (-15 to -10). Texas contains zones 6b all the way to 10a.

Latitude, elevation, and rainfall all play a part in determining how cold it gets in these areas. The data is compiled over several decades, though it’s worthwhile noting that, because of climate change, these numbers might be shifting.

How to Use Hardiness Zones

The sad truth is that some plants simply can’t grow in extreme conditions. A palm tree won’t grow in Alaska, and lilacs will struggle in Florida.

Seed packets and nursery plants will almost always give you a range of hardiness zones where the plant will thrive. Your zone will also tell you when to get certain plants in the ground. For example, broccoli should be planted in zones 3-4 in May, but as late as October in zones 9-10.

Plants are, generally speaking, either cold hardy or tender. The hardier they are, the more extreme winter temperatures they can survive. Lilacs and peonies are cold hardy. Aloe and canna lily are not. Figuring out which perennials, shrubs, and trees will grow year after year in your yard is much easier once you know your zone.

Beyond the Zone

Your zone isn’t the end-all, be-all of gardening knowledge. It’s also important to understand the length of your growing season (the time between the last and first frosts of each year) as well as your average rainfall and percentage of sunny days.

For gardeners in the western half of the United States, you might find that the Sunset Climate Zones are a more accurate predictor of plant failure. Rather than merely focusing on winter lows, this system looks at the whole picture, including “length of growing season, timing and amount of rainfall, winter lows, summer highs, wind, and humidity.”

Unfortunately, because the USDA system is by far the more popular, you’ll have a hard time finding Sunset Climate Zone data printed directly on seed packets. Instead, you’ll have to do some digging to figure out if a particular plant variety will thrive in your area.

Add comment