Understanding USDA Plant Hardiness Zones

Your USDA plant hardiness zone is a number (and an "a" or "b") that tells you how cold winter typically gets where you live, and so which perennials, shrubs and trees can survive there. The current 2023 USDA map divides the country into 13 zones based on the average coldest night of the year, each 10°F wide and split into two 5°F halves. Zone 5b, for example, averages a winter low of −15°F to −10°F. Find yours by ZIP code with our lookup below, then read on for how the map works, how zones in the West have changed, and what the map doesn't tell you about water.

Our lookup uses the 2023 USDA zone data. If you're on a zone boundary, confirm it on the official USDA map, which lets you zoom to your street.

What a hardiness zone actually is

The USDA map is based on the average annual extreme minimum winter temperature: the single coldest night of each year, averaged over 30 years. USDA displays it as 10-degree zones and 5-degree half zones.

Plant tags and catalogs list a range, such as "zones 4–8." If your zone falls inside that range, the plant can normally survive your winter. The lower number is the one that matters most for cold climates; the upper number usually reflects heat or lack of winter chill.

The zones at a glance (2023 map)

Zones 1 and 2 cover the coldest parts of Alaska; zones 12 and 13, added in 2012, appear only on the maps for Hawaii and Puerto Rico.

Zones for Western xeriscaping cities

These are the 2023 zones for central ZIP codes; suburbs at higher elevation are often a half zone colder.

Local pages with zone, frost dates and filtered plant lists are coming for each metro; the Denver zone page is the first.

How the map is made

The 2023 map was produced by the USDA Agricultural Research Service with Oregon State University's PRISM Climate Group. According to USDA's map creation notes:

• It uses 1991–2020 temperature data, the same 30-year period climatologists use for current U.S. climate "normals." The 1990 map was drawn from only 1974–1986 data.
• A GIS-based algorithm interpolates between weather stations while accounting for elevation and nearby water, so the map shows warmer urban heat islands and colder mountaintops that older maps smoothed over.
• In the West, it draws on stations run by the NRCS, Forest Service, Bureau of Reclamation and BLM to better define mountain zones.

Two things follow from the method. Your zone describes a statistical winter, not a guaranteed one; a once-a-decade cold snap can undershoot it. And the "average coldest night" says nothing about summer heat, rainfall or soil.

How the map has changed

USDA notes that the 2023 map is generally about one quarter-zone warmer than the 2012 map across much of the country, mostly because of the newer averaging period (1991–2020 instead of 1976–2005), with some changes from better data and interpolation. Many locations crossed into the next warmer half zone as a result; some moved colder. USDA also cautions that because the map is built from extreme cold events, zone changes aren't reliable evidence of climate change on their own.

For Front Range gardeners the practical change is concrete: central Denver is now 6a, while many older books and plant tags still call it zone 5.

The takeaways:

• Use the current map. Our lookup and the USDA site both use 2023 data.
• Treat old plant guides with caution. A "not hardy in your area" note written against the 1990 map may be out of date.
• Don't over-trust a warmer number. Averages still include harsh outlier winters. For expensive trees and large shrubs, stay within your zone rating; experiment only with inexpensive perennials.

Hardiness is only half the story: water

Two gardens can share a zone and live on different planets. Santa Fe and Columbus, Ohio, are both zone 6b on the 2023 map; the winter floor is the same, but the summer water budget is not.

Your zone tells you what survives winter; your rainfall tells you what survives summer without a hose. Xeriscaping means designing for both: plants rated for your zone, chosen from the dry side of the palette. That's the approach behind our zone 5 and zone 6 plant guides, with zone 4, zone 7 and zones 8–10 guides on the way.

What else the zone map doesn't tell you

• Heat. A plant that survives a zone 9 winter in Seattle may struggle in a zone 9 summer in Austin. The American Horticultural Society's heat zone map, based on the number of days above 86°F, covers the hot end.
• Humidity and disease. Lavender that thrives in dry Colorado air can rot in Gulf Coast humidity at the same zone rating.
• Soil and drainage. Many xeric plants die of wet winter soil at temperatures they'd survive in gravel. CSU Extension recommends a simple drainage test: dig an 18-inch hole, fill it with water, and if water stands more than 30 minutes, drainage is poor. See our soil prep guide.
• Snow cover. Reliable snow insulates plants; a snowless −15°F night is harder on them than a snow-covered −20°F one.
• Wind and winter sun. Dry winter wind and bright sun desiccate evergreens even when temperatures are within their range.
• Frost dates. Zones describe winter lows, not the length of the growing season.

Microclimates: your yard has several zones

The map stops at your property line. Inside it:

• South- and west-facing walls store heat and can support plants a little outside your rating. Our full-sun plants thrive there.
• Low spots and frost pockets collect cold air and run colder than your official zone.
• Under eaves and evergreens stays drier in winter, which helps plants that die from winter wet.
• Boulders and paving radiate stored heat; rock gardens take advantage of it.

How to use your zone

1. Look it up, including the a/b half.
2. Filter by zone. Buy plants rated to your zone or colder. For trees and big shrubs, consider one half zone colder for a safety margin.
3. Filter by water. From the zone-safe list, choose the low-water natives and adapted plants. Our perennials, grasses, shrubs and trees guides list zones for every plant.
4. Use microclimates on purpose. Warm pockets for borderline plants, cold pockets for the toughest ones.

From our installs

On the Front Range, zone numbers explain only part of what we see. Plants rated well within our zone still die when they're set in clay that stays wet through a freeze-thaw winter, and a warm, snowless January followed by a hard freeze does more damage than a steady cold winter. We choose plants rated at least a half zone colder than the site for anything structural, put borderline plants against south walls in raised, gritty soil, and give evergreens a deep drink during dry winter warm spells in their first few years. (Xeris Landscaping is owned by Xeriscaping.com's founder.)

Frequently asked questions

What zone am I in?

Enter your ZIP code in our zone lookup above. It uses the 2023 USDA data. For boundary addresses, zoom in on the official USDA map.

Did my zone change on the 2023 map?

Possibly. USDA says the 2023 map is about a quarter zone warmer than the 2012 map across much of the country, so many places moved into the next warmer half zone. Denver, for example, is now 6a.

Can I grow a plant rated one zone warmer?

Sometimes, in a protected microclimate with sharp drainage, knowing a hard winter may kill it. Take that risk with $12 perennials, not $300 trees.

Are hardiness zones the same as frost dates?

No. Zones describe winter minimums; frost dates describe when the growing season starts and ends. You need both: zones for what to plant, frost dates for when.

Why do plants that share a zone behave so differently?

Zones only measure winter cold. Heat, rainfall, humidity, soil and elevation do the rest, which is why a Phoenix garden and a coastal Southern California garden in neighboring zones share few plants.

Know your number? Start with why xeriscaping works, then browse plants for your zone.