Last Frost Date

Hardiness zones, and what changed on the 2023 map

Last checked October 25, 2026

Plant tags say "hardy to zone 5" or "zones 7 to 10." That zone comes from the USDA Plant Hardiness Zone Map, which was redrawn in November 2023. Here's what it measures, and what changed.

What a hardiness zone measures

A hardiness zone is about one thing: how cold the coldest night of winter usually gets. The USDA takes the lowest temperature of each winter, averages it over 30 years, and sorts the country into zones by that average.

Each zone covers 10°F. Zone 1, found in Alaska, is the coldest; zone 13 is the warmest, and zones 12 and 13 appear only in Hawaii and Puerto Rico. Each zone is split into two halves, "a" (the colder 5 degrees) and "b" (the warmer 5). For example:

The USDA is careful to say these are averages of the coldest night, not the coldest it has ever been. A plant at the edge of its range can thrive for years and then die in one rare cold snap.

What zones are for, and what they aren't

Zones tell you which perennials survive your winters: shrubs, trees, bulbs and flowers that live year after year. They're the right number to check before you buy a rose bush or a fig tree.

Zones don't tell you when to plant. A zone says nothing about how early spring warms up or how late fall frost comes. For that you need frost dates. The two can disagree a lot. Kansas City, Missouri and Bend, Oregon are both in zone 6b, yet Kansas City's typical last frost comes on April 4 and Bend's on June 6, 63 days later. Their winters bottom out at about the same temperature; their springs warm up on very different schedules.

So most gardeners need both: the zone for what to grow, and the frost dates for when.

What changed on the 2023 map

The USDA released the new map on November 15, 2023, its first update since 2012. It was made with the PRISM Climate Group at Oregon State University. Three things changed:

The USDA summed up the result two ways. Its announcement said about half of the country shifted to the next warmer half zone and the other half stayed in the same half zone. Its map pages describe the new map as generally about a quarter zone warmer than the 2012 map throughout much of the country.

We counted for ourselves, using the ZIP code lists Oregon State publishes for both maps. Of the 40,257 ZIP codes listed on both maps, 57 percent moved to a warmer half zone in 2023 (560 of them by a full zone or more), 42 percent stayed where they were, and 1 percent (444) moved to a colder one. The largest shares of ZIP codes moving warmer were in District of Columbia (100 percent), Delaware (87 percent), Tennessee (86 percent) and Alabama (80 percent). The smallest shares were in Hawaii (13 percent), Puerto Rico (19 percent) and North Dakota (20 percent).

Not every change is about the weather. The USDA notes that some shifts come from the extra stations and better mapping, and that these can make a zone cooler as well as warmer. Alaska shows this most: its 2023 map is warmer partly because the new data captures how, in winter, warm air sits over cold air that settles into the valleys.

Does this prove the climate is changing?

The USDA says no, not on its own. Climate change is usually measured from trends in average temperatures over 50 to 100 years. The hardiness map averages one extreme night a year over 30 years, and that night swings a lot from winter to winter. The map's makers cautioned against treating a zone change as reliable evidence either way.

If your zone changed

The USDA's advice is plain: a new zone doesn't mean you should start removing plants. What has thrived in your yard will most likely keep thriving.

Where the change matters is in choosing new plants. If you moved from 6a to 6b, plants rated for 6b are now within your zone on paper. But a plant at the edge of its range is still at risk in an unusually cold winter. If losing it would hurt, choose something rated a half zone colder than you are.

Other things that decide whether a plant survives

The USDA lists several things a zone can't capture: wind, soil type, soil moisture, humidity, pollution, snow cover and winter sunshine. A plant that is barely hardy may come through a winter under deep snow and die in a dry, windy one. Summer heat matters too, and the hardiness map says nothing about it.

Your yard also has its own small climates. A spot against a south-facing wall may be a half zone warmer than the open lawn; a low spot where cold air pools may be colder.

Finding your zone

Look up your ZIP code to see your zone on the 2023 map, with your zone on the 2012 map beside it. The zone comes straight from the USDA's ZIP code list, prepared by Oregon State. The USDA's own interactive map, at planthardiness.ars.usda.gov, lets you click on any spot for its zone, which helps if your ZIP code covers both hills and valleys.

Sources

More guides: What your last frost date really means · How to protect plants from a late frost · Starting seeds indoors: when and how · What to plant, month by month