USDA Planting Zones Explained (Find Yours by ZIP)

If you have ever picked up a perennial at a garden center and seen “hardy in zones 4-8” on the tag, you have met the USDA Plant Hardiness Zone Map. It is the single most used piece of gardening data in the United States, and also one of the most misunderstood. This guide explains what the zones actually measure, what changed in the 2023 update, how to read designations like 7a and 7b, and how to use your zone (and its limits) to make better planting decisions.

Find your zone first

Everything below makes more sense once you know your own zone. Enter your ZIP code and we will return your USDA zone, your average frost dates, and a planting calendar built from them.

What USDA hardiness zones actually are

A USDA hardiness zone is defined by one number: the average annual extreme minimum winter temperature at your location, averaged over a 30-year period. In plain terms, it answers the question “how cold does a typical winter get at its coldest moment?”

The map divides the US into 13 zones in 10 degree Fahrenheit steps. Zone 1 bottoms out around -60°F in interior Alaska, while Zone 13 in Puerto Rico rarely drops below 60°F. Each zone is further split into two 5 degree half zones, labeled a (colder) and b (warmer). The current map, produced by the USDA Agricultural Research Service with Oregon State University’s PRISM Climate Group, is based on weather data from 1991 through 2020.

Why minimum winter temperature? Because for perennials, shrubs, and trees, the coldest night of winter is usually the survival test. A lavender plant or fig tree that sails through summer will die in a winter that dips below its cold tolerance. Hardiness zones exist so plant breeders and nurseries can label plants with the coldest zones they reliably survive, and so you can match those labels to your location.

What changed in the 2023 map update

In November 2023 the USDA released the first new map since 2012, rebuilt on the 1991-2020 climate normals. The headline result: about half of US locations shifted roughly half a zone warmer, typically moving from an “a” half zone into the neighboring “b” (for example, 6a to 6b). The new map also used far more weather stations and higher-resolution modeling, so some of the change reflects better measurement rather than pure warming, as the USDA itself notes.

What this means for you:

• Recheck your zone, even if you have gardened in the same place for decades. Many gardeners are half a zone warmer than the map they memorized.
• Old advice may be conservative. A plant guide written for the 2012 map may list your area as colder than the current data says.
• Do not overreach. A half zone shift in the 30-year average does not prevent an old-fashioned cold snap. If a plant is rated exactly to your new, warmer zone, it is still a borderline plant.

How to read a zone designation like 7a vs 7b

Each full zone spans 10 degrees; the letter splits it into 5 degree halves:

• Zone 7 overall: average annual minimum of 0 to 10°F
• Zone 7a: 0 to 5°F (the colder half)
• Zone 7b: 5 to 10°F (the warmer half)

The half zone distinction matters most at the edges of a plant’s rated range. A crape myrtle rated to Zone 7 will likely be fine in 7b but may suffer dieback in a hard 7a winter. When a plant tag gives a range like “zones 5-9,” the first number is the coldest zone where it reliably survives winter, and the second is the warmest zone where it performs well (heat, chill hours, and dormancy needs set the upper end).

Here is the full range of US half zones, each linked to its frost dates and planting calendar:

What your zone tells you, and what it does not

Your zone is one input, not a complete climate profile. It is worth being precise about the difference.

Your zone does tell you:

• Which perennials, shrubs, trees, and bulbs can survive your typical winter
• How your winters compare to other regions when reading catalogs, plant tags, and growing guides
• A rough sense of season length, since colder zones generally have shorter frost-free seasons

Your zone does not tell you:

• Frost dates. Zones measure the depth of winter cold, not when frost arrives or leaves. Two towns in the same zone can have last frosts weeks apart. For annual vegetables, average frost dates are the numbers that actually set your schedule.
• Summer heat. A zone 8 in humid Georgia and a zone 8 on the cool Oregon coast have wildly different summers. The American Horticultural Society publishes a separate Heat Zone map for this reason.
• Rainfall and humidity. Phoenix and parts of coastal Texas share zone numbers with places that get triple their rain.
• Snow cover, wind, and soil. Reliable snow acts as insulation; exposed, windy sites are effectively colder than their zone suggests.
• Microclimates. A south-facing wall, an urban heat island, or a frost pocket at the bottom of a hill can shift your effective conditions by a half zone or more in either direction, a point university extension services make repeatedly.

How to use your zone for planting decisions

A simple decision framework:

1. For perennials, shrubs, and trees: filter by zone. Only buy plants rated to your zone or colder. If you are in 7a, a plant rated to zone 7 is a maybe; a plant rated to zone 6 is a safe bet. Treat anything rated exactly to your zone as needing a protected spot, mulch, and some luck.
2. For annual vegetables and flowers: schedule by frost dates. Winter minimums are irrelevant to a tomato that lives from May to October. Timing works backward and forward from your average last spring frost: start tomatoes indoors about 6 weeks before it, direct sow beans after it, sow peas a few weeks before it. Our zone pages and crop guides do this arithmetic for you.
3. Use half zones for edge cases. Deciding whether to risk a fig, camellia, or rosemary outdoors year-round is exactly when the a/b distinction earns its keep.
4. Adjust for your microclimates. Note where snow melts first and where frost lingers in your yard, and plant borderline specimens in the forgiving spots.
5. Recheck after the next map update. Zones are a moving 30-year average, and the 2023 revision will not be the last.

Where the data comes from

Our zone assignments come from the USDA 2023 Plant Hardiness Zone Map, and our frost dates come from NOAA’s 1991-2020 climate normals. For deeper reading, university extension programs publish excellent explainers, including Oregon State University Extension on what zones do and do not tell you, the University of Minnesota Extension on hardiness in cold climates, and the University of Wisconsin-Madison Horticulture program on the map’s history and methodology.

The short version: find your zone by ZIP above, use it to filter what you plant permanently, and use your frost dates to decide when to plant everything else.