Planting Zones & Frost Dates

Your hardiness zone tells you one thing: how cold your winter usually gets. That’s the whole job. It won’t tell you when to sow tomatoes, when your soil warms up, or whether August will cook a lettuce bed in four days.

Frost dates answer the timing questions. Zones answer the “will this perennial live through winter” question. You need both, and they come from two different places.

What the USDA zone map actually measures

It measures the average annual extreme minimum winter temperature. Every word there is carrying weight.

• Average — not the coldest night ever recorded. A once-in-thirty-years cold snap drops below your zone, and your zone never promised otherwise.
• Annual extreme minimum — the single coldest night of each winter, averaged over 30 winters.
• Winter — nothing at all about summer.

The current edition is the 2023 map. USDA’s Agricultural Research Service built it with Oregon State University’s PRISM Climate Group, using 1991–2020 data from more than 13,000 weather stations. The 2012 map used 7,983.

More stations and a warmer 30-year window moved the lines. Roughly half the country landed one half zone warmer than on the 2012 map; the other half stayed where it was. Across the contiguous United States the 2023 map runs about 2.5°F warmer overall.

The 13 zones, in °F and °C

Each numbered zone covers a 10°F band. Each one splits into an “a” half and a “b” half of 5°F each, so 7a is the colder half of zone 7 and 7b the warmer half. A shrub rated hardy to zone 6 should survive a normal winter in 6a and anywhere warmer.

What the map can’t tell you

Almost everything else. The zone map does not account for:

• Summer heat and humidity — it is a winter measurement only
• Rainfall, drought, or how much you irrigate
• Soil type, drainage, or pH
• Day length and total sunshine hours
• Wind, snow cover, and winter sun
• Frost dates — the map contains none

USDA puts it plainly: “Many other environmental factors, in addition to hardiness zones, contribute to the success or failure of plants. Wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine can greatly affect the survival of plants.” That’s why something can be hardy in your zone and still die in July. Parts of coastal Washington and parts of central Texas both sit in zone 8: they share a winter floor and almost nothing else.

How to find your zone

Type your ZIP code into the search box on the USDA Plant Hardiness Zone Map. Thirty seconds, no account.

Write down the half zone, not the round number. The bottom of 6a sits five degrees below the bottom of 6b, and on a marginal fig that gap is the whole argument. If a tag says zone 6 and you’re 6a, you’re at the edge of its range, not the middle.

Why your yard may sit a zone away from the map

Because the map is drawn from grids, and your yard is not a grid. USDA describes the problem directly: “Microclimates, which are fine-scale climate variations, can be small heat islands—such as those caused by blacktop and concrete—or cool spots (frost pockets) caused by small hills and valleys.”

Some of that is baked into the map at map scale. USDA notes that “cities tend to hold more heat because they encompass large areas of concrete and blacktop, so a city or town might be assigned to a zone warmer than the surrounding countryside,” and that “higher elevations tend to be colder than surrounding lower areas.” Below that scale, these are the spots that behave off-map:

• A south-facing masonry wall. Brick and concrete soak up sun all day and give it back at night. USDA lists “a sheltered area in front of a south-facing wall” as a classic warm pocket.
• The bottom of a slope. Cold air is denser, so it slides downhill and pools — USDA’s “low spot where cold air pools first.” That’s a frost pocket, and it can run colder than the yard fifty feet uphill.
• Mid-slope, on the same hill. Usually the safest ground you own — cold air drains past it instead of settling.
• Under a porch roof or a mature evergreen. Overhead cover slows heat loss to an open night sky. A few degrees, exactly when you need them.

USDA’s own conclusion is the honest one: “No hardiness zone map can take the place of the detailed knowledge that gardeners learn about their own gardens through hands-on experience.”

Frost dates: what “average” actually means

An average last-spring-frost date is a 50% probability. It’s the date after which your odds of another freeze drop below one in two. It is a coin flip, not a promise.

Same at the other end. The average first-fall frost date is when a freeze becomes more likely than not. In half of years it comes earlier.

Why the 10% and 30% dates matter more

Because tomatoes don’t get a second try. NOAA publishes freeze dates at several probability levels from the 1991–2020 Climate Normals — in its own words, “the dates past which the chances of freezing temperatures drop below 40, 30, 20, and 10 percent.”

• The 50% date is for hardy things — peas, spinach, onion sets, potatoes. Losing the top growth costs you a week, not the crop.
• The 30% date is a reasonable line for hardened-off transplants you’re willing to cover.
• The 10% date is where tender crops belong — tomatoes, peppers, basil, squash, melons. It falls later than the 50% date by an amount that varies a lot by station, so look yours up rather than assuming a fixed gap. It’s the difference between planning and gambling.

Where to get your real numbers

Two sources, both official, both free.

• NOAA’s National Centers for Environmental Information publishes the freeze/frost normals by station. Their explainers on the last spring freeze and the first fall freeze are the plain-language versions.
• Your state Cooperative Extension service, which converts those numbers into county-level planting dates for crops that actually grow where you live. USDA’s land-grant university directory will point you at your state’s.

Pick the nearest reporting station and check its elevation. An airport twenty miles off and three hundred feet lower is describing a different spring than your backyard.

Spring: counting back from your last frost

Seed packets give a head start in weeks. You subtract those weeks from your last frost date to get a sowing date.

Worked example. Say your average last frost is May 10, and you’re starting tomatoes at the short end of the usual 6–8 weeks indoors. Six weeks is 42 days. Count back 42 days from May 10 and you get March 29. That’s your sowing date. Transplants then go out after the frost risk passes — for tomatoes, the 10% date, not the 50% one.

The ranges below are typical planning figures, not local instructions. Check them against your state Extension’s vegetable planting guide before you commit a whole flat.

Fall: counting back from your first frost

Different math. Here you use days to maturity from the seed packet, then add roughly two weeks for the fall factor.

The fall factor exists because autumn days shorten and temperatures drop, so growth slows. A crop that matures in 65 days in June takes longer in September. Two weeks is a common Extension allowance rather than a fixed constant — your state’s fall planting guide may use a different one.

Worked example. Broccoli transplants listed at 65 days, plus 14 days of fall factor, is 79 days. If your average first frost is October 25, count back 79 days and you land on August 7. That’s when the transplants go in the ground. Sow two weeks late and frost takes the crop the week before it heads up.

Add 10 to 14 days on top if you want more than one picking from cut-and-come-again greens.

Four caveats worth keeping

• A zone is a survival floor for perennials, not a planting calendar. Plenty of dead tomatoes have been planted on perfectly correct zone information.
• Averages fail in individual years. A 30-year normal describes 30 winters. It says nothing about the one you’re standing in.
• Your yard is not the map. A frost pocket, a wall, a slope or a city block can move you a half zone or more in either direction, and no ZIP code lookup can see it.
• Your state Extension wins every disagreement. They publish county-level dates for the crops and cultivars that work locally. National tables, this one included, are a starting point.

Keep two numbers somewhere you’ll find them: your half zone and your 10% last-frost date. Most spring decisions come down to those, plus a sheet to throw over things when the forecast turns.

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