Planting Zone vs Frost Date: What a Hardiness Zone Can and Can’t Tell You

Your planting zone and your frost dates answer different questions. A USDA hardiness zone tells you how cold the coldest night of a typical winter gets, which decides whether a perennial, shrub or tree survives. Your frost dates tell you when the frost-free season starts and ends, which decides when to plant. Use the zone to choose plants and the frost dates to time them.

The two are only loosely related. We matched the 2023 zone of 38,384 US ZIP codes with NOAA frost dates: inside one half-zone, the last spring frost still varies by three to four weeks.

What a hardiness zone measures

The USDA Plant Hardiness Zone Map rests on one number: the average annual extreme minimum winter temperature, that is, the coldest night of each winter, averaged over 1991–2020.² Each zone spans 10°F, and the “a” and “b” halves split it into 5°F steps: 7a is the colder half of zone 7 and 7b the warmer half.³ The 2023 edition was mapped by Oregon State University’s PRISM Climate Group from 13,625 weather stations, on a grid of cells about half a mile across.⁴

USDA is clear about what the map is for. All of its zone maps, it says,

should serve as general guides for growing perennial plants

The map says nothing about spring or fall; USDA notes that it can’t account for wind, soil moisture, snow or an early-fall cold snap.³ The 2023 map runs about a quarter zone warmer than the 2012 map across much of the country, partly because of newer data and partly because of better mapping methods.⁴ USDA adds that a changed zone is no reason to remove plants that have done well.³

What a frost date measures

A frost date comes from daily temperature records at a weather station. NOAA’s 1991–2020 U.S. Climate Normals give each station’s last spring and first fall freeze at several levels of odds.¹ NC State Extension explains the thresholds in those tables: 32°F is a freeze, 28°F a hard freeze, and 36°F is used for frost.⁶ Our frost vs. freeze guide explains the difference and the weather alerts for each. The dates on this site use 32°F at the station nearest your ZIP code.

The “average” (median) date is a coin flip: half of years still bring a later frost, so tender plants usually wait for the safer 10% date. Our frost dates by ZIP code show all four levels of odds. The days between your last and first frost are your frost-free growing season, and University of Vermont Extension explains why that matters for annual vegetables and flowers:

it’s important when selecting plants and seeds to know the length of your growing season

A tomato that needs 120 days won’t ripen in a 100-day season, UVM notes,⁷ whatever the zone.

Same zone, different frost dates: what our data shows

To measure how well a zone predicts frost, we took every ZIP code in the PRISM Climate Group’s ZIP listing for the 2023 map (lower 48 states)⁵ and matched it with the NOAA median frost dates from the station nearest it.¹ 38,384 ZIP codes had both. We checked 16 of the zones against USDA’s own map viewer, and all 16 matched.

Zones do follow frost in broad strokes. One half-zone warmer moves the median last frost about 9 days earlier and the first frost about 7 days later. But inside one half-zone, from 4a to 9a, the middle 80% of ZIP codes spread across 18 to 32 days in spring and 14 to 24 days in fall. About one ZIP code in seven is more than two weeks from its zone’s median last frost. The frost-free season varies even more: 33 to 60 days inside a half-zone.

Cities that share a zone but not a season

Kansas City and Bend, Oregon, are both zone 6b. Kansas City’s median last frost is April 4 and its frost-free season runs 211 days. Bend’s station, at 3,660 feet, has a median last frost of June 6 and a 105-day season. Philadelphia and Reno share zone 7b, yet Reno’s last frost comes four weeks later. In zone 9a, Charleston, South Carolina, gets a 321-day season and Seattle gets 247.

Same frost date, different zones

It works the other way, too. Des Moines (zone 5b), Omaha (6a), Amarillo (7a) and Corvallis, Oregon (8b) all have a median last frost between April 15 and April 18, yet their zones are 30°F apart in winter cold. Of the 3,553 ZIP codes whose median last frost falls between April 10 and 16, at least 1% fall in each of seven half-zones, from 5b to 8b.

Frost-date-by-zone charts, tested

Some garden sites publish frost dates by zone. We tested two against NOAA normals for every ZIP code in each zone. Garden Design lists a date range per zone;⁸ Lowe’s gives one date per zone, with the same April 15 last frost for zones 5, 6 and 7.⁹

Across zones 3 to 9, Garden Design’s last-frost range contained a ZIP code’s median last frost 36% of the time, and the Lowe’s date was within a week of it 33% of the time. The misses are not harmless. In zone 5, April 15 comes before the median last frost in almost every ZIP code (99.5%), and in 64% of them a later frost arrives in at least 9 years out of 10.

When to use your zone and when to use your frost dates

Find both numbers for your address

Look up your zone with the ZIP code search on USDA’s official Plant Hardiness Zone Map, which is the authoritative source; the zones in this guide come from its published ZIP data. Our planting zone lookup shows the same zone next to your 2012 zone and frost dates. Then look up your last and first frost dates by ZIP code. Our methodology page explains how each ZIP code is matched to a NOAA station.