Garden Bed Math

📘 Know-howZones5 min read

Zone vs frost date: two different numbers for one garden

Quick answer

Your zone tells you how cold the coldest winter night gets, on average. Your frost dates tell you when freezing nights stop in spring and start again in fall. They come from different data and answer different questions. Boston and Spokane are both zone 7a, yet Spokane's typical last spring frost comes 22 days later and its first fall frost 28 days sooner. Use the zone to pick plants that live through winter and the frost dates to time planting.

Source: USDA Plant Hardiness Zone Map (2023), checked October 6, 2026; PRISM 2023 USDA hardiness zone GIS datasets and zip-code listing, checked October 6, 2026; NOAA 1991-2020 U.S. Climate Normals, annual/seasonal, checked October 7, 2026.

Updated October 8, 2026 Sources checked October 7, 2026 Printable checklist

A hardy shrub on one side and tender tomato transplants under upturned buckets on the other after a cold night

The basics

Two numbers, two questions

The USDA zone comes from one reading a year: the coldest night of each winter, averaged over 1991 to 2020 and sorted into 10°F zones and 5°F half zones. It answers one question: can this plant live through my winter?

Source: USDA Plant Hardiness Zone Map (2023), checked October 6, 2026; USDA ARS, how the 2023 map was made, checked October 6, 2026.

Frost dates come from NOAA's climate normals for the same 30 years. For each weather station, NOAA works out the date of the last 32°F night in spring and the first one in fall. They answer a different question: when can tender plants go out, and how long is the season between freezes?

Source: NOAA 1991-2020 U.S. Climate Normals, annual/seasonal, checked October 7, 2026; NOAA 1991-2020 annual/seasonal normals documentation, checked October 7, 2026.

Same zone

Same zone, different calendar

Zip code 02108 in Boston and 99201 in Spokane are both listed as zone 7a. At the nearest NOAA station to each, the typical last spring frost is April 4 in Boston and April 26 in Spokane. The typical first fall frost is November 9 in Boston and October 12 in Spokane. That's 219 days between the typical frosts in Boston and 169 in Spokane.

Source: PRISM 2023 USDA hardiness zone GIS datasets and zip-code listing, checked October 6, 2026; NOAA 1991-2020 U.S. Climate Normals, annual/seasonal, checked October 7, 2026.

The gap gets wider in mild places. Seattle (98101) and Charleston, South Carolina (29401) are both zone 9a. Seattle's nearest station has 249 days between typical frosts and Charleston's city station has 329.

Source: PRISM 2023 USDA hardiness zone GIS datasets and zip-code listing, checked October 6, 2026; NOAA 1991-2020 U.S. Climate Normals, annual/seasonal, checked October 7, 2026.

Same USDA zone, different frost-free seasons (typical NOAA dates) Boston, MA, zone 7azip 02108 Apr 4Nov 9219 days Spokane, WA, zone 7azip 99201 Apr 26Oct 12169 days Seattle, WA, zone 9azip 98101 Mar 13Nov 17249 days Charleston, SC, zone 9azip 29401 Feb 4Dec 30329 days JFMAMJJASOND
Typical last spring and first fall frost at the NOAA station nearest each zip code, 1991 to 2020 normals. Zones from the 2023 zip code listing.

Same date

Different zones, same spring

It works the other way too. Minneapolis (55401) is zone 5a, Columbus, Ohio (43215) is 6b and Spokane is 7a. That's two whole zones, or 20°F of difference on the coldest night. Yet the typical last spring frost at their nearest stations falls on April 23, April 20 and April 26.

Source: PRISM 2023 USDA hardiness zone GIS datasets and zip-code listing, checked October 6, 2026; USDA Plant Hardiness Zone Map (2023), checked October 6, 2026; NOAA 1991-2020 U.S. Climate Normals, annual/seasonal, checked October 7, 2026.

Three zones, last spring frost within a week (typical NOAA dates) Minneapolis, MN, zone 5azip 55401 Apr 23Oct 16176 days Columbus, OH, zone 6bzip 43215 Apr 20Oct 24187 days Spokane, WA, zone 7azip 99201 Apr 26Oct 12169 days JFMAMJJASONDTwo whole zones apart on the winter map, 6 days apart on the spring calendar
Three zip codes two whole zones apart whose typical last spring frost falls within 6 days. NOAA 1991 to 2020 normals at the nearest station.

Plan the year

Which number to use, step by step

  1. Find your zone

    Type your zip code into the zone finder. Note the zone, letter and all.

    Source: PRISM 2023 USDA hardiness zone GIS datasets and zip-code listing, checked October 6, 2026.

  2. Check perennials against the zone

    For shrubs, trees, bulbs and other plants that stay in the ground all year, compare the zone range on the tag with your zone. That's a winter question, and the zone answers it.

  3. Find your frost dates

    Type the same zip code into the frost date lookup. It shows the typical and the 1 in 10 dates at the three nearest NOAA stations.

    Source: NOAA 1991-2020 annual/seasonal normals documentation, checked October 7, 2026.

  4. Time planting from the frost dates

    Seed packets that say "after the last frost" or "6 weeks before the last frost" mean your date from the lookup, not anything tied to the zone.

  5. Write both on one card

    Print the frost date card, put your zone at the top beside the NOAA dates, and add the frosts you see each year.

A young shrub in a nursery pot beside a freshly dug planting hole with a spade in the soil
A shrub going in for good is a zone question. A tomato going out in May is a frost-date question.

The reason

Why the two drift apart

The zone looks at one extreme night. Frost dates look at the edges of the cold season. A place with a mild coldest night but a long, cool spring, like Spokane next to Boston, can share a zone and still have a much shorter season. A place with a sharp cold snap but a quick spring can sit in a colder zone with an early last frost.

Height, distance from water and city heat move frost dates from one station to the next, even within a few miles. The frost date lookup shows three stations for that reason.

Source: NOAA 1991-2020 U.S. Climate Normals, annual/seasonal, checked October 7, 2026.

Print it, tick it off

Zone and frost date card

Fill this in once, keep it with your seed packets, and check it before each planting.

  • My zone from the 2023 zip code listing
  • The NOAA station I chose, and why (closest, same height, same setting)
  • Typical last spring frost and the 1 in 10 later date
  • Typical first fall frost and the 1 in 10 earlier date
  • Days between the typical dates
  • Perennials checked against the zone, annuals timed from the frost dates
  • This year's real last and first frost, written down

Open the printable zone and frost date checklist

What people learn the second year

Tips and tricks

  • Look both up on the same day. Run the zone finder and the frost lookup with the same zip code and copy both onto one card. Most mix-ups start with a zone from one site and a date from another.
  • Ignore frost dates printed by zone. A chart that gives one date per zone can be weeks off for your town. Use the date for your zip code.
  • Count back from the date, not the zone. When a packet says to start seeds indoors some weeks before the last frost, count back from your own typical date.
  • Write down the real frosts. After a few seasons, your notes show whether your yard runs earlier or later than the station.

Common questions

Questions

Is my hardiness zone the same as my frost date?

No. The zone comes from the average coldest winter night. Frost dates come from when 32°F nights end in spring and begin in fall. Two places in the same zone can be weeks apart on frost dates.

Source: USDA Plant Hardiness Zone Map (2023), checked October 6, 2026; NOAA 1991-2020 annual/seasonal normals documentation, checked October 7, 2026.

What is the last frost date for zone 7?

There isn't one. Boston and Spokane are both zone 7a, and their typical last spring frosts at the nearest NOAA stations are April 4 and April 26. Look up your own zip code instead.

Source: PRISM 2023 USDA hardiness zone GIS datasets and zip-code listing, checked October 6, 2026; NOAA 1991-2020 U.S. Climate Normals, annual/seasonal, checked October 7, 2026.

Which number do seed packets mean?

Packets that time planting around "the last frost" mean your local last frost date. Zone ranges on packets and tags are about winter survival for plants that stay in the ground.

Do frost dates change when zones change?

They're separate products. The 2023 zone map and the frost dates on this site both use 1991 to 2020 data, but a zone move doesn't shift your frost dates by any set amount.

Source: USDA ARS, how the 2023 map was made, checked October 6, 2026; NOAA 1991-2020 U.S. Climate Normals, annual/seasonal, checked October 7, 2026.

The arithmetic, done for you

Tools for this job

Keep reading

More guides

Sources

Planning guide, not a guarantee. See every source on the site on the sources page. Spotted an error? Tell us.

Photos: T / Unsplash