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GuideEntry 31Reviewed 2026-10-16

USDA hardiness zones

A hardiness zone is a number for how cold winter usually gets. It tells you which perennials are likely to survive; it does not tell you when to plant tomatoes. This page explains the zones, the 2023 USDA map and how zones relate to frost dates, from USDA's Agricultural Research Service, NOAA and extension services.

What a zone measures

The USDA Plant Hardiness Zone Map is built on one statistic: the average annual extreme minimum winter temperature, the average of each year's single coldest temperature over 30 years (USDA ARS). The map runs from zone 1, the coldest, to zone 13, the warmest. Each zone covers 10°F, and each is split into an "a" and "b" half of 5°F; in USDA's own example, "7a and 7b are 5-degree F increments representing the colder and warmer halves of zone 7."

USDA presents the map as a guide to choosing perennial plants likely to thrive at a location, and Iowa State calls it "a standard guide to a plant's cold tolerance." Nurseries and catalogs list zones with plant descriptions for this reason. The figure is an average, not a record: USDA notes the zones "are based on the average lowest temperatures, not the lowest ever."

Zone temperature ranges

Zone"a" half (°F)"b" half (°F)
1−60 to −55−55 to −50
2−50 to −45−45 to −40
3−40 to −35−35 to −30
4−30 to −25−25 to −20
5−20 to −15−15 to −10
6−10 to −5−5 to 0
70 to 55 to 10
810 to 1515 to 20
920 to 2525 to 30
1030 to 3535 to 40
1140 to 4545 to 50
1250 to 5555 to 60
1360 to 65above 65

Ranges as tabulated by Wikipedia (secondary source); they follow USDA's 10°F zones and 5°F halves, and USDA's statement that zone 12 covers average extreme minimums above 50°F and zone 13 above 60°F. Zones 12 and 13 appear only on the maps for Hawaii and Puerto Rico.

USDA hardiness zones on a temperature scale, and what zones and frost dates each answerTop: a horizontal bar of thirteen colored bands numbered 1 to 13 above a temperature axis running from minus 60 to plus 70 degrees Fahrenheit, each band 10 degrees wide. Zone 7 is enlarged below the bar into two halves, 7a from 0 to 5 degrees and 7b from 5 to 10 degrees. Zones 12 and 13 are outlined with a dashed line. Bottom: two boxes. The left box, Hardiness zone, shows a thermometer and asks whether a perennial will survive a typical winter. The right box, Frost dates, shows a January to December strip with a last spring frost marker, a first fall frost marker and a green growing-season bar between them, with a Virginia zone 7a example. Six numbered markers match the list below the drawing. Average annual extreme minimum winter temperature 12345612137891011 −60°F−30°F0°F30°F60°F 7a7b 0°F5°F10°F Hardiness zoneFrost dates Will this perennialsurvive a typicalwinter here?For plants that stayin the ground all year JFMAMJJASOND season Zone 7a, Virginia (VCE):last frost Apr 15–25first fall frost Oct 15–25 1 2 3 4 5 6
Drawing by The More One Sows, based on the cited sources. Colors are illustrative, not USDA's map colors. Not to scale.
  • 1. Each zone spans 10°F of average annual extreme minimum winter temperature, from zone 1 (coldest) to zone 13 (warmest) (USDA ARS)
  • 2. Half zones: 7a covers 0 to 5°F and 7b 5 to 10°F (USDA ARS example; ranges as in the table above)
  • 3. Zones 12 (above 50°F) and 13 (above 60°F) appear only on the Hawaii and Puerto Rico maps (USDA ARS)
  • 4. A hardiness zone answers one question: is a perennial likely to survive a typical winter here?
  • 5. Frost dates answer a different one: when to sow and transplant, and how long the season is (University of Idaho, UVM)
  • 6. Example: zone 7a in Virginia, average last spring frost April 15–25 and first fall frost October 15–25 (Virginia Cooperative Extension); other states differ

The 2023 map: what changed

USDA released the current map on November 15, 2023, the first update since 2012. It was produced by USDA's Agricultural Research Service (ARS) with Oregon State University's PRISM Climate Group, uses temperature data from 1991 to 2020, and is drawn on a grid of cells about half a mile on a side. The release says about half the country moved to the next warmer half zone and the other half stayed in the same half zone.

EditionWeather dataNotes
1960 (revised 1965)Not stated in the sources openedIssued from the U.S. National Arboretum with uniform 10°F zones (Wikipedia)
19901974–1986Shorter data period, made without GIS (USDA ARS); first with "a" and "b" halves (Wikipedia)
20121976–2005Zones 12 and 13 added (USDA ARS); 7,983 stations (2023 release)
20231991–202013,412 or 13,625 stations (see below); Alaska detail improved from 6¼ to ¼ square mile

USDA attributes the differences mainly to the newer data period, more weather stations and better mapping methods. Some places moved to a cooler zone; the University of Minnesota notes that some areas of that state "may report a lower plant hardiness zone number." The largest changes from new data and methods were in upland Alaska.

Regional examples from extension: nearly all of Iowa is now zone 5, with the northern half 5a and the southern half 5b (Iowa State). In Illinois, the 5b/6a boundary moved about 60 miles north, from near Springfield to near Peoria, and a small area of 7b appears in the state for the first time (Illinois Extension).

Where the sources disagree

QuestionWhat the sources say
How many stations?13,412 (USDA's November 2023 release); "a total of 13,625 stations" (USDA's Map Creation page); "over 13,000" (UMN)
How much warmer?About half the country one half zone warmer (2023 release); "generally about one quarter-zone warmer" than 2012 across much of the US (Map Creation page). An average of a quarter zone is consistent with half of places shifting a half zone, but the two pages frame it differently.
Is it a climate-change signal?USDA and UMN: changes in zones "are not reliable evidence of whether there has been global warming," because the map uses 30-year averages of extreme cold, not long-term average temperatures. Illinois Extension headlines the shift as reflecting a changing climate and quotes the state climatologist linking warming winters to human-caused warming. Wikipedia describes a northward shift "reflecting a warming trend."
Can zones time vegetable planting?University of Idaho: "Knowing your hardiness zone will not tell you how many frost-free days you will have." Virginia Cooperative Extension does key its planting tables to zones 6a–8b, but within one state, with its own frost-date range for each zone, and says actual frost dates "will vary due to local conditions."

Zones vs frost dates

Hardiness zoneFrost dates
MeasuresAverage of each winter's lowest temperatureDate of the last 32°F night in spring and first in fall, with a probability attached
Published byUSDA ARS with Oregon State's PRISM groupNOAA NCEI Climate Normals (1991–2020), as agricultural normals including freeze chances, first fall freeze and growing season length
AnswersWill a perennial survive winter here?When to sow and transplant; how long the season is
Food-garden usePerennials that stay in the ground over winter; tender perennials are often sold or treated as annuals in colder zones (UVM)Annual vegetables and herbs, spring and fall sowings; UVM's example is a tomato's days to maturity

As horticulturist John Porter puts it, the map "shares data on what the average coldest temperature is," while "to know when to plant we look at the average freeze and frost dates." Frost dates carry probabilities: Porter's Omaha example gives April 21 as the 50% date for a 32°F frost and May 4 as the 10% date. UVM adds the season-length check: count the days between your last and first frost dates and compare them with "the number of days to maturity on seed packets"; a 120-day tomato in a 100-day season may meet a freeze before it ripens.

How to use your zone

  1. Look up your zone with the "Quick Zip Code Search" on USDA's interactive map, or click your location on the map. Note the half zone, such as 6a.
  2. Recheck it even if you knew your old zone: UVM advises checking the 2023 map before buying seeds or plants.
  3. Compare with a plant's zone range. Illinois Extension's example: a plant hardy in zones 4 to 8 should thrive in zone 6, while one hardy only in zones 6 to 8 may struggle in an unusually cold winter.
  4. Adjust for your yard. USDA names heat islands from blacktop and concrete, low frost pockets where "cold air pools first," and warmer spots in front of south-facing walls, all too small to show on the map.
  5. Remember what the map leaves out: summer heat and moisture, dry soil going into winter, wind, snow, winter sun, how long a cold spell lasts, and early-fall or midwinter cold snaps (USDA ARS).
  6. Get your frost dates separately for timing annual crops: see frost dates explained, then enter them in the planting calendar.

Zone pages

A new zone number is not a reason to pull plants. USDA: "What has thrived in your yard will most likely continue to thrive." Illinois Extension says the same, and suggests a warmer zone may be a chance to try overwintering new perennials.

Sources

  1. USDA Agricultural Research Service — USDA Plant Hardiness Zone Map (interactive map and ZIP code search)
  2. USDA Agricultural Research Service — Plant Hardiness Zone Map: How to Use the Maps
  3. USDA Agricultural Research Service — Plant Hardiness Zone Map: Map Creation
  4. USDA Agricultural Research Service — USDA Unveils Updated Plant Hardiness Zone Map (news release, November 15, 2023)
  5. University of Minnesota Extension — A new plant hardiness zone map from the USDA (Julie Weisenhorn, 2023)
  6. University of Illinois Extension — Shifting USDA plant hardiness map reflects changing climate, changes for plants (Emily Steele, 2024)
  7. Iowa State University Extension and Outreach — New Plant Hardiness Zone Map Released from the USDA (Aaron Steil, 2023)
  8. University of Vermont Extension — USDA Hardiness Zones (Deborah J. Benoit, 2023)
  9. University of Idaho Extension — Spring Vegetable Planting Guide for Idaho (BUL 965, Andy West, 2020, PDF)
  10. Virginia Cooperative Extension — Virginia's Home Garden Vegetable Planting Guide: Recommended Planting Dates and Amounts to Plant (426-331, Jadrnicek and Hessler, 2025)
  11. The Garden Professors — Planting Prognostication: Understanding last frost and planting dates (John Porter)
  12. NOAA NCEI — When to Expect Your Last Spring Freeze
  13. Wikipedia — Hardiness zone (secondary source)

All sources were opened and checked in October 2026. Rules and figures change; confirm with the original before acting.