How to read your hardiness zone — and what it can't tell you

Last updated October 19, 2026 · Sources: cited below

Your hardiness zone answers one narrow question: how cold does the coldest night of a typical winter get here? It is a very good answer to that question, and a poor answer to almost every other gardening question. This guide explains exactly what the number and letter mean, how to read a plant label against it, and where the zone stops being useful — with examples from the 512 cities on this site.

What the number measures

The 2023 USDA Plant Hardiness Zone Map is built from the average annual extreme minimum temperature for 1991–2020. For each year, take the single coldest temperature recorded; then average those 30 yearly lows. That average decides the zone. The map was produced by the USDA Agricultural Research Service with the PRISM Group at Oregon State University, using data from more than 13,000 weather stations and a model that accounts for elevation, terrain and coastlines.^1,2

Zones are 10°F apart, numbered from 1 (coldest) to 13 (warmest). Each is split into an “a” half (colder) and a “b” half (warmer), 5°F apart. Zone 1a starts at −60°F, so the lower limit of any half-zone is easy to work out:

The important word is average. USDA is explicit that the zone “does not represent the coldest it has ever been or ever will be in an area.”³ Roughly half of winters will dip below the zone's average low, and a once-a-decade cold snap can go well below it. A zone rating is therefore a statement about typical risk, not a guarantee.

How to read a plant label

When a tag says a shrub is “hardy to zone 6,” it means the plant is expected to survive the average coldest night of zone 6 (−10 to 0°F) when it is established and in the ground. Read it as follows:

1. Find your half-zone. Use the ZIP code lookup. Our city pages also list every ZIP code in the city, because 170 of the 512 cities here span more than one half-zone.
2. Compare numbers, not names. A plant rated to zone 5 is rated for a colder winter than yours if you are in zone 6 or 7 — that is a safety margin. A plant rated to zone 8 is “zone-pushing” in zone 7.
3. Treat the colder half of a zone carefully. A label that says “zone 7” is often written for the whole zone; in 7a (0 to 5°F) a borderline plant has less margin than in 7b.
4. Remember the label assumes good conditions. Well-drained soil, a healthy established plant, and normal autumn hardening. Young plants, wet winter soil and plants in above-ground containers are more exposed.

Annual vegetables such as tomatoes, beans and basil are a different matter: they die at the first frost whatever your zone, so the zone tells you nothing about when to plant them. That is what frost dates are for — see Frost dates explained.

What your zone does not tell you

Extension horticulturists repeat the same warning: hardiness maps describe winter survival only, and ignore summer heat, drought, rainfall, humidity and soil.⁴ University of Arizona Cooperative Extension adds that plants which do well in one zone are often adapted to the next warmer one, but not always, because milder winters tend to come with hotter summers and erratic spring frosts.⁵

Our own data shows how loosely a zone predicts the growing season. In zone 6b, the typical (30% risk) last spring frost ranges from Apr 8 in Boston, MA to Jun 14 in Bend, OR — 67 days apart, for 46 cities in the same half-zone. In zone 6a the spread is 54 days, from Chicago (Apr 17) to Flagstaff (Jun 10). The difference is mostly elevation and dryness: Boston sits at sea level beside the ocean, while Bend is at about 3,600 feet in the high desert, where clear, dry nights stay frosty well into June.

Growing season length shows the same pattern. Boston and Bend share zone 6b, but the median frost-free season (NOAA, 50% probability) is 218 days in Boston and only 105 days in Bend. A gardener moving between the two would face very different vegetable choices despite an identical zone.

Other things the zone leaves out:

• Chill hours. Many fruit trees need a certain amount of winter cold to flower. Warm zones can lack enough of it even when they are “hardy.”
• Snow cover. Steady snow insulates roots; the same air temperature on bare ground is harsher.
• Winter wind and sun. Evergreens can dry out in sunny, windy cold even when the temperature is within their rating.
• Timing of cold. An early autumn freeze or a hard freeze after a warm spell in late winter can injure plants that would survive the same temperature in mid-January.
• Your own yard. Low spots, slopes, walls and pavement shift temperatures by several degrees. See Microclimates.

A worked example: Denver

Denver shows how the zone and the calendar answer different questions. Most of the city is zone 6a on the 2023 map (it was 5b in 2012), meaning the coldest night of an average winter is −10 to −5°F. That tells you a shrub rated to zone 5 has a comfortable margin, a zone 6 shrub is a reasonable bet, and a zone 7 shrub is a gamble.

It tells you nothing about tomatoes. For that you need Denver's frost dates: at the Denver-Stapleton station, the 30%-risk last spring frost is May 8 and the first fall frost is Oct 3, giving a median frost-free season of about 156 days. Chicago, also zone 6a, has a season of about 208 days. Same zone, different gardens.

Common misconceptions

• “My zone tells me when to plant.” It doesn't. Planting dates come from frost dates and soil temperature, which vary a lot within one zone.
• “Higher zone number means a longer season.” Usually, but not reliably. A coastal zone 9 city may have cool summers that slow tomatoes, while an inland zone 7 city ripens them easily.
• “If my zone changed, my old plants are now wrong.” No. The plants in your garden survived real winters; the new label describes a slightly milder average.
• “The zone is the coldest it ever gets.” It is an average of yearly lows. Record cold is lower.

Alaska, Hawaii and Puerto Rico

The 2023 map covers all three, and our ZIP lookup includes their PRISM listings. Alaska gained much finer detail in 2023, which USDA notes made parts of the state map warmer, especially in mountains above cold valley air.¹ In the ZIP listings, Alaska's ZIP codes run from zone 1a to 8b, Hawaii's from 10b to 13a and Puerto Rico's from 11b to 13b. Where most people in Hawaii and Puerto Rico live, frost is essentially absent and planting is timed by rainfall and heat. For those places, our city pages say so plainly rather than listing frost dates.

ZIP code zones versus the map

The ZIP code lookup on this site uses the official ZIP code listings published by the PRISM Group at Oregon State University for the 2023 map.² Each ZIP code gets a single zone, based on the ZIP's area. That is convenient but coarse: a ZIP code spanning a river valley and a ridge has one label but two climates. When a city spans several half-zones, our city pages show the share of land in each, and the headline zone is the one covering the most land.

This site is independent and does not reproduce the official map. For the interactive official map, go to the USDA site at planthardiness.ars.usda.gov.

A practical checklist

1. Look up your ZIP and note the half-zone and its temperature range.
2. For perennials, trees and shrubs: buy plants rated for your half-zone or colder. Use plants rated one half-zone warmer only in sheltered spots, and accept some risk.
3. For vegetables and annuals: ignore the zone and use your city's frost dates instead.
4. Check summer heat and rainfall: a plant hardy in your winter may still struggle in your summer.
5. Watch your own garden. Over a few seasons, your observations beat any map.

If your zone changed in 2023, read why the 2023 map looks warmer before changing what you plant.

More guides: Frost dates explained · Why the 2023 hardiness zone map looks warmer · Microclimates · Succession and fall planting