Quick answer
A plant hardiness zone is a band on a map that groups places by the average coldest winter night of the year, so you can judge whether a perennial plant is likely to survive the winter where you live.
In the United States the standard is the USDA map. It says nothing about summer heat, rainfall, soil or the small warm and cold spots inside a single yard.
Quick facts
The key facts about Plant Hardiness Zone, with sources:
- Set by
- USDA Agricultural Research Service, with Oregon State University’s PRISM Climate Group 2
- What a zone measures
- The average annual extreme minimum winter temperature, shown as 10°F zones split into 5°F half zones 1
- Current edition
- Released 15 November 2023, the first update since 2012 2
- Data period
- 30-year averages for 1991 to 2020; the 2012 map used 1976 to 2005 3
- Weather stations
- 13,412 in the 2023 map, up from 7,983 in the 2012 map 2
- Change since 2012
- About half the country moved to the next warmer half zone; the other half stayed in the same half zone 2
Key takeaways
- A hardiness zone answers one question: how cold does winter usually get here? It is the average of each year’s lowest temperature, according to the USDA 1.
- The 2023 USDA map uses 1991 to 2020 data and moved about half the country to a warmer half zone, so an old zone number may now be wrong 2.
- Zones say nothing about summer heat, rainfall, humidity, wind or soil. University of Arizona Extension calls them a starting point for choosing plants 4.
- One yard can hold several microclimates, from a hot south wall to a cold low spot, and a zone number cannot show that 5.
- Use the zone on a plant tag to rule plants out, then check the sun, water and soil at the spot before you buy.
A winter number on a map
The USDA Plant Hardiness Zone Map is described by the agency as the standard gardeners and growers use to decide which perennial plants are most likely to thrive at a location.
The zones rest on a single temperature statistic: the average annual extreme minimum winter temperature 1. Put simply, pick the coldest night of each winter, then average those nights over many years.
The map divides the country into 10°F zones, and each zone is split into 5°F half zones 1. Zone 7 is warmer than zone 6, and zone 7b is warmer than zone 7a. The letter “a” is the colder half and “b” is the warmer half.
A perennial plant is one that lives through more than one year. The zone matters most for plants that stay in the ground all winter, such as shrubs, trees and perennial flowers. Annuals live one season, so winter hardiness hardly applies to them.
This page explains the term. It does not list plants by zone, because those lists differ by region and by cultivar. For a planting job, read our planting and garden beds guide, and for what a nursery promises after planting, see plant warranties.
How the 2023 map was made
The data. USDA and Oregon State University’s PRISM Climate Group built the 2023 map from 30 years of weather station records, covering 1991 to 2020. It uses 13,412 stations, compared with 7,983 for the 2012 map 2.
The map-creation page of the USDA site gives a slightly different figure of 13,625 stations for the data set 3. We print the news release figure in the tables and flag the difference here.
The estimate. PRISM estimated the average annual extreme minimum temperature for each grid cell, and the cells are about half a mile on a side.
The method accounts for elevation, distance to water and terrain, including coastal effects and temperature inversions 3.
That is why cities, high ground and places downwind of large lakes can land in warmer or cooler zones than their surroundings.
The review. USDA says climatologists, agricultural meteorologists and horticultural experts reviewed the draft and rechecked zones that looked odd 3.
The zones. The 2012 and 2023 editions added zones 12 and 13 for places with average extreme minimums above 50°F and 60°F. These appear only on the Hawaii and Puerto Rico maps 3.
- Collect30 years of winter lows from thousands of weather stations
- AverageThe lowest temperature of each year, then the mean across years
- ModelElevation, water and terrain fill the gaps between stations
- BandCells fall into 10°F zones and 5°F half zones
- ReviewExperts recheck zones that look out of line
Half zones from 6a to 9b, with example towns
| Half zone | Average yearly lowest temperature | Example Yavapai County, Arizona towns (zones updated to 2023 data) |
|---|---|---|
| 6a | −10°F to −5°F | None listed in the bulletin |
| 6b | −5°F to 0°F | None listed apart from a split-zone town |
| 7a | 0°F to 5°F | None listed |
| 7b | 5°F to 10°F | Prescott, Prescott Valley |
| 8a | 10°F to 15°F | Chino Valley, Dewey |
| 8b | 15°F to 20°F | Bagdad, Camp Verde, Clarkdale, Cottonwood, Jerome, Mayer |
| 9a | 20°F to 25°F | Black Canyon City, Congress, Cornville |
| 9b | 25°F to 30°F | None listed |
A band, not a promise
The bulletin’s ranges are labelled with the 2012 criteria, while its town zones were updated for the 2023 map 4. The USDA still describes the 2023 map as 10°F zones and 5°F half zones 1. Check the live USDA map for your exact spot instead of reusing a table.
Sources: [4]
Example: what a zone range on a plant tag spans
Plant tags often print a range such as “zones 6 to 9.” The first number is the cold limit. Arizona Extension explains that the first set of numbers on a tag is the cold hardiness range 4. The arithmetic below uses the half-zone bands in the table above. The tags are invented examples.
| Tag reads (example) | Coldest average low it covers | Warmest average low it covers | Width of the range |
|---|---|---|---|
| Zones 6 to 9 | −10°F (bottom of 6a) | 30°F (top of 9b) | 40°F |
| Zones 7b to 8b | 5°F | 20°F | 15°F |
| Zones 8 to 9 | 10°F | 30°F | 20°F |
A wide range does not mean the plant is easy. Arizona Extension warns that plants that do well in one zone are often adapted to the next warmer zone, but not always, because a warmer zone usually has hotter summers that some plants tolerate and others do not 4.
The tag range is also the grower’s claim, so ask a local nursery or your Cooperative Extension office how the plant performs nearby.
Where the term shows up: tags, catalogs, bids and planting plans
Plant tags and catalogs. Arizona Extension says most nursery tags refer to the USDA hardiness zone, and some now show a heat zone too, as in “3-8, 8-1” 4. The first pair is cold hardiness. The second pair is heat.
Planting plans and bids. A designer may list plants “for zone 6.” Ask which map edition the plan used. After November 2023 many places moved up a half zone, so an older list can be off 2. Our landscaping estimates guide shows what a plant line on a bid should state.
Warranties. A plant warranty often excludes winter loss in a year with an unusual cold snap. The USDA notes that the yearly lowest temperature “can be highly variable from year to year,” depending on local weather patterns 2. Read the terms on the plant warranties page first.
Grass. Lawn grasses are matched to regions, not to the zone number alone. See cool-season vs. warm-season grass.
What the zone does well, and where it falls short
What it does well
- Gives one shared, free way to compare winter cold across the country 1.
- Rules out plants that cannot take your winter lows, which is the most common cause of plant loss in a bed.
- Uses a long, well-documented data set with many more stations than the 2012 edition 2.
- Lets a nursery, a designer and a buyer talk about the same number.
What it cannot do
- It leaves out summer heat. Arizona Extension notes that milder winters usually come with higher summer temperatures that some plants cannot stand 4.
- It says nothing about rain, humidity or soil. The American Horticultural Society heat map even assumes adequate water 4.
- It cannot see one yard’s warm and cold pockets 5.
- It averages the coldest night, so a record-cold year can still kill a plant rated for your zone 4.
Limits and open points
The zone is an average, not a floor. Arizona Extension says the zones rest on averages from several years. In years that run warmer or colder than the long-term average, plants that usually do well can struggle 4.
The map is not a climate-change gauge. The USDA says temperature updates to the zones “are not necessarily reflective of global climate change.” The developers explain that changes also come from better mapping methods, many more weather stations and the changeable nature of extreme minimum temperatures 2.
Map editions differ. The 2023 map is generally about one quarter-zone warmer than the 2012 map across much of the country, while some areas became cooler because of added data and better interpolation 3. A list written for the 2012 map may not fit.
Other zone systems answer other questions. The heat zones and Sunset climate zones are covered in the comparison section. We did not find a single national system that combines cold, heat and moisture.
Plant performance has other causes. Drier air, strong seasonal winds and alkaline or salty soil can create a hostile place for plants that are cold hardy in a given zone 4. A plant that dies is not always a zone mistake.
What a zone mistake does to cost and plant life
We do not print prices on a glossary page. A zone mismatch changes a project in these ways.
Replacement. A plant that cannot take your winter lows may need replacing. Arizona Extension notes that some plants sold as perennials may act as annuals because of cold and heat extremes 4. That turns a one-time purchase into a repeat one.
Plant size and age. A larger, older plant costs more to lose than a small one. Ask a company how it handles a plant that fails in the first winter. Common warranty terms are explained on that page.
Protection work. Windbreaks, mulch, siting against a warm wall and watering through dry winters all add labor or materials. See the cost hub and our landscaping cost estimator for what moves a bid.
No standard lifespan. How long a plant lives depends on the plant, the site and the weather. We do not give one figure for every yard.
How to use a zone when you choose plants
The is below a reading order, not a recipe. Each step comes from the USDA, Arizona Extension or Washington State University pages cited here.
- Find your zone on the USDA map at planthardiness.ars.usda.gov, which USDA publishes free to download and print 1. Many websites also list zones by ZIP code, but Arizona Extension points out that a town or ZIP code can contain different microclimates 4.
- Check which edition any plant list or tag was written for. If it predates November 2023, compare it with the current map 2.
- Rule out plants whose cold limit on the tag is warmer than your zone. Treat a plant at the edge of its range as a risk.
- Walk the yard and note warm and cold spots. Washington State University suggests mapping sun and shade at 9 a.m., noon and 3 p.m. and pushing a garden thermometer about 2 inches into the soil in several places to find the hottest and coldest spots 5.
- Ask local nursery staff, neighbors and your Cooperative Extension office what grows well in your microclimate, as Arizona Extension advises 4.
- Match the plant to the sun, water and soil you have. See sun exposure for landscape plants and soil testing.
Keeping your zone information current
A zone number does not need care, but your information about it does. Re-check the USDA map when it is updated, because the 2023 edition was the first revision since 2012 2.
Keep a simple garden record of which plants came through winter. Arizona Extension says “nothing substitutes for experience and actual plant performance” 4. A note about which plant died in which spot can teach you more than any map.
Watch the soil as well as the air. Washington State University suggests measuring soil temperature to find the hottest and coldest spots when you want to grow a plant that is slightly out of your zone 5.
If you move a plant that struggled, read transplanting landscape plants first.
Warning signs of a zone or microclimate mismatch
Washington State University says gardeners often bring a struggling plant to the Master Gardener clinic only to learn that, once other causes are ruled out, the plant dislikes the spot even in the correct zone 5. These signs point to a mismatch.
Winter damage. Dead tips, split bark or a plant that fails to leaf out in spring after a cold winter.
Summer stress. Scorched leaves or poor growth in a hot summer, which suggests heat is the limit and not cold 4.
Patterns by location. The same kind of plant does well in one corner and poorly in another. WSU lists wind, soil moisture, a hot south-facing slope and a cold glen as common causes 5.
This page is not a diagnosis. If a mature tree or a large shrub is declining, ask your local Cooperative Extension office or a certified arborist. To find a company, read how to choose a landscaping company.
Call 811 before you dig a planting hole
A zone number is only a starting point for choosing plants 4. Before digging for any new tree or shrub, call 811 so buried utility lines are marked first. Keep large trees well away from overhead power lines.
Who sets the map, and what is law
| Topic | What is stated | Type | Source |
|---|---|---|---|
| The USDA map | Published by USDA ARS as the standard for choosing perennials; free to download and print | Agency reference | USDA 12 |
| Half zones | 10°F zones with 5°F half zones | Map convention | USDA 1 |
| Data period | 1991 to 2020 for the 2023 edition; 1976 to 2005 for 2012 | Map method | USDA 3 |
| Tag labeling | Most nursery tags refer to USDA zones; some add heat zones | Trade practice, as described by one extension office | Arizona Extension 4 |
| Planting rules, setbacks, banned plants | Set by local ordinance, HOA or state law, not by the map | Local or state rule | Check locally |
Hardiness, heat and climate zones compared
| System | Built by | What it measures | Watch for |
|---|---|---|---|
| USDA hardiness zone (this page) | U.S. Department of Agriculture | Average coldest winter night of the year | No heat, rain or soil data 41 |
| Heat zone | American Horticultural Society | Average days a year above 86°F; assumes adequate water | Based on 1974 to 1995 data published in 1997 4 |
| Sunset climate zone | Sunset magazine, in the Western Garden Book | Winter lows, summer highs, elevation, nearness to coast or mountains, rainfall, humidity, aridity and growing season | A Western U.S. system 4 |
| Microclimate | The site itself | Local conditions on a small scale, from a few square feet to a city lot | Not on any map 54 |
| Grass adaptation region | Extension turf programs | Which lawn grasses suit a region | See cool vs. warm season grass |
USDA hardiness zone
- How cold does winter get?
- Average yearly lowest temperature
- 10°F zones, 5°F half zones
AHS heat zone
- How hot does summer get?
- Days a year above 86°F
- Assumes enough water
Sunset climate zone
- What is the whole climate like?
- Cold, heat, rain, humidity, season length
- Written for the West
Myths about hardiness zones
- Myth If a plant is rated for my zone, it will thrive.
- Reality The zone covers winter cold only. Heat, wind, dry air and alkaline or salty soil can still harm a plant that is cold hardy in your zone 4.
- Myth My zone moved, so my climate changed by that much.
- Reality The USDA cautions that zone changes are not necessarily a reliable sign of climate change, because mapping methods and station counts also changed 2.
- Myth Everyone in my ZIP code has the same zone.
- Reality Arizona Extension says towns and ZIP codes contain different climates and microclimates, so zones are a starting point 4.
- Myth A plant rated for a warmer zone will always do well.
- Reality Arizona Extension says this is often but not always so, because a milder zone brings hotter summers and erratic frosts and warm spells 4.
What changes by region: your zone, your elevation and your yard
The zone itself is the regional variable. It can shift over a short distance when elevation, water or terrain changes. The Arizona table above shows towns in one county spread across 7b, 8a, 8b and 9a 4.
Cities, high ground and places downwind of large lakes can sit in warmer or cooler zones than their surroundings 3.
For a hot, dry region, a cold, short-season region and a humid region, the same zone number can mean very different plant lists, so ask your Cooperative Extension office for a local list.
For turf rebates and watering rules that also vary by place, use the research pages below.
When the zone matters most, and your next step
The zone matters most when you plant shrubs, trees or perennials that must live through winter, and when a plant sits near the edge of its range. It matters less for annuals and for plants you will lift or bring indoors.
Your next step is to look up your zone on the current USDA map, rule out plants that cannot take your winter lows, and then walk your yard for warm spots, cold spots, sun and water.
Ask a local nursery or Extension office about plants that have done well in your area. For help with plant selection, read native landscaping, drought-tolerant plants and xeriscaping.
If you want a company to plan and plant a bed, find a landscaping company. HyreYard does not sell or plant anything.
- If Will it survive winter here?
- Then Use the USDA hardiness zone on the current map, then check your microclimate 15
- If Will it cope with summer?
- Then A hardiness zone cannot say. Look for a heat zone or ask local growers 4
- If Is my list older than November 2023?
- Then Compare it with the 2023 map, because about half the country moved up a half zone 2
- If Does one spot in my yard keep failing?
- Then Check wind, drainage, sun and soil before blaming the zone 5
What changed in the 2023 edition, in numbers
The 2023 map added more weather stations and moved to a newer 30-year period. USDA says the new map is more accurate and more detailed, and that Alaska’s detail improved from roughly 6¼ square miles per area to ¼ square mile 2. The chart shows the station counts.
The change is not even. About half the country moved to the next warmer half zone, while the other half stayed in the same half zone 2. Check your own spot, not the national headline.
Questions about Plant Hardiness Zone
What is a plant hardiness zone?
It is a band on a map that groups places by the average coldest winter night of the year. The USDA map uses 10°F zones split into 5°F half zones. It tells you whether a perennial plant is likely to survive winter cold in a place. It does not cover heat, rainfall or soil.
What does the plant hardiness zone number mean?
The number shows how cold winter usually gets. The USDA bases each zone on the average annual extreme minimum winter temperature. A higher number is warmer, and the letters a and b split each zone into colder and warmer halves of 5°F each. Zone 7b is warmer than 7a.
What changed in the 2023 USDA hardiness zone map?
USDA released it on 15 November 2023. It uses 1991 to 2020 data and 13,412 weather stations, up from 7,983. About half the country moved to the next warmer half zone, and the other half stayed. USDA says the changes should not be read as a reliable sign of climate change.
How do I find my hardiness zone?
Use the USDA Plant Hardiness Zone Map at planthardiness.ars.usda.gov, which is free. Many sites also list zones by ZIP code. University of Arizona Extension warns that a ZIP code can hold several microclimates, so treat the number as a starting point and ask local growers what works in your yard.
Is hardiness zone the same as heat zone?
No. A hardiness zone covers winter cold. The American Horticultural Society heat zone counts the average number of days a year above 86°F and assumes enough water. University of Arizona Extension says some plant tags now show both numbers, such as 3-8 for cold and 8-1 for heat.
Can I grow a plant rated for a colder or warmer zone?
Sometimes. Arizona Extension says adjoining zones grade into one another, so a few plants not rated for your zone may survive, and plants from the next warmer zone often do well. It is not guaranteed, because a warmer zone usually has hotter summers that some plants do not tolerate.
What is a microclimate in a yard?
It is the local climate on a small scale. Washington State University says a microclimate can be as small as a few square feet in a bed or as large as a city lot. Slopes, buildings, paving, hedges and low spots where cold air settles all create them.
Does the hardiness zone tell me when to plant?
No. The zone describes winter lows, not planting dates. Planting time depends on your plant, your local frost dates, soil and rainfall. Ask your Cooperative Extension office for local timing. We do not print one planting date for every yard, because it varies by climate.
Why did my zone change?
The 2023 map used newer data, from 1991 to 2020, and many more weather stations. USDA says changes also reflect better mapping methods and the variable nature of extreme minimum temperatures. The new map is generally about one quarter-zone warmer than the 2012 map across much of the country.
Do zones tell me which plants will die in a hard winter?
Not exactly. The zone is based on average yearly lows. Arizona Extension says that in years that run colder than the long-term average, plants that usually do well may struggle. A tag rated for your zone is a guide, not a guarantee against a record cold year.
Sources
- USDA Agricultural Research Service, USDA Plant Hardiness Zone Map (home page), retrieved .
- USDA Agricultural Research Service, USDA unveils updated Plant Hardiness Zone Map (news release, 15 November 2023), retrieved .
- USDA Agricultural Research Service, Plant Hardiness Zone Map: Map Creation, retrieved .
- University of Arizona Cooperative Extension, Yavapai County, Hardiness, Heat, and Climate Zones (Bulletin 75, Mary Barnes, November 15, 2023), PDF read as text with pdftotext, retrieved .
- Washington State University Extension, Skagit County Master Gardeners, Microclimates (Claudia Wells, June 7, 2019), PDF read as text with pdftotext, retrieved .
Expert review
Written by the HyreYard Research Desk. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Zone definition, half zones, edition dates, data periods and the climate caveat come from USDA pages loaded on 2026-10-14.
USDA pages give two station counts for the 2023 map (13,412 in the news release, 13,625 on the map-creation page); we print the news release figure.
Half-zone temperature bands, heat zones and Sunset zones come from a University of Arizona Extension bulletin read as text from a PDF; its ranges are labelled with the 2012 criteria.
Microclimate points come from a Washington State University Extension handout, also read as text. We give no plant lists by zone and no planting dates because they vary by region.
Suggest a correction. We fix errors and say what changed.