Plant tags often say something like “zones 5 to 9.” That refers to the USDA Plant Hardiness Zone Map, the standard gardeners in the United States use to judge whether a perennial plant is likely to survive the winter where they live. This guide explains what the zones mean, how to find yours and what the map does not tell you.
Key takeaways
- The USDA map is based on the average annual extreme minimum winter temperature.
- Zones are 10 degrees Fahrenheit apart, with 5-degree half zones marked a and b.
- Hardiness is only one factor: heat, rainfall, soil and sun also matter.
What the map measures
According to the USDA Agricultural Research Service, the Plant Hardiness Zone Map is based on the average annual extreme minimum winter temperature at a location. In other words, it tells you roughly how cold the coldest night of the winter usually gets. That matters because cold is what kills many perennials, shrubs and trees outside their range.
How zones are numbered
The map shows zones in 10-degree Fahrenheit steps, with 5-degree half zones. Half zones are marked with the letters a and b, where a is the colder half. A plant rated for zones 5 to 9 is expected to survive typical winters in any of those zones.
Finding your zone
The USDA’s map website lets you type a ZIP code to find your zone, or click on an interactive map. The current map is the 2023 edition, which replaced the 2012 edition. Many homeowners found their zone changed between editions, so it is worth checking again if you last looked years ago.
Using zones when you shop
- Check the zone range on the plant tag or in a reliable plant database.
- Prefer plants rated comfortably within your zone, not at its edge.
- For expensive trees and shrubs, look for local experience from your cooperative extension office.
- Be cautious with plants rated only for warmer zones, even if a mild winter lets them survive for a while.
What the zones do not tell you
The map covers winter cold only. It does not account for summer heat, humidity, rainfall, soil type, wind or how much sun a spot gets. Two places in the same zone can be very different: one humid and rainy, the other dry and windy. A plant that survives winter can still fail because of heat, drought or poor drainage.
Hardiness at a glance
| Question | Answer |
|---|---|
| What is measured | Average annual extreme minimum winter temperature |
| Zone width | 10 degrees Fahrenheit |
| Half zones | 5 degrees Fahrenheit, labeled a and b |
| Current edition | 2023 |
| Where to check | USDA map site, by ZIP code |
Microclimates in your yard
Your yard has its own small variations. A sheltered spot by a south-facing wall may be warmer; a low spot where cold air settles may be colder. Many gardeners use these microclimates to grow plants at the edge of their zone, but it is a gamble with expensive plants.
Tip: When a plant is marginal for your zone, try one in the most sheltered spot of your yard before planting a whole bed of them.
Hardiness and water-smart planting
EPA’s WaterSense program suggests determining your USDA hardiness zone and then choosing plants listed as low water use or drought tolerant for your area. Its What to Plant page links to state and regional plant lists, many from cooperative extension services and native plant societies. See Water-Wise Landscaping: Xeriscaping Basics for Any Region.
Which plants fit your zone
Native plants from your region are, by nature, adapted to local winters; see Native Plants for Your Yard: Why They Help and How to Choose Them. Beyond that, good nurseries usually stock plants suited to the local zone, but garden centers sometimes sell plants that will not survive winter, especially seasonal annuals and tropicals. Read the tag.
Heat and drought matter too
In hot regions, summer heat and drought can limit plants more than winter cold. Many regional plant lists, including those linked from EPA’s WaterSense program, note which plants tolerate heat and dry spells. Cooperative extension services often publish recommended plant lists by region, which combine hardiness with local experience of heat, soil and pests. Use the zone to narrow the list, and local advice to choose from it.
Common mistakes
- Using an old zone from a previous map edition.
- Treating the zone as a guarantee.
- Ignoring summer heat and drainage.
- Planting marginal trees in exposed spots.
Frequently asked questions
How do I find my hardiness zone?
Enter your ZIP code on the USDA Plant Hardiness Zone Map website.
What do a and b mean?
They are 5-degree Fahrenheit half zones; a is the colder half.
Does my zone apply to annuals?
Annuals complete their life in one season, so zones matter mainly for perennials, shrubs and trees.
For layout ideas, see Landscape Design Basics for Homeowners.