USDA Zones: US Planting Guide

What USDA zones actually measure — and what they don’t

USDA Plant Hardiness Zones measure one thing: the average annual extreme minimum winter temperature at your location. They tell you whether a perennial plant survives your coldest night of the year.

That’s it. Zones don’t measure summer heat, rainfall, soil type, humidity, or frost-free days. A Zone 6 plant in Nashville and a Zone 6 plant in Denver face similar winter cold but dramatically different summers — yet both technically match on the USDA scale.

This narrow definition makes zones useful for one specific question: “Will this perennial or shrub survive my winters?” It makes them less useful for questions about which vegetables to grow or when to start seeds.

The USDA Plant Hardiness Zone Map divides the US into 13 zones, each representing a 10°F range of average annual extreme minimum temperature. Each zone is subdivided into an “a” half (colder) and a “b” half (warmer), creating 26 sub-zones in total.

The 2023 map update: what changed

The USDA released an updated map in November 2023, replacing the 1990 version that many gardeners still reference. The new map incorporates 30-year climate data from 1991–2020 instead of 1976–2005.

Key changes:

• About 50% of the continental US shifted warmer by roughly half a zone
• The shift was most pronounced in the western US, especially the Pacific Northwest and intermountain regions
• No areas moved colder in the 2023 update

If your gardening books or seed catalogs reference the 1990 map, your zone may now be half a step warmer than listed. Check the current map at planthardiness.ars.usda.gov using your zip code.

Zone breakdown for the continental US

Zone 6 covers the largest geographic footprint of any single zone in the continental US — the reason most plant labels lead with “Zones 4–6” or “Zones 5–9” as their benchmark range.

How to read a plant label using zones

Plant nursery tags, seed packets, and catalogs list hardiness zones like this: “Hardy to Zone 5” or “Zones 6–9.”

“Hardy to Zone 5” means the plant survives winters down to -20°F. Plant it in Zone 5, 6, 7, 8, or any warmer zone and it should overwinter without protection. Plant it in Zone 4 (-30°F winters) and it will likely die.

“Zones 6–9” means the plant needs winters cold enough to provide dormancy (can’t survive year-round 60°F-plus temperatures) but can’t handle Zone 5’s cold. Zone 5 growers can sometimes overwinter these with heavy mulch; Zone 10 growers cannot grow them at all.

The zone listed is always about minimum winter survival, not ideal conditions. A hydrangea rated for Zones 3–9 grows in Zone 3 — it just won’t grow as large or flower as long as the same plant in Zone 6.

Zones vs. last frost dates: two different tools

Zones and last frost dates are separate — and equally important — planning tools. Confusing them is one of the most common new-gardener mistakes.

• USDA zones: tells you what survives winter cold
• Last frost date: tells you when to plant outdoors in spring

A Zone 6 garden in Chicago has a last frost around May 15. A Zone 6 garden in the Pacific Northwest might have its last frost in late March. Same zone, 6-week difference in planting windows.

For vegetables and annuals, the last frost date matters more than the USDA zone.

Microclimates: your yard probably has at least two zones

Your zip-code zone describes the general area. Your actual yard may behave differently in several spots.

Warmer microclimates:

• South-facing brick or stone walls absorb heat during the day and radiate it overnight
• Covered patios and overhangs reduce frost deposit
• Areas sheltered from north wind

Colder microclimates:

• Low-lying spots where cold air settles (frost pockets)
• Open areas exposed to north or northwest wind
• East-facing slopes that warm quickly in morning but don’t hold heat

A south-facing stone wall in a Zone 6 yard can create effectively Zone 7b conditions — letting you grow camellias or other borderline plants that shouldn’t survive by the zip-code zone alone.

A $15 digital min/max thermometer at a few spots around your yard over one winter gives you a more accurate picture than any map.

What zones don’t tell you

Zones measure winter minimum temperatures. They tell you nothing about:

• Summer heat: a camellia rated for Zone 7 grows in Zone 7 Nashville (hot, humid summers) and Zone 7 Seattle (cool, mild summers) — very different summer conditions with the same winter zone rating
• Summer humidity: crucial for disease-prone plants like roses
• Soil drainage: affects root survival more than air temperature in many cases
• Frost-free days: matters for squash, melons, and other long-season crops that need 90+ days without frost
• Rainfall patterns: a Zone 8 California coast and Zone 8 Charleston, SC receive very different annual rainfall

For complete planting zone information, combine your USDA zone with last frost date, average summer temperatures, and rainfall data from your local Cooperative Extension office.

Zones by major US city

How zones affect planting decisions

Perennials and shrubs: Buy plants rated to your zone or colder. A Zone 5-rated lavender grows in Zones 5–9. A Zone 8-rated camellia dies in Zone 6 winters.

Trees: Buy trees rated at least one zone colder than your zone for maximum survival. A Zone 5-rated tree handles Zone 5 winters; a Zone 4-rated tree has additional cold buffer for unusual winters.

Roses: Look at zone ratings for hybrid tea roses carefully — many are rated Zone 6–9 but need winter protection in Zone 6. Species roses and shrub roses like rugosas are often rated Zone 3–4.

Vegetables: Annual vegetables don’t have zone ratings — they have days-to-maturity and frost tolerance. A Zone 3 gardener can grow the same tomato variety as a Zone 9 gardener; the Zone 3 gardener just has fewer frost-free days to do it.

The AHS Plant Heat Zone Map

The American Horticultural Society’s Heat Zone Map complements the USDA system by rating plants for summer heat tolerance. It divides the US into 12 heat zones based on the average number of days per year with temperatures above 86°F — the point where many plants experience heat stress.

• Heat Zone 1: fewer than 1 day above 86°F per year (parts of Alaska)
• Heat Zone 12: more than 210 days above 86°F per year (desert Southwest)

Most of the interior South and Southwest falls in Heat Zones 7–12. The Pacific Coast and New England fall in Heat Zones 1–4.

Plants increasingly carry dual ratings like “USDA Zones 5–9, AHS Heat Zones 1–9.” This tells you both the cold floor and the heat ceiling the plant tolerates.

How to use zones to avoid wasting money

The biggest waste in the garden — buying a Zone 8 plant for a Zone 6 garden and watching it die in February. Three rules that prevent this:

1. Always verify the zone rating before buying a perennial or shrub. The plant tag or label should list it. If not, ask at the nursery or look up the botanical name at a university extension database.
2. Buy one zone colder when the climate data for your area shows occasional extreme winters. Zone 6 gardens in the Midwest can see Zone 4 temperatures in a polar vortex year.
3. Test borderline plants in a protected spot first — against a south wall, mulched heavily — before committing to a full planting.