USDA hardiness zones: what the number means and how to use it

Q: Why does my ZIP code give a different zone than my garden?

The official map is drawn at a fine scale and your garden can sit in a cold pocket or a warm walled courtyard. ZIP-level lookups are an average; use the official map and your own observations for the final call.

Q: What does the zone not tell me?

Heat, rainfall, frost-free days, soil, wind and summer stress. Two gardens in the same zone can have very different growing seasons.

A USDA zone is the average annual extreme minimum temperature at your location. It answers one question — will this plant survive your winter — and says nothing about heat, rain, frost dates or soil. Zone 7 in a coastal garden and zone 7 in a high desert are not the same growing conditions.

Zone ranges

Zone	Average annual minimum	In °C
1	-60 to -50 °F	-51 to -46 °C
2	-50 to -40 °F	-46 to -40 °C
3	-40 to -30 °F	-40 to -34 °C
4	-30 to -20 °F	-34 to -29 °C
5	-20 to -10 °F	-29 to -23 °C
6	-10 to 0 °F	-23 to -18 °C
7	0 to 10 °F	-18 to -12 °C
8	10 to 20 °F	-12 to -7 °C
9	20 to 30 °F	-7 to -1 °C
10	30 to 40 °F	-1 to 4 °C
11	40 to 50 °F	4 to 10 °C
12	50 to 60 °F	10 to 16 °C
13	60 to 70 °F	16 to 21 °C

Each zone has an a and b half: 5°F (2.8°C) colder or warmer. Zone 6b means the average winter minimum sits between −5 and 0°F (−21 to −18°C).

How many plants in our dataset suit your zone

Counts from the planner's curated plant list (68 taxa), by the coldest zone each one is listed for. A plant listed for zones 4–8 also grows in 5, 6 and 7.

Zone	Plants that list this zone	Examples
Zone 3	28 of 68	Pagoda dogwood, white fringetree, witch hazel
Zone 4	42 of 68	Serviceberry, redbud, smooth hydrangea
Zone 5	59 of 68	Japanese maple, flowering dogwood, boxwood
Zone 6	63 of 68	Serviceberry, panicle hydrangea, coneflower
Zone 7	66 of 68	Crape myrtle, switchgrass, pink muhly
Zone 8	65 of 68	Rosemary, Texas sage, desert willow
Zone 9	42 of 68	Crape myrtle, rosemary, pink muhly
Zone 10	9 of 68	Desert willow, fig ‘Chicago Hardy’, yucca

How to find your zone

Use the ZIP lookup in the planner for a first answer, or open the official USDA map (2023 edition) and zoom to your address.
Check the map twice: once for the garden, once for the coldest corner (a north-facing wall, a low spot where cold air pools).
Record the a/b half. Choose plants at or above your zone for perennials and shrubs; treat anything one zone colder as an experiment.
Add the other limits: summer heat, rainfall, first and last frost dates, soil pH. A zone 8 plant that hates humid summers will still fail in a zone 8 coastal garden.

What the zone cannot tell you

Factor	Why it matters	Where to find it
Summer heat	Heat delays fruiting and scorches leaves; cold-hardy plants can still fail in hot summers.	Local extension service, AHS heat-zone maps.
Frost-free days	Decides whether a crop ripens before the first autumn frost.	NOAA/local climate normals.
Rainfall and irrigation	Winter hardiness does not mean summer survival without water.	Local climate data, your own rain gauge.
Soil	pH and drainage decide nutrient uptake and root health.	A soil test through your extension service.
Microclimate	A wall, slope, or city centre can shift the effective zone by half to a full zone.	Your own minimum thermometer readings.

FAQ

What do USDA zones mean?

Average annual extreme minimum temperature, in 10°F bands with 5°F a/b halves. It predicts winter cold survival only.

Why does my ZIP code give a different zone than my garden?

ZIP lookups are an average; the official map and your own cold-spot observations are more precise.

What does the zone not tell me?

Heat, rainfall, frost-free days, soil and wind — all of which decide whether a plant thrives rather than merely survives.