Hardiness Zones

Your hardiness zone is the most useful single number for choosing perennials. It tells you how cold your winters get, and every plant profile on this site lists the zones where that plant survives.

Choose Your Zone

Each zone page lists the perennials that grow there, along with planting and winter care advice for that climate. Zones 3 to 9 cover most of the United States and southern Canada.

How to Read a Plant’s Zone Range

Plant tags and our profiles give a range, such as “Zones 3 to 8.” The first number is the coldest zone where the plant reliably survives winter. The second number is the warmest zone where it does well, usually because hotter places have too much summer heat and humidity, or winters too mild for plants that need a cold rest.

Catmint: zones 3 to 82345678910

If your zone sits inside the range, the plant is a safe choice. If your zone is the first number, the plant is at its cold limit: it will usually survive, but give it your best-drained, most sheltered spot. Lavender, rated for zones 5 to 9, is a good example. It grows easily in zone 7 but needs sharp drainage and a warm wall to get through zone 5 winters.

Zones 3 to 9 at a Glance

The zone number comes from the average of the coldest temperature recorded each winter over 30 years. It is not the coldest temperature ever recorded, so a rare cold snap can still go below your zone’s range.

Average Annual Extreme Minimum Temperature by Zone

Zone	Fahrenheit	Celsius	Plants
Zone 3	-40 to -30 °F	-40 to -34.4 °C	9
Zone 4	-30 to -20 °F	-34.4 to -28.9 °C	13
Zone 5	-20 to -10 °F	-28.9 to -23.3 °C	14
Zone 6	-10 to 0 °F	-23.3 to -17.8 °C	14
Zone 7	0 to 10 °F	-17.8 to -12.2 °C	14
Zone 8	10 to 20 °F	-12.2 to -6.7 °C	14
Zone 9	20 to 30 °F	-6.7 to -1.1 °C	9

What Your Zone Does Not Tell You

A zone measures winter cold and nothing else. Two gardens in the same zone can behave very differently, so treat your zone as a starting point and adjust for the conditions below.

Snow Cover

A steady blanket of snow insulates the soil and keeps roots far warmer than the air above. A zone 5 garden with reliable snow can be kinder to perennials than a zone 6 garden where winters swing between bare frozen ground and thaws.

Winter Drainage

Wet soil kills more perennials in winter than cold does, because roots sitting in icy water rot. Plants from dry climates, such as lavender, Russian sage and sedum, survive colder zones in gravelly, fast-draining soil than in heavy clay.

Summer Heat

Hardiness zones say nothing about heat. The American Horticultural Society’s Plant Heat Zone Map fills that gap by counting the days each year above 86°F (30°C), which helps explain why a plant rated to zone 8 can still struggle in a hot, humid summer.

Microclimates

Your own garden has warmer and colder pockets. Beds against a south-facing wall, near paving or in a city can run half a zone warmer, while low spots where cold air settles and open, windy sites run colder. Plant borderline perennials in the warmest spots.

Containers

Roots in a pot are exposed to cold on every side, so potted perennials are far less hardy than the same plants in the ground. For pots left outside all winter, choose plants rated at least two zones colder than yours. Our guide to overwintering perennials in pots explains how to protect the rest.

USDA Zones and Canadian Zones

The USDA map uses a single measure: the average coldest winter temperature. Canada’s official map, published by Natural Resources Canada, combines seven climate factors, including frost-free days, summer rainfall, snow depth and wind, and numbers its zones from 0, the harshest, to 9.

Because the two systems measure different things, a Canadian zone number does not always match the USDA number for the same town. Most plant tags and nursery catalogs, including the ranges on this site, use USDA zones. Natural Resources Canada also publishes USDA-style zones for Canadian locations, and those are the numbers to compare with plant labels.

The 2023 USDA Map Update

The USDA released its current map in November 2023, built from 1991 to 2020 weather data and 13,412 weather stations, up from 7,983 for the 2012 map. About half the country shifted into the next warmer half zone, with average winter lows 0 to 5°F warmer than before.

The USDA cautions against reading the change as a measure of climate change, since winter lows vary a lot from year to year and the new map uses better data and mapping methods. If your zone moved, the perennials that thrived in your garden before will keep thriving. A warmer number mostly opens the door to plants that were borderline.

Frequently Asked Questions

Quick answers to the questions gardeners ask most about hardiness zones.

What Do the Letters A and B Mean?

Each USDA zone is split into two halves 5°F apart. The “a” half is colder and the “b” half is warmer, so zone 5a has average winter lows of -20 to -15°F and zone 5b has lows of -15 to -10°F. Plant ranges almost always use whole zones.

Can I Grow Perennials Rated for a Warmer Zone?

Sometimes, but treat them as an experiment. Plants one zone outside their range can survive in a sheltered, well-drained spot with good snow cover or a thick winter mulch. Expect losses in a hard winter, and do not plant anything you cannot afford to replace.

Should I Use My Old Zone or the New One?

Use the current 2023 zone when you shop, since it reflects more recent weather. If your zone moved up half a step, you do not need to change anything already growing well. Be a little cautious with plants at the cold edge of their range for a few winters.

Do Hardiness Zones Matter for Annuals?

Not much. Annuals live for one season and die at frost whatever your zone, so zones mainly matter for perennials, shrubs and trees that must survive winter. For the difference, see what perennials are and how they compare with annuals.