Planting Zones: A US Guide

What USDA hardiness zones actually measure

Your USDA hardiness zone is a single number that tells you the average coldest temperature your location experiences each winter. Nothing more, nothing less.

The USDA Plant Hardiness Zone Map divides the US into 13 primary zones. Each zone represents a 10°F range of average annual minimum winter temperature. Zone 1 is the coldest, with average lows below -60°F in parts of Alaska. Zone 13 is the warmest, with lows above 60°F in parts of Hawaii and Puerto Rico.

Every zone is also split into an “a” and “b” half-zone, each representing a 5°F range. Zone 6a averages -10°F to -5°F. Zone 6b averages -5°F to 0°F.

The zone tells you whether a perennial plant, shrub, or tree will survive your winters. A lilac rated for Zones 3-7 will survive a Zone 6 winter. Plant it in Zone 8 and it may not get cold enough dormancy to bloom properly.

The 2023 USDA map update and what changed

The USDA released a major update to the Hardiness Zone Map in November 2023. The new map uses temperature data collected from 1991 through 2020 at 13,412 weather stations across the country.

The previous map was based on data from 1976 to 2005. The newer, warmer baseline reflects rising winter temperatures over recent decades. About half of the country shifted half a zone warmer compared to the 1990 map. Many gardeners in the Mid-Atlantic and Southeast moved from Zone 6 to Zone 7 or from Zone 7 to Zone 7b.

This matters practically. If you purchased a plant labeled “Zone 7 hardy” in 2015 based on the old map and you lived in former Zone 6b, that plant was borderline. Under the 2023 map, the same location may now be mapped as Zone 7a, making that plant a solid fit.

Check your current zone directly at the USDA website (planthardiness.ars.usda.gov) before buying any perennial, tree, or shrub.

Zone breakdown for every region of the US

About 80% of American gardeners fall between Zones 5 and 9. Most plants sold at US nurseries carry zone ratings designed for this range.

What zones do not tell you

Your USDA zone is a starting point, not the whole picture. Several factors matter just as much or more for plant survival:

Frost dates. A Zone 6 gardener in Nashville has their last spring frost around April 5 on average. A Zone 6 gardener in Denver averages May 7. Both are Zone 6, but the Denver gardener plants tomatoes five weeks later. Use the Old Farmer’s Almanac frost date tool (almanac.com) to look up your specific dates.

Microclimate. A south-facing brick wall in Zone 6 can create a Zone 7 microclimate. A low-lying frost pocket in Zone 7 can experience temperatures cold enough to kill Zone 5 plants. Walk your yard and notice where frost lingers on cold mornings — that spot is your hardest zone.

Summer heat. Plants rated for Zone 9 sometimes fail in Zone 8 parts of the Deep South not because winters are too cold but because summers are too hot and humid. Peonies and delphinium need cold winters to reset their growth cycle and struggle in the Southeast regardless of zone.

Soil drainage. A plant that survives -10°F in well-drained soil may die at 0°F in wet clay. Wet soil freezes solid and expands, rupturing roots. Good drainage often matters more than one zone’s difference in temperature.

Rainfall and irrigation. Desert Zone 9 plants thrive in Phoenix but may rot in humid Zone 9 Houston without sharp drainage. The heat-stress zone system from the American Horticultural Society addresses summer heat, but most nursery tags use only USDA winter hardiness zones.

How to push your zone with microclimates

Experienced gardeners regularly grow plants one half to one full zone warmer than their official rating by using microclimates and protective techniques.

South-facing walls. Brick and stone walls absorb solar heat during the day and release it at night. Planting marginally-hardy shrubs like gardenias or crepe myrtles against a south-facing wall adds 5-10°F of protection on cold nights.

Deep mulching. Applying 4-6 inches of wood chip mulch over the root zone of borderline-hardy perennials insulates the soil and prevents repeated freeze-thaw cycles that damage roots. This technique reliably protects plants rated one half-zone warmer than your zone in most winters.

Planting on a slope. Cold air sinks. A garden on a gentle south-facing slope stays warmer than a flat yard or a low spot that collects cold air. Even a 3% slope makes a measurable difference on nights when temperatures dip near the critical threshold.

Container protection. Container plants are more vulnerable than in-ground plants because their roots are exposed to ambient air temperature rather than insulated by soil. Move containers into an unheated garage or basement when temperatures drop below your plant’s rated minimum.

Using zones when buying plants

Always check the zone rating on nursery tags before you purchase any plant you expect to return year after year. Here is how to read them:

• Zone 3-9 means the plant is very cold-hardy and widely adaptable. It will survive winters from Minnesota to the Gulf Coast.
• Zone 7-11 means the plant cannot handle hard freezes. It is suitable for the South, Pacific Coast, and warm states only.
• Zones 5-9 is the most common range in US nurseries. These plants cover the majority of the country.

If a tag says only “Zones 5 and above,” that means the plant has no upper zone limit — it tolerates heat and humidity as long as winters are cold enough.

When buying online, pay extra attention to zone ratings. A beautiful photo of a bougainvillea may not disclose that it is only hardy to Zone 9. If you are Zone 6, it is an annual or a houseplant in your climate.

Zones for vegetables and annuals

Vegetables and annual flowers are not rated by USDA hardiness zones. They complete their life cycle in one season and die at the first hard frost regardless of zone. What matters for vegetables is your frost dates and your season length.

Cool-season vegetables (lettuce, spinach, broccoli, peas) tolerate light frost down to about 28°F. In Zone 7 and warmer, you can grow these from September through November and again from February through April.

Warm-season vegetables (tomatoes, peppers, cucumbers, squash) die at any frost. Plant them outdoors only after your last spring frost date. In Zone 5, that is typically late May. In Zone 8, it may be late March.

Season length by zone:

• Zone 4: About 90-110 frost-free days. Stick to fast-maturing varieties.
• Zone 6: 150-170 days. Good range for most vegetables.
• Zone 8: 200-220 days. Two full vegetable seasons possible.

For more on starting your vegetable plot using zone and frost date information, read How to Start a Vegetable Garden. Container growers in cold zones can extend the season — Container Gardening for Small Spaces covers the details.

Perennials and shrubs by zone: what to expect

If you are building a perennial border, zone ratings are your primary guide for plant selection. Here are reliable performers by zone:

Zone 4-5 (northern states):

• Coneflower (Echinacea purpurea) — Zones 3-8
• Black-eyed Susan — Zones 3-9
• Karl Foerster grass — Zones 4-9
• Siberian iris — Zones 3-9

Zone 6-7 (middle states):

• Salvia nemorosa — Zones 4-8
• Catmint — Zones 3-8
• Rudbeckia — Zones 3-9
• Ornamental grasses — most Zones 5-9

Zone 8-9 (southern and western states):

• Lantana — Zone 8-11 (perennial in the South)
• Gardenias — Zones 7-11
• Society garlic — Zones 7-10
• Agapanthus — Zones 7-11

Best Spring Perennials covers reliable zone-by-zone choices for early-season color. For gardeners working on improving soil before planting, How to Start Composting can help build the humus-rich beds that help borderline-hardy plants survive winter.

Checking local conditions beyond the zone map

Your official zone is an average. It does not tell you about the worst winter your area can deliver.

Nashville, Tennessee is Zone 6b, averaging winter lows around 0°F to -5°F. But in February 2021, Nashville hit -8°F — solidly in Zone 5a territory. Gardenias, crepe myrtles, and fatsia that had survived a decade of mild winters died to the ground.

To protect yourself, look at the historic record for your city, not just the average. The NOAA Climate Data Online tool (ncdc.noaa.gov) lets you pull the lowest recorded temperature for your area going back 100 years. Build your permanent plant choices around the coldest temperature your site has ever recorded, not the average.

For pest problems that arrive with spring warmth regardless of zone — like beetles or aphids — How to Control Japanese Beetles is a good reference for US gardeners in Zones 5-9 where this pest is most active.

Pollinator gardens depend on native plants matched to your region. Attract Pollinators to Your Garden covers zone-appropriate native choices for bees and butterflies across the country.

Zone rating changes you need to know about

Three specific US regions saw significant zone changes in the 2023 USDA update that are worth knowing:

1. Mid-Atlantic coast: Much of coastal Virginia, Delaware, and southern New Jersey moved from Zone 7 to Zone 7b or even Zone 8a. Crape myrtles and camellias are now more reliably rated for this region.

2. Pacific Northwest: Parts of coastal Oregon and Washington shifted from Zone 8a to Zone 8b. More tender Mediterranean perennials can now survive winters outdoors in Portland and Seattle.

3. Great Lakes: Some areas of southern Michigan and Ohio shifted from Zone 5b to Zone 6a, expanding the range of plants gardeners in these areas can reliably grow.

If you bought a home in one of these regions in the last decade and relied on the old zone map, it is worth re-checking your zone and reassessing your plant choices accordingly.