What a hardiness zone tells you about a tree, and what it leaves out

A USDA hardiness zone tells you how cold the coldest night of the year gets where you live, averaged over 30 winters, and so whether a species can survive your winters. It says nothing about summer heat, wet or dry ground, frost after the buds open, the warm and cold spots in your own yard, or where a tree’s seed was collected. On the 2023 map each zone is a band of 10 degrees Fahrenheit, split into halves of 5 degrees.

What the zone number measures

The map starts from one number a year: the lowest temperature at each place. The 2023 edition averages those lows over 1991 to 2020. Zone 1 is the coldest and zone 13 the warmest, and each splits into a colder half, a, and a warmer half, b. An average is not a floor:

“Hardiness zones in this map are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future.” (USDA Agricultural Research Service, How to Use the Maps)

A plant at the cold limit of its rating can thrive for several years and then die in a cold snap lasting a day or two. The 2023 map runs about a quarter zone, 2.5 degrees, warmer than the 2012 map across much of the country, because it averages 1991 to 2020 instead of 1976 to 2005.

By ZIP code, the eastern states run from zone 3 in Maine’s north woods to zone 11 in the Florida Keys:

What the zone predicts

Every woody species has a lowest temperature it survives, and the zone tests your winters against it. Each plant has a range, such as zones 5 to 9 for pawpaw, and the first number is the one the map checks. The second marks the warm end, where summer heat and too little winter chill take over, and the map measures neither. Mountain camellia, which is sensitive to heat, stops at zone 8.

Holly and some magnolias can come through several mild winters near Chicago and then be injured by a typical one. American holly is hardy well into zone 5 with shelter from winter wind. The site check grades a species against your zone this way:

• One zone colder than its range: a struggle, with tip dieback in hard winters.
• One zone warmer: a struggle with summer heat and drought.
• Two zones out either way: a failure.

Hardiness is not native range, one reason the range maps are wrong about where a tree will grow. Bald cypress is hardy far north of its native range.

What the zone leaves out

Wind, soil moisture, humidity, snow and winter sun are among the other factors in survival. Summer heat and moisture balance are particularly important.

Summer heat

The American Horticultural Society’s Plant Heat Zone Map of 1997 sorts the country into 12 zones by the average number of days a year above 86 degrees. That is the point at which plants begin to suffer heat damage. Heat kills slowly: a frozen plant dies quickly, while a heat-stressed one can decline for years. Few nursery labels give a heat zone.

Wet and dry ground

The zone cannot tell a swamp from a sand ridge. Sweetbay magnolia, rated for zones 5 to 10, thrives where lawns puddle, and mountain laurel, rated for 4 to 9, fails in wet soil. Dry ground matters too. A plant hardy in your zone can be injured if a dry late autumn sends it into dormancy short of water.

Late spring frost

The map records how deep the winter goes, not how the temperature swings. Injury comes more often from the swings than from the cold:

“Contrary to popular belief, plant damage is not generally caused by an unusually cold winter. Low-temperature injury is more often associated with extreme temperature fluctuation than with prolonged cold weather.” (The Morton Arboretum, Winter injury to trees and shrubs)

In April 2026, Trenton-Mercer Airport in Ewing Township, New Jersey, a zone 7 town, set daily records of 88 degrees on the 15th and 27 degrees on the 21st. That freeze was among the most widespread disasters on record for the state’s fruit crops. The shagbark hickory above, hardy to zone 4, shows the damage the next day.

Early flowers take the same risk. Oglethorpe oak flowers in early April, and late frost often damages its female flowers, so acorn crops are usually small. White fringetree blooms late enough to dodge frost.

Winter chill

Chilling is counted in the hours between 32 and 45 degrees, and a bud stays dormant until its chilling requirement is met. A plant short of chill can break bud unevenly, and the zone does not measure chill.

Warm and cold spots, from a city to a yard

The map is drawn in cells about half a mile across, and microclimates can be too small to show on it. Cities hold heat in their concrete and blacktop, and mountains run colder than the lowlands around them. By the 2023 ZIP code listing, downtown Washington is zone 8a, while Manassas, Virginia, about 30 miles southwest, is 7a.

Inside one yard, the ground in front of a south-facing wall is a warm pocket, and a low spot where cold air pools first is a cold one. A species at its cold limit belongs in the warm pocket, and an early-flowering one belongs out of the low spot.

Where the seed came from

A zone rating covers a whole species, and a species is many local populations. Black walnut, rated for zones 4 to 9, grows where minus 45 degrees has been recorded, but few of its races can take that cold. Its stands differ three to five times more from one another than trees within a stand do in leaf-out and winter dieback. Seed from the south grows later into the season, one day longer for every 24 miles south it came from.

A seed transfer guide for the species finds cold tolerance rising with the latitude of the seed’s source. It counts a move of 200 miles north as safe, and at the northern edge of the range it recommends local seed. The spring schedule is inherited too, one reason two oaks of one species can break bud two weeks apart. Blue Whale Trees grows its cherrybark oak from one parent tree at the northern edge of the species, chosen in part for its hardiness there.

Using your zone with our tools

Use the zone with our tools in this order:

1. Find your half zone with the ZIP code lookup, which reads the 2023 map’s ZIP code listing.
2. Check the first number of a plant’s zone range. At or below your zone, the species takes your winters.
3. Check the second number. If your zone is higher, expect summers hotter than the species likes.
4. Describe the spot in the site check: zone, sun, soil moisture and the distances to a wall and a power line.
5. At a species’ cold edge, choose the warm pocket and northern seed.
6. Choose among the species that pass with the tree finder, which filters by zone, size, sun and soil moisture.

Common questions

What does zone 7a mean?

On the 2023 USDA map, zone 7a means the coldest night of the year averaged 0 to 5 degrees Fahrenheit from 1991 to 2020, and 7b means 5 to 10 degrees.

Can I grow a tree rated one zone warmer than mine?

Our site check calls that a struggle rather than a failure: expect tip dieback in hard winters, and plant it where a building or woods breaks the winter wind.

Did my zone change with the 2023 map?

Perhaps: the 2023 map runs about a quarter zone warmer than the 2012 map across much of the country. Keep what already thrives, since it will most likely keep thriving.

Why did a tree hardy in my zone die over winter?

Usually because of a swing in temperature rather than the depth of the cold: a hard freeze after a mild fall, a thaw followed by a sharp drop, or a dry autumn before dormancy.

Sources

• How to Use the Maps: USDA Agricultural Research Service: what a zone measures, the 1991 to 2020 averages and half zones, losses to a rare cold snap at the edge of a rating, dry soil in late autumn, the other factors and microclimates, and keeping plants after a zone change.
• Map Creation: USDA Agricultural Research Service: the half-mile grid, the quarter-zone shift from the 2012 map and its causes, and warmer cities and colder mountains.
• 2023 USDA Plant Hardiness Zone GIS Datasets: PRISM Climate Group, Oregon State University: the zone and half-zone bands and the ZIP code listing behind the table and the Washington and Manassas zones.
• Know Your Heat Zone: UF/IFAS Extension Duval County: the 1997 heat zone map, its 12 zones by days above 86 degrees, heat damage that lingers for years, and labels without a heat zone.
• Winter injury to trees and shrubs: The Morton Arboretum: temperature swings rather than deep cold behind winter injury, and holly and some magnolias hurt by a typical winter after several mild ones.
• April 2026 and 2025-26 Snow Season Overview: New Jersey State Climate Office, Rutgers University (Dave Robinson): the record 88 and 27 degrees at Trenton-Mercer Airport in April 2026 and the damage the freeze did.
• Understanding dormancy and chilling hours in perennial fruit crops: University of Minnesota Extension (Madeline Wimmer): chilling hours between 32 and 45 degrees, dormancy held until the chilling is met, and uneven budbreak without it.
• Black Walnut: USDA Forest Service, Silvics of North America: few races that take minus 45 degrees, variation among stands in leaf-out and winter dieback, and longer growth from southern seed.
• Black Walnut: Guidance for Seed Transfer Within the Eastern United States: USDA Forest Service, Tree Planters’ Notes (Pike and Woeste, 2022): cold tolerance by latitude of origin, a safe move of 200 miles north, and local seed at the northern edge.