Before buying a plant, before digging a hole, before ordering seeds — check the zone. The USDA Plant Hardiness Zone Map is a free, publicly available tool that tells gardeners what plants are most likely to survive winter in their location. It is not a guarantee, and it is not the only factor that matters, but it is the foundation on which every planting decision should rest.
This guide explains how the zone system works, what it measures, what it does not measure, and how to use it alongside other climate data to make smarter choices in the garden.
What the USDA Hardiness Zone Map Measures
The map is based on one number: the average annual extreme minimum temperature. The USDA collects temperature data from weather stations across the country and calculates the average of the coldest temperature recorded each year over a 30-year period. That average minimum becomes the basis for zone assignment.
The full map runs from Zone 1 (average annual minimum of -60°F to -50°F, found in interior Alaska) to Zone 13 (60°F to 70°F, found in Puerto Rico and the southernmost tip of Florida). Most of the contiguous United States falls in Zones 3 through 10.
Each zone spans 10°F, and each zone is further divided into an "a" half (the colder half) and a "b" half (the warmer half), each spanning 5°F. Zone 6b, for example, has average annual minimums between -5°F and 0°F.
What the Zone Map Does Not Measure
The hardiness zone is a cold-tolerance indicator only. It says nothing about:
- Summer heat. A Zone 7 garden in the Pacific Northwest and a Zone 7 garden in the mid-Atlantic have the same winter lows, but entirely different summers. The American Horticultural Society's Heat Zone Map is a useful companion tool for summer-sensitive plants.
- Rainfall and humidity. Plants described as suited to Zone 8 may need supplemental irrigation in dry climates and may struggle with fungal disease in humid ones.
- Soil type. Drainage, pH, and organic content all affect whether a plant survives and thrives. A perennial rated for Zone 5 may die in a waterlogged Zone 6 garden.
- Microclimates. A south-facing slope, a sheltered courtyard, or proximity to a large body of water can shift the effective zone by half a zone or more.
How to Find Your Zone
The USDA maintains an interactive zone map at planthardiness.ars.usda.gov. Enter a ZIP code and the map returns the zone for that location. Most seed packets, plant tags, and nursery catalogs list hardiness zones directly.
The map was most recently updated in 2023, incorporating data from 1991–2020. Many locations shifted half a zone warmer compared to the previous 1976–2005 dataset, reflecting observed warming trends.
Zone-by-Zone Overview
Zones 1–3: Extreme Cold
Zones 1 through 3 cover Alaska, northern Canada, and the highest elevations of the Rocky Mountains. Average annual minimums drop to -40°F and below. The growing season may be as short as 60–90 days. Cold-hardy annuals, short-season vegetables, and a small selection of native perennials and woody plants are the primary options. Siberian iris, cold-hardy willows, and vegetables like kale, radishes, and short-season potatoes are reliable performers.
Zones 4–5: Cold Climates
Much of the Upper Midwest, New England, and mountain regions of the West fall into Zones 4 and 5. Average minimums range from -30°F to -10°F. The growing season runs roughly 120–160 days. These zones support a wide range of vegetables, ornamental perennials, and fruit trees — including apples, pears, and cold-hardy cherries. Gardeners in Zone 4 and 5 should time planting carefully around last frost dates. For more detail, see the guide to seasonal planting calendars.
Zones 6–7: Moderate Climates
Zones 6 and 7 cover a broad swath of the country including the mid-Atlantic, the Midwest transition zone, the Southeast piedmont, and parts of the Pacific Northwest. Average minimums range from -10°F to 10°F. These zones offer the widest plant selection of any region in the country. Most garden vegetables, herbs, perennial flowers, and fruiting shrubs perform well. The challenge in Zone 7 is often summer heat rather than winter cold.
Zones 8–9: Mild Winter Regions
The Gulf Coast, coastal California, the Pacific Northwest lowlands, and the Southwest fall primarily in Zones 8 and 9. Average minimums range from 10°F to 30°F. Winters are mild enough to grow cool-season crops year-round. Summer heat becomes the limiting factor for many plants. Gardeners in these zones can grow citrus outdoors with some protection in Zone 8, and reliably in Zone 9.
Zones 10–13: Frost-Free and Tropical
South Florida, Hawaii, and parts of Southern California and the Rio Grande Valley occupy Zones 10 through 13. Frost is rare or nonexistent. The growing calendar is inverted compared to northern zones — cool-season crops like lettuce and broccoli are planted in fall and winter, while summer heat forces a period of reduced planting. Tropical fruits, palms, and tender perennials grown elsewhere as annuals thrive here year-round.
Using Zone Information Alongside Frost Dates
Hardiness zones describe winter lows, but planting timing is governed by frost dates — the average date of the last spring frost and the first fall frost. These dates vary within zones and between years. A Zone 6b garden in a valley may experience frost weeks later in spring than a Zone 6b garden on a hillside nearby.
Local extension service frost date data, combined with zone information, gives the most accurate planting calendar. For vegetable gardening, frost dates are often more useful than zone ratings because most vegetables are annuals grown for one season only. The companion planting guide includes additional timing notes for interplanted crops.
Interpreting Plant Labels
When a plant label says "Hardy to Zone 5," it means the plant can survive average annual minimum temperatures of -20°F to -10°F. Plants labeled with a zone range — "Zones 4–9," for example — indicate both a cold tolerance (the lower number) and a heat tolerance (the upper number, beyond which the plant may not thrive).
Perennials, shrubs, and trees are rated by hardiness zone. Annual flowers and vegetables are not zone-rated because they complete their life cycle in a single season and do not need to survive winter.
Microclimates and Zone Pushing
Experienced gardeners learn to exploit microclimates — small areas where conditions differ from the surrounding landscape. Common microclimates include:
- South-facing walls that absorb and radiate heat, often one zone warmer than surrounding areas
- Low spots where cold air pools on still nights, creating frost pockets that may be colder than the surrounding zone
- Urban heat islands that push city gardens half a zone warmer than surrounding suburban areas
- Proximity to large lakes or ocean coastlines that moderate both summer and winter extremes
Zone pushing — growing plants rated one zone colder than your location, with additional protection — is a common practice. Heavy mulching, burlap wraps, and frost cloth can help marginally tender plants survive. Success depends on the severity of the winter, the plant's overall health, and site conditions.
Frequently Asked Questions
How often does the USDA update the hardiness zone map?
The map has been updated periodically since its introduction in 1960. The most recent update was published in 2023, using temperature data from 1991 to 2020. Previous updates occurred in 1990 and 2012.
Can a garden span two hardiness zones?
Yes. Large properties, especially those with significant elevation changes or microclimates, can span zone boundaries. It is useful to identify both zones and choose plants accordingly — or use the colder zone as the conservative benchmark.
Do hardiness zones account for snow cover?
No. The zone map reflects air temperature data only. In practice, snow cover acts as insulation for plant roots and soil, allowing some plants to survive temperatures colder than their zone rating suggests. Gardeners in snowy climates sometimes successfully grow plants one zone colder than labeled.
Is the hardiness zone the same as the growing zone?
The terms are often used interchangeably, but technically, "hardiness zone" refers specifically to the USDA map. "Growing zone" is an informal term that may refer to the same map or to a broader set of climate indicators. When in doubt, verify which system is being referenced.
Why did my zone change on the new map?
The 2023 update showed approximately half of the country shifting half a zone warmer, reflecting changes in observed minimum temperatures over the 30-year data period. This does not necessarily mean local climate has changed dramatically — it may reflect the inclusion of more weather stations or changes in data averaging methodology.