Hardiness Zones vs. Frost Dates: Which One to Use

Your USDA hardiness zone tells you which perennials survive your winters. Your frost dates tell you when to plant vegetables and annuals. They measure different things, and cities in the same zone can have last frosts weeks apart.

Two different questions

Gardeners often treat hardiness zones and frost dates as interchangeable. They answer different questions:

• Hardiness zone: "Will this plant survive my winters?" It's for perennials, shrubs, trees, and bulbs that stay in the ground year-round.
• Frost dates: "When does my growing season start and end?" It's for timing vegetables, annual flowers, and seed starting.

What a hardiness zone measures

The USDA Plant Hardiness Zone Map divides the country by average annual extreme minimum temperature: the coldest night of the winter, averaged over many years. Each zone covers a 10°F range, split into "a" and "b" halves of 5°F each. Zone 6b, for example, means the coldest winter night typically falls between −5°F and 0°F.

The current map, released in 2023, is based on 1991–2020 weather data, the same period as the frost dates on this site. Compared with the previous map from 2012, about half of the country shifted into the next warmer half-zone.

A zone says nothing about when spring arrives, how long the season lasts, or how hot summers get. It only describes the coldest night.

Same zone, different seasons

Because zones measure only winter lows, places in the same zone can have very different growing seasons. Here's the most extreme pair among sizable US cities:

Evansville, IN and Bend, OR are both in Zone 6b. Yet Evansville's typical last spring frost is March 29, while Bend's is June 6: 69 days apart. A tomato transplanted on the same calendar date would be safe in one and frozen in the other.

Other things zones ignore:

• Summer heat. Coastal Pacific Northwest and parts of the Southeast share warm zone numbers but have completely different summers. Heat-loving crops that thrive in one may sulk in the other.
• Season length. Mountain and high-plains towns can have mild-enough winter lows for their zone but short, late-starting growing seasons.
• Rainfall and humidity, which matter as much as cold for many plants.

Which one to use when

• Buying a perennial, shrub, fruit tree, or bulb: check its hardiness zone rating against yours. Plant labels list a zone range, like "Zones 4–8."
• Planting vegetables and annual flowers: use your frost dates. A tomato doesn't care how cold January gets; it cares whether May has a frost.
• Starting seeds indoors: frost dates. See when to start seeds indoors.
• Overwintering crops like garlic or fall spinach: both. The fall frost date tells you when to plant, and your zone tells you how hard the winter will be.

Find both for your location

Every FrostCal city page shows its USDA zone alongside its frost dates. You can also browse typical frost dates by zone, which helps for a rough idea, though your own city's dates will always be more precise than a zone average. Look up your ZIP code to see both.