Frost Dates by ZIP Code

What frost dates are

A frost date is a statistic, not a forecast. The last spring frost date is when the last 32°F night of spring usually happens at a place; the first fall frost date is when the first one of fall usually arrives. The stretch between them is the frost-free growing season. The National Weather Service defines the growing season the same way, from the median last spring freeze to the median first fall freeze.

Gardeners use these two dates to time seed-starting, spring sowing, setting out tomatoes and planting for fall.

Where the dates come from

The dates on this site come from NOAA’s U.S. Climate Normals for 1991-2020. Normals are NOAA’s official 30-year averages and statistics for almost 15,000 US weather stations, recalculated every 10 years. Besides averages, they include probabilities for key variables, among them the dates by which the last spring freeze and the first fall freeze have occurred in a given share of years.

Each ZIP code uses the best-matching nearby NOAA station with freeze records, named with its distance and elevation difference.

Risky, typical and safe dates

A single average hides how much one spring differs from the next. NOAA’s Climate.gov notes that its average last-freeze date marks the point where the chance of a later freeze falls below 50%, and that this may feel too risky for tomato seedlings. In its Boulder, Colorado example, the 50% date is May 6 but the 10% date is not until May 18.

So the site shows three spring dates:

• Risky: frost still comes later than this in 9 years out of 10.
• Typical: the median. A later frost comes in half of years.
• Safe: a later frost comes only 1 year in 10.

Purdue’s Indiana planting calendar uses the same idea, mapping the date with a 10% chance of a later frost and noting that the 50% date falls about 2 weeks earlier. In fall the order flips: the risky date is when an early frost arrives 1 year in 10.

Use the date that matches the plant. Hardy crops such as peas, spinach and onions are planted weeks before the typical date. Tender crops are planted after it: Purdue sets out tomatoes, beans and sweet corn after the average last frost when air temperatures stay above 50°F, and cucumbers, peppers, eggplant and melons at least 2 weeks later.

32°F frost and 28°F hard freeze

Not all frosts are equal. The National Weather Service in St. Louis defines a freeze as 32°F or below, which can significantly damage nearly all seasonal plants, and a hard freeze as 28°F or below, which can kill nearly all of them. UGA Extension’s scale runs from a light freeze (29-32°F), which kills tender plants, through a moderate freeze (25-28°F) that damages most plants, to a severe freeze at 24°F and below.

The difference matters for cool-season crops, which often survive a 32°F night, so the site also shows 28°F dates. UGA notes broccoli, pumpkins, carrots and beets handle a light frost, while tomatoes, cucumbers, beans, melons and squash do not.

Why frost forms above 32°F

Official temperatures are measured in a shaded, ventilated shelter about 2 meters (roughly 6 feet) above the ground. On clear, calm nights the ground radiates heat into the sky and the coldest air settles at the surface. The National Weather Service in Omaha says the air right at the surface can be up to 10°F colder than at 4-5 feet; its Louisville office measured differences of 3-5°F between half a meter and 2 meters. That is why frost advisories go out for lows of 33-36°F, and why a breeze or clouds can prevent frost on a cold night.

Why airports and your yard differ

The official temperature for a city usually comes from its airport, the NWS in Aberdeen, South Dakota notes, and many frost records are airport stations. Your garden can differ:

• Low ground is colder. Cold, dense air drains downhill and pools in valleys, hollows and river bottoms, which frost first (NWS, UGA).
• Towns are milder. NWS Grand Forks finds that sites in town tend to see frosts and freezes a little later in the year than rural sites.

If your garden sits in a low spot or open countryside, lean toward the safe date. A sheltered city yard may run ahead of it.

How to protect plants from frost

When frost is forecast after planting, Extension services agree on the basics:

1. Water the soil a day or two before. Moist soil holds and releases more heat than dry soil (UGA).
2. Cover before sunset. Use sheets, blankets, towels or frost cloth that reach all the way to the ground, so they trap heat rising from the soil. Stakes keep heavy, wet fabric off the plants (UGA).
3. Keep plastic off the leaves. Plastic touching foliage transfers cold and can freeze the leaves, and plastic left on into a sunny morning can cook plants (UGA, UVM). Use it only over hoops or a frame.
4. Uncover in the morning once temperatures are back above freezing, so plants get light and the soil can warm again.

For longer protection, Colorado State finds floating row covers give up to about 10°F of frost protection, cold frames 3 to more than 6°F, and water-filled plant protectors around tomatoes and peppers protect down to the mid-teens. Remove row covers from squash and other insect-pollinated crops once they flower.

Frost is not the only cold that hurts warm-season crops. Colorado State warns that a week of daytime highs below 55°F can stunt tomatoes, peppers and melons, and Illinois Extension notes that basil can be damaged at about 50°F. Watch the forecast as well as the calendar.