Your Frost Dates Shape Everything You Can Grow

Every location in the continental US has two critical dates: the average last spring frost and the average first fall frost. The gap between them — your frost-free growing season — determines which crops can complete their lifecycle outdoors.

A tomato needs 70-90 frost-free days to fruit. Sweet corn needs 60-100. Winter squash needs 80-120. If your frost-free season is only 100 days, long-season varieties won't mature before the first fall freeze kills the plant. Short-season varieties — bred specifically for northern climates — can produce within that window.

NOAA's RCC-ACIS publishes a gridded daily temperature record built from 30 years of weather station observations. Frost dates read from it are statistical probabilities, not fixed calendar dates.

The frost-free growing season varies dramatically across the US, driven by latitude, elevation, proximity to water, and regional climate patterns:

90–130 days

Short Season

Northern tier states (MN, WI, ME, MT) and high elevations. Fast-maturing varieties are essential. A cold frame can add 2-4 weeks on each end (Iowa State and WVU Extension figures — see the season-extension guide).

150–200 days

Moderate Season

Mid-Atlantic, Midwest, and Pacific Northwest. Most standard vegetable varieties mature comfortably. Succession planting allows multiple harvests.

240–365 days

Long / Year-Round

Deep South, Gulf Coast, Southern California, and Florida. Year-round growing possible. Heat tolerance becomes the constraint instead of frost.

Pick your state for its frost window, then your county for the local 28°F hard-freeze dates — closer to your own ground than a statewide range, and a step toward the frost date at your exact address.

Most gardening websites provide frost dates by ZIP code. The problem: ZIP codes are mail delivery routes, not climate zones. A single ZIP code can span valleys and hilltops with frost date differences of 2-3 weeks.

• Elevation: Cold air pools in valleys. A parcel at the bottom of a slope can frost 2-3 weeks later in spring and 2-3 weeks earlier in fall than one at the top — even within the same ZIP code.
• Urban heat island: Cities retain heat from pavement and buildings. Urban parcels may have frost-free seasons 1-2 weeks longer than rural parcels at the same latitude and elevation.
• Water proximity: Large water bodies moderate temperature extremes. Parcels near the Great Lakes, ocean coastlines, or large rivers often have delayed spring warming but also delayed fall frost — shifting the growing window later in both directions.

NOAA's gridded daily record captures these spatial gradients on a 4 km grid, providing frost estimates that account for the local landscape rather than averaging across a ZIP code.

Pro Tip

A cold frame can buy 2-4 weeks at each end of the season — the Iowa State and WVU Extension figures behind our season-extension guide. If you're 10 days short of growing a longer-season crop, season extension may be all you need.

Growable Ground reads frost dates from the NOAA RCC-ACIS gridded daily record (1991–2020) at your parcel's exact coordinates. These dates feed into multiple scoring dimensions:

• Frost-free season length: The number of days between last spring frost (50% probability) and first fall frost. Crops that need more days than your season provides score lower.
• Planting calendar: Your last spring frost date anchors the planting schedule. "Start indoors 6 weeks before last frost" becomes a specific calendar date for your location.
• Growing degree day accumulation: Frost dates define the start and end of the heat accumulation window. GDD (from PRISM climate data) measures whether enough warmth accumulates during your frost-free period for crops to mature.
• Frost pocket detection: Growable Ground uses USGS elevation data to identify parcels at the bottom of terrain depressions — frost pockets where cold air collects. These locations may experience frost dates outside the regional average.