Concept

Frost Date

Also known as: last frost, first frost, average frost date, frost-free date

The average date of the last spring frost and the first fall frost in a given location — the two dates that bracket the conventional outdoor growing season for frost-tender crops. Published as 50%-probability statistical averages by national meteorological services, refined locally by experienced gardeners observing their own microclimates. Knowing your frost dates is the single piece of bioregional information that organizes the rest of the gardening calendar.

The two dates a gardener learns first about their location:

Last spring frost (LSF) — the average date after which frost-tender crops can go outside without protection
First fall frost (FFF) — the average date when those same crops are likely to be killed

The interval between LSF and FFF is the frost-free growing season — the conventional outdoor window for frost-tender annuals: tomatoes, peppers, eggplant, cucumbers, squash, beans, melons, basil.

What “average” actually means

Published frost dates are 50%-probability statistical averages. Half the years, frost will happen later in spring or earlier in fall than the published date. Experienced gardeners typically wait 1–2 weeks past LSF for the heat-loving crops, and harvest or protect 1–2 weeks before FFF.

Some sources publish 10% / 50% / 90% probability dates: “10% chance of frost after May 5, 50% after April 20, 90% after April 5.” This is more useful than a single number.

Where to find your dates

United States: NOAA, the National Gardening Association, the Old Farmer’s Almanac, and most state Extension services publish frost date tables by ZIP code or weather station.
United Kingdom: Met Office, RHS regional guides.
Canada: Environment Canada climate normals.
Elsewhere: national meteorological agencies; most publish equivalent data.

The published date is for the nearest weather station, which may be at an airport on flat ground a long way from your specific microclimate. Local refinement matters.

Microclimate corrections

Your specific site may differ from the published date by 1–4 weeks in either direction:

South-facing slopes warm earlier in spring, hold later in fall
Frost pockets (low spots, north sides, near cold-air drainage paths) frost earlier in fall and later in spring
Urban heat islands push both dates outward by 1–3 weeks
Proximity to large water bodies moderates both ends of the season
Elevation — every 250 feet of elevation gain shifts dates roughly 1 week in the wrong direction

Two gardens half a mile apart can have frost-free seasons differing by three weeks. The published date is a starting point.

The calendar built on these dates

Once LSF is known, the rest of the spring calendar derives:

8 weeks before LSF — start tomatoes, peppers, eggplants indoors
6 weeks before LSF — start cucurbits (cucumbers, squash, melons) indoors
4 weeks before LSF — direct-sow cold-hardy greens, peas, radishes
2 weeks before LSF — start hardening off frost-tender seedlings
LSF + 1 week — transplant frost-tender crops

The fall calendar inverts: count backwards from FFF.

Sources

NOAA Climate Data Online — authoritative U.S. dataset
The Old Farmer’s Almanac frost-date calculator
Eliot Coleman, The New Organic Grower — chapter on planning around frost dates

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Hardiness Zone hardiness zones describe winter extremes; frost dates describe season length; both are needed for full climate context

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