Frost Date Probability Calculator

Frost Date Probability

Frost dates follow a normal distribution around the average frost date for your location. On the average date itself the odds are about even, and they fall away in one direction and climb in the other.

The simplified model, and which way it runs

Spring and fall are mirror images, and getting them the wrong way round is the easiest mistake on this page.

Last spring frost. Risk is high before the average date and falls after it. That is the point of the date: it is the day after which frost becomes unlikely, which is why it governs when you plant. Roughly 80% at 14 days before, 50% on the date, 30% at 14 days after, under 10% a month after.

First fall frost. Risk is low before and climbs after. It is the day after which frost becomes likely, which is why it governs when you harvest. Roughly 10% at 14 days before, 50% on the date, 75% at 14 days after, 90% a month after.

In both cases the average date is the 50% line. What changes is which side of it is the dangerous one.

USDA frost date zones (last spring frost):

First fall frost (similar inverse):

How frost forms: Light frost typically requires:

• Air temperature ≤ 32°F (0°C)
• Calm wind (less than 5 mph)
• Clear sky (radiative cooling)
• Low humidity (no cloud insulation)
• High pressure ridge

A “killing freeze” requires sustained 28°F (-2°C) for several hours.

Microclimate adjustments:

• Hilltops / ridges: 2-4°F warmer than valleys (cold air sinks)
• Valley bottoms: 4-8°F colder than hilltops (frost pockets)
• Near large water bodies: moderates extremes by 5-10°F
• Urban heat island: 3-7°F warmer than rural
• South-facing slopes: 2-4°F warmer than north-facing
• Concrete/dark surfaces: retain heat longer

The dropdown asks for this in days, not degrees, because that is the form the calculation needs. The two are related by how fast the season is moving: in spring, temperatures climb roughly half a degree Fahrenheit per day in most temperate zones, so a spot that runs 3-5°F warmer than the regional average clears its last frost about a week earlier. A frost pocket that runs 5°F colder holds its last frost about ten days longer. Those are the conversions behind the options.

Frost categories:

Crop frost tolerance:

Planting strategy:

• Direct sow tender crops AFTER your average last frost + 1-2 weeks for safety
• Transplant tender starts AFTER soil reaches 60°F+ (often 2 weeks past last frost)
• Cover with row cover or frost blanket if cold snap forecast
• Watch the local forecast. Frost can arrive 2+ weeks past the average date in a cold year

Frost protection methods:

• Row covers / frost blankets: 2-6°F protection
• Cold frame: 5-10°F protection
• Floating mulch: 2-4°F protection
• Sprinkler watering through freeze: protects to 24°F (latent heat release)
• Wind machines (orchards): 2-4°F protection
• Plastic tunnels: 8-15°F+ protection