Frost Date Probability Calculator
Estimate frost probability by date for gardening planning.
Calculate first and last frost risk windows from your location and average frost dates.
Frost Risk Assessment
Frost Date Probability
Frost dates follow a normal distribution around the average frost date for your location. The further from the average, the lower the probability.
The simplified model: Probability ≈ 50% on average date, ~10% at 14 days before, ~90% at 14 days after
USDA frost date zones (last spring frost):
| Zone | Average Last Frost |
|---|---|
| 3a-3b (cold north) | May 25 - Jun 5 |
| 4a-4b | May 10 - May 25 |
| 5a-5b | Apr 20 - May 10 |
| 6a-6b (mid-Atlantic) | Apr 5 - Apr 25 |
| 7a-7b | Mar 25 - Apr 10 |
| 8a-8b (south) | Feb 25 - Mar 15 |
| 9a-9b | Jan-Feb (rarely if ever) |
| 10a-10b | Frost-free most years |
First fall frost (similar inverse):
| Zone | Average First Frost |
|---|---|
| 3a-3b | Sep 1 - Sep 15 |
| 4a-4b | Sep 15 - Oct 1 |
| 5a-5b | Sep 30 - Oct 15 |
| 6a-6b | Oct 15 - Oct 30 |
| 7a-7b | Oct 25 - Nov 10 |
| 8a-8b | Nov 5 - Nov 25 |
| 9a-9b | Dec or rarely |
| 10a+ | Frost-free |
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
Frost categories:
| Type | Temp Range | Damage |
|---|---|---|
| Light frost | 32-29°F (0 to -2°C) | Tender plants damaged |
| Moderate freeze | 28-25°F (-2 to -4°C) | Most annual crops killed |
| Hard freeze | 24-20°F (-4 to -7°C) | Even some perennials damaged |
| Killing freeze | Below 20°F (-7°C) | Fall garden ends |
Crop frost tolerance:
| Tolerance | Crops |
|---|---|
| Very hardy (down to 20°F) | Spinach, kale, leeks, parsnips, brussels |
| Hardy (down to 28°F) | Cabbage, carrots, lettuce, peas, radish |
| Tender (damaged at 32°F) | Tomato, pepper, basil, beans, zucchini |
| Very tender (damaged at 40°F) | Watermelon, sweet potato, okra, eggplant |
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 local forecast — frost can occur 2+ weeks past average date in cold years
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