Every planting decision you make depends on two dates: the last spring frost and the first fall frost. The window between them is your growing season. Everything else, from when to start seeds indoors to when to harvest the last tomato, works backward from these two numbers.
What frost dates actually mean
A frost date is a statistical average, not a guarantee. When someone says "the last frost in Zone 6b is April 15," they mean that historically, there is a 50% chance of frost occurring after that date. Some years it comes earlier, some years later.
NOAA calculates frost dates from decades of weather station data. They publish several probability thresholds:
- 10% probability — frost is very unlikely after this date (safe for tender crops)
- 50% probability — the "average" last frost (most commonly cited)
- 90% probability — frost is still quite possible (safe only for cold-hardy crops)
Most planting guides use the 50% date. If you are risk-averse or growing expensive transplants, use the 10% date instead.
Last spring frost
This is the date after which freezing temperatures (32°F / 0°C) are unlikely. It controls when you can:
- Transplant warm-season crops outdoors — tomatoes, peppers, squash, and basil all die at frost. Wait until after your last frost date, then add a safety margin of 1-2 weeks.
- Direct-sow tender seeds — beans, cucumbers, and corn germinate poorly in cold soil. The soil needs to warm up after the last frost, which typically takes an additional 1-2 weeks.
- Start seeds indoors — count backward from your transplant date. If tomatoes need 6-8 weeks indoors and you transplant 2 weeks after last frost, start seeds 8-10 weeks before your last frost date.
First fall frost
This is the date after which freezing temperatures become likely. It determines:
- When to harvest — anything frost-sensitive needs to be picked before this date. Tomatoes, peppers, and basil will blacken overnight.
- Whether a fall planting is worth it — if you want to grow lettuce in autumn, count backward from the first frost to see if there are enough days left in the season.
- When to plant garlic — garlic goes in 4-6 weeks before the first frost so it can establish roots before the ground freezes.
Growing season length
The number of days between your last spring frost and first fall frost is your growing season. This single number tells you which crops are realistic:
- Under 90 days — stick to fast crops: radish, lettuce, peas, bush beans. Start everything else indoors well ahead of time.
- 90-120 days — most standard vegetables work if you start warm-season crops indoors. Choose "early" or "short season" tomato varieties.
- 120-150 days — the sweet spot. Most crops succeed with normal timing.
- Over 150 days — you can grow long-season crops like sweet potatoes, watermelon, and winter squash directly outdoors.
How to find your frost dates
Your frost dates depend on your specific location, not just your USDA hardiness zone. Two gardens in the same zone can have frost dates weeks apart due to elevation, proximity to water, or urban heat islands.
The most reliable sources:
- Vernalo — enter your location in the app and your frost dates are calculated automatically from NOAA data covering 9,200+ US locations and 1,000+ international cities.
- Your local extension office — county-specific data, often with microclimate notes.
- NOAA climate normals — the raw data, published every 10 years.
- Your own records — after a few seasons, your garden journal becomes the most accurate source for your specific plot.
Frost tolerance by crop
Not all plants respond to frost the same way. Understanding tolerance categories helps you push the season in both directions:
- Hardy (survives hard frost, below 28°F / -2°C) — kale, spinach, garlic, onion, peas. These can go out 4-6 weeks before the last frost.
- Semi-hardy (survives light frost, 28-32°F / -2 to 0°C) — lettuce, carrot, beet, chard, broccoli. Plant 2-4 weeks before last frost.
- Tender (damaged at 32°F / 0°C) — tomato, pepper, cucumber, basil, squash. Wait until after last frost, ideally with a 1-2 week buffer.
- Very tender (damaged below 40°F / 4°C) — sweet potato, okra, watermelon. Need warm soil, not just frost-free air.
Extending your season
Frost dates are averages, not walls. Several techniques let you plant earlier in spring or harvest later in fall:
- Row covers — lightweight fabric draped over crops adds 4-8°F of protection. Enough to survive a light frost.
- Cold frames — a bottomless box with a glass or plastic lid. Creates a microclimate 10-15°F warmer than ambient.
- Wall o' Water — water-filled plastic teepees around individual plants. The thermal mass of the water buffers temperature swings.
- Mulch — a thick layer of straw or leaves insulates soil, keeping root zones warmer longer into fall.
These tools effectively shift your frost dates by 2-4 weeks in either direction, which can mean the difference between a 90-day and a 120-day growing season.