Last updated: October 2026
When should I start seeds indoors?
Short answer: count backwards from your last spring frost date — 10–12 weeks for onions, leeks and celery, 8–10 weeks for peppers and eggplant, 6–8 weeks for tomatoes, broccoli, cabbage and basil, and 4–6 weeks for lettuce and kale. Spinach and summer squash are direct-sown outdoors. The calculator above does the counting for you.
How to use your last frost date
Your last spring frost date is the average date of the last freezing night (32°F / 0°C) in your area — and nearly every seed packet's timing is anchored to it ("start indoors 6–8 weeks before last frost"). Here's the process:
- Find your date. Search "[your town] last frost date" — the Old Farmer's Almanac planting calendar publishes frost-based dates for most US ZIP codes.
- Enter it above and calculate — you get exact indoor start dates and outdoor transplant dates for 15 vegetables, based on the midpoint of the standard range for each crop.
- Start warm-season crops later. Tomatoes, peppers, eggplant, cucumbers, squash and basil go out only after the last frost, when soil and nights are warm.
- Start cool-season crops early. Broccoli, cabbage, kale, lettuce, spinach, onions and leeks tolerate — and prefer — cool weather, so they go out before the last frost.
The dates in the calculator use the midpoint of each range (e.g. "6–8 weeks" computes as 7 weeks). The full range is shown in the Notes column so you can fine-tune for your microclimate.
Hardening off: the step most beginners skip
Indoor seedlings grow up sheltered — no wind, no direct sun, constant temperature. Putting them straight into the garden causes transplant shock: wilted, sun-scorched seedlings that stall for weeks. Hardening off prevents it:
- 7–10 days before your transplant date, set seedlings outside in shade for 2–3 hours.
- Each day, add an hour and move them into progressively more sun.
- Leave them out overnight only in the final 2 days, and only if nights stay above 50°F (10°C).
- Transplant on a cloudy day or in the evening to ease the transition.
Frequently asked questions
What is my last frost date?
Your last spring frost date is the average calendar date of the last freezing night (32°F / 0°C) in your area. Almost every seed packet's timing — "start indoors 6–8 weeks before last frost" — is anchored to it. Search your town name plus "last frost date" to find yours.
What if I don't know my last frost date?
Search "[your town] last frost date" — gardening sites and almanacs publish it for most US ZIP codes and many cities worldwide. As a safe fallback, use the 30% probability date (the date after which frost has only a 30% chance) rather than the 50% average.
What happens if I start seeds too late?
Late-started warm-season crops like tomatoes and peppers may not ripen before the first fall frost, and cool-season crops planted late can bolt in summer heat. If you're behind, buy transplants from a garden center instead of starting seed.
What does hardening off mean?
Hardening off is a 7–10 day acclimation period before transplanting: set seedlings outside in shade for a few hours daily, gradually increasing sun and time. Skipping it causes transplant shock — wilted, sunburned seedlings that stall for weeks.
Is my frost date data private?
Completely. Every calculation runs locally in your browser with JavaScript — your frost date is never sent to a server, stored, or tracked.