When should I start seeds indoors?
Count back from your average last spring frost. Every crop has a customary lead time — tomatoes six weeks before the frost date, peppers eight, onions ten, squash only three — and a customary planting-out date relative to the same frost date: tender things a week or two after it, hardy things a few weeks before. The calculator applies those offsets to your date, which is the whole trick; the offsets themselves are the same ones printed on seed packets and extension charts, collected in one place.
The "last frost date" is an average — the date by which, in half of past years, the last 32 °F night had already happened. Half the time it hasn't. So treat the transplant dates here as earliest sensible, watch the ten-day forecast, and keep row cover handy for the week after. Your USDA zone gives a typical date (the calculator fills it in); the real number for your town from your extension office or NOAA's climate normals is better, and a neighbor who's gardened there twenty years is better still.
Seed starting dates by zone — a few crops
| Crop | Zone 5 last frost ≈ Apr 30 | Zone 6 last frost ≈ Apr 17 | Zone 7 last frost ≈ Apr 3 |
|---|---|---|---|
| Tomato | Sow Mar 19 · out May 7 | Sow Mar 6 · out Apr 24 | Sow Feb 20 · out Apr 10 |
| Pepper | Sow Mar 5 · out May 14 | Sow Feb 20 · out May 1 | Sow Feb 6 · out Apr 17 |
| Broccoli | Sow Mar 19 · out Apr 16 | Sow Mar 6 · out Apr 3 | Sow Feb 20 · out Mar 20 |
| Lettuce | Sow Apr 2 · out Apr 16 | Sow Mar 20 · out Apr 3 | Sow Mar 6 · out Mar 20 |
| Pea | Direct Mar 19 | Direct Mar 6 | Direct Feb 20 |
| Bean, bush or pole | Direct May 7 | Direct Apr 24 | Direct Apr 10 |
| Cucumber | Sow Apr 9 · out May 14 | Sow Mar 27 · out May 1 | Sow Mar 13 · out Apr 17 |
Engine output at each zone's typical last frost. "Sow" = indoors under lights; "out" = transplant; "Direct" = sow in the ground.
A tomato sown in January for an April planting-out spends two months leggy and root-bound on a windowsill and transplants worse than one sown in March. The lead times here are what the plant needs to be the right size on planting day — 6–8 in tall, a few true leaves, not yet flowering — under decent light. If your only light is a window, sow a week later than the chart, not earlier; if you have a shop light an inch above the seedlings, the chart is right.
Hardening off and planting out
Indoor seedlings have never met wind, full sun or a cold night. Starting 7–10 days before the transplant date, put them outside in shade for an hour and work up to a full day of sun and a night out. Plant in the evening or on a cloudy day, water in, and don't fertilize for a week. For the tender crops — tomatoes, peppers, squash, basil — the transplant date assumes soil has warmed too; a cold wet week after the frost date is a reason to wait, not a reason to hurry.
The fall garden: count back from the first frost
The half of the calendar most charts leave off. Fall crops are sown in late summer and need to mature before the first fall frost slows them down, so the offset runs the other way: weeks before first frost. For a first frost around Oct 20 that's roughly broccoli Jul 28, lettuce Aug 25, spinach Sep 1, radish Sep 15 and garlic Sep 22. Fall lettuce and spinach are often the best of the year — cool nights, no bolting — and garlic planted a month before frost is next July's harvest.
Direct sowing: the soil thermometer beats the date
Peas, spinach and radishes germinate in soil at 40 °F and go in a month or more before the frost date; beans, corn and squash rot in soil below about 60 °F and go in a week or two after it. The dates here are when those soil temperatures usually arrive; a $15 soil thermometer tells you when they actually have. Sow a second batch of the quick crops every two or three weeks for a continuous harvest — the dates are the start of the season, not the only day.
Sources: Johnny's Selected Seeds, Seed-Starting Date Calculator (offsets); University of Maryland Extension, Vegetable Planting Calendar; Iowa State Extension, Planting and Harvesting Times for Garden Vegetables; Clemson HGIC 1256, Planning a Garden. Frost dates by zone are the common almanac approximations; use your local average.