Back-calculate from the frost date
Most crops are planned around the last spring frost date: warm-season crops (tomatoes, peppers, basil) go out only after it, so their seeds start before it by 4–10 weeks depending on the crop; cool-season crops (lettuce, peas, cabbage) can be direct-sown before it. The calculator works the math both ways.
How to find your frost date
- Your local extension service publishes frost probabilities — use the 10% frost risk date (i.e., the date by which 90% of years have had their last frost).
- Zip-based tools (e.g. the National Gardening Association's frost map) give a good default.
- Microclimate matters: urban gardens and south-facing beds run ~2 weeks earlier; hilltops later.
Seed-starting basics
- Light: seedlings need 14–16 h of strong light — a south window is often not enough; grow lights should be 2–4 in above the plants.
- Heat: most seeds germinate fastest at 20–25°C (70–80°F); a heat mat speeds slow crops (peppers, eggplant).
- Hardening off: 7–10 days before transplant, move seedlings outside for increasing hours — skipping this is why transplants flop.
- Don't start too early: leggy, root-bound seedlings transplant worse than fresh ones. The calendar's dates are the sweet spot.
Dates are typical for temperate climates; adjust ±2 weeks by your local conditions and each seed packet's "days to maturity".