Seed Starting Date
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It counts backward from your area's average last frost date by the number of weeks the specific plant needs indoors before transplanting, which is typically 6-8 weeks for tomatoes and peppers but only 3-4 weeks for quick-growing plants like lettuce.

It's the average date in spring after which frost is no longer likely in your area, based on historical weather data. Transplanting tender seedlings before this date risks a late frost killing them, so it anchors the entire seed-starting schedule.

Slow-growing, heat-loving plants like peppers need a long head start indoors to reach transplant size before the season is warm enough outside, while fast-germinating cool-season crops like lettuce or peas can be direct-sown or started with much less lead time.

It's a statistical average, meaning frost after that date still happens some years, roughly 10-30% of the time depending on your location's climate variability. Many gardeners hedge by waiting a week past the average date, or keeping frost cloth on hand.

Seedlings started too far ahead of the transplant window often become leggy and root-bound waiting for outdoor conditions to be safe, which weakens the plant and can delay its recovery after transplanting compared to a seedling started at the right time.