Your local frost dates determine everything: when you can plant, how long your season lasts, and which crops can survive your winters.
Two key dates:
Last spring frost — the average date of the final frost in spring. Planting tender crops (tomatoes, peppers, squash, cucumbers, basil) before this date risks killing them. Planting after this date is safe.
First fall frost — the average date of the first frost in fall. Tender crops must be harvested or protected before this date.
How to find your dates: Search "[your city] last frost date" or use the Old Farmer's Almanac frost date tool. Your USDA hardiness zone is related but different — zones describe winter cold, not growing season length.
Counting backward from last frost: Seed packets say "start indoors 6–8 weeks before last frost." If your last frost is May 15 and the packet says 8 weeks, start seeds indoors March 20. Count exactly — starting too early produces root-bound transplants that struggle after planting.
Cool-season vs. warm-season crops:
Cool-season crops (lettuce, spinach, kale, broccoli, peas, carrots) can tolerate frost and actually prefer temperatures of 50–70°F. Plant these 4–6 weeks before last frost in spring, and again in late summer for fall harvest.
Warm-season crops (tomatoes, peppers, squash, corn, basil, cucumbers, melons) need soil temperature above 60°F and cannot survive frost. Wait until 1–2 weeks after last frost to transplant.
Extending the season: Row covers (lightweight fabric) protect plants from frosts down to about 28°F, extending your season by 2–4 weeks on both ends. Cold frames (low glass or plastic structures) add another 2–4 weeks. Season extension is often the difference between a short and a long productive season.
The fall harvest window: Count backward from first fall frost. If first frost is October 15 and broccoli takes 75 days to mature, plant no later than July 31 to get a harvest before frost.