When to Start Seeds: Timing Your Whole Garden to the Last Frost

Every vegetable-planting decision hangs on one date: your average last spring frost. Start too early and tender seedlings get killed by a late cold snap or grow leggy waiting to go out. Start too late and slow crops like tomatoes never ripen. Get the frost date right and everything else is just counting weeks forward or backward from it.

Why the last frost date runs everything

Plants split neatly into two camps. Cool-season crops (peas, lettuce, spinach, the cabbage family) tolerate light frost and actually prefer cool weather — they can go outside weeks before the last frost. Warm-season crops (tomatoes, peppers, beans, squash, corn) are killed or stunted by cold, so they wait until after it. Because that one date sorts everything, seed packets and planting guides express timing as “weeks before/after last frost” rather than fixed calendar dates — the same offset works whether you garden in a warm or cold region.

Counting backward: when to start seeds indoors

Starting indoors gives slow crops a head start so they mature before autumn. Count back from the last frost:

Crop	Start indoors	Move outdoors
Onions (from seed)	~10 weeks before	~4 weeks before frost
Peppers, eggplant	~8 weeks before	~2 weeks after frost
Tomatoes	~6–8 weeks before	~1 week after frost
Broccoli, cabbage, kale	~5–6 weeks before	~2–4 weeks before frost
Lettuce	~4–5 weeks before	~3 weeks before frost
Cucumber, squash	~3–4 weeks before	~1–2 weeks after frost

Don’t start too early: a tomato seedling started 12 weeks out becomes a root-bound, leggy plant stuck on a windowsill waiting for the weather. Matching the start date to the transplant date keeps plants stocky and healthy.

Counting forward: sowing outdoors

Many crops skip the indoor stage entirely. Cool-season direct sowing happens before the frost — peas and spinach about 4–6 weeks before, lettuce a few weeks before. Warm-season direct sowing waits until after it — beans, corn, squash and cucumbers at or just after the last frost, when the soil has warmed. Root crops (carrots, beets) and legumes (peas, beans) are almost always direct-sown because they resent having their roots disturbed by transplanting.

Turn the offsets into real dates

The one thing you have to supply is your local average last-frost date — look it up from a regional extension service or a frost-date tool for your area. Once you have it, the planting date calculator turns each crop’s offset into an actual calendar date to start seeds, transplant and direct-sow. From there, the plant spacing and seed spacing calculators tell you how many plants and seeds you’ll need for your beds — all worked out privately in your browser.

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These are general guidelines that vary with region, cultivar and the season’s weather, and extension sources differ by a week or two on some crops. Anchor to your local last-frost average and the seed packet, and give tender crops a safety margin in a cold spring. Timing figures adapted from extension-service planting schedules.