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.

A worked example

Suppose your average last spring frost is 1 May. Working the offsets from the table gives a concrete sowing calendar with no guesswork:

Task	Offset	Date (last frost = 1 May)
Start onions indoors	~10 weeks before	late February
Start peppers indoors	~8 weeks before	early March
Start tomatoes indoors	~7 weeks before	mid-March
Sow peas & spinach outdoors	~5 weeks before	late March
Start brassicas indoors	~5 weeks before	late March
Direct-sow beans, corn, squash	at/after frost	early May
Transplant tomatoes outdoors	~1 week after	second week of May
Transplant peppers outdoors	~2 weeks after	mid-May

Shift the frost date and every row shifts with it, that is the whole point of frost-relative timing. A gardener whose last frost falls on 15 April simply slides the same calendar two weeks earlier; one at 20 May slides it later. The offsets stay put.

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.

For warm-season direct sowing, soil temperature matters more than the calendar. Beans and squash germinate slowly and may rot in cold, wet ground even after the frost date has passed; corn is similar. If spring is running cold, waiting an extra week for the soil to warm usually beats sowing on the nominal date. A cheap soil thermometer removes the guesswork, most warm-season seeds want soil above roughly 16 °C (60 °F) to germinate reliably.

Hardening off before transplanting

Indoor-raised seedlings have never met wind, direct sun or cool nights, and moving them straight outdoors can shock or scorch them. Hardening off bridges that gap: over about 7-10 days, set the seedlings outside for a lengthening spell each day, starting with an hour or two in shade and building up to a full day and eventually an overnight, before they go into the ground. Skipping this step is one of the most common reasons healthy-looking seedlings stall or bleach after transplanting. Build the hardening-off window into your plan so the seedlings are ready exactly when their transplant date arrives, not a week after.

Warm-season crops: read the nights, not just the frost date

The last frost date marks when a killing frost becomes unlikely, but tender crops care about cold nights too, not only actual frost. Tomatoes, peppers and cucumbers stall when nights sit in single digits Celsius even without frost, so the “~1-2 weeks after frost” offsets in the table build in a buffer for exactly that. In a cold spring, wait for settled warm nights rather than rushing plants out on the calendar date; a plant held back a week catches up fast, while one chilled early may sulk for a month.

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.