Seed Starting Date Calculator

Start with your last frost date

Every seed-starting date is counted backward from one number: the average last frost date for your area. That is the date after which, on average, the risk of a killing frost has passed. Look it up by zip code on a garden extension site or the Farmers' Almanac planting guide. Once you have it, subtract the number of weeks each crop needs. Tomatoes and peppers are slow, so they start six to ten weeks before. Cucumbers and squash are fast, so they start two to three weeks before.

The word average matters. A last frost date is a midpoint, not a guarantee, and a late cold snap can still come after it. That is why most gardeners transplant warm-season crops a week or two after the date, not on it, and keep a row cover handy for a cold night. Cool-season crops are the reverse, they can go out two to four weeks before the date because they shrug off a light frost.

Worked example: tomatoes in zone 6

Say your last frost date is May 15. Tomatoes want six to eight weeks indoors. Count back six weeks from May 15 and you land on April 3. Count back eight weeks and you land on March 20. Aim for the middle, around March 27, so the plants are sturdy but not leggy. Harden them off over a week, then transplant about a week after the last frost, around May 22.

The window is a range for a reason. Start too early and the seedlings stretch tall and pale waiting for warm weather, root-bound in their cells. Start too late and the plants are small when the season is already underway, so you lose weeks of growth. If you can only remember one number, remember six weeks for tomatoes and adjust from there.

Worked example: peppers and onions

Peppers are slower than tomatoes. At eight weeks before a May 15 frost date, pepper seeds go in on March 20. Onions and leeks are slower still, ten to twelve weeks, so they start around February 20 to March 6. Eggplant follows peppers at eight weeks. Brassicas like broccoli and cabbage sit in the middle at four to six weeks, the same window as tomatoes, but they tolerate a light frost, so they can go outside two weeks before the last frost instead of after it.

The pattern is simple once you see it: the slower a plant grows and the more warmth it needs, the earlier you start it. Onions and celery lead the pack, peppers and eggplant follow, tomatoes and brassicas come next, and the fast warm-weather crops bring up the rear. Start them all at once and half will be ready weeks before you can plant them out.

Weeks before last frost, by crop

The lead times the calculator uses, counted back from your last frost date:

These windows come from extension guidance such as the South Dakota State University seed-starting guide and the University of Maine cooperative extension, which both count from the last spring frost.

Hardening off and transplanting

Seedlings raised indoors are soft. The wind, sun and cold swings outside will set them back or kill them if you move them out in one step. Hardening off is the week of transition: set the plants outside in a sheltered spot for two hours the first day, then three or four the next, working up to a full day and then an overnight stay. Bring them in if a frost is forecast during that week.

Transplant on a cloudy day or in the evening so the roots settle before the sun hits them. Water them in well, and for a week or two after planting, keep an eye on moisture, because a transplant is still establishing and dries out faster than an established plant. A little shade cloth or a cut-off milk jug over each plant helps on the hottest first days.

Direct sowing versus starting indoors

Not everything belongs in a tray. Root crops like carrots, beets, radishes and turnips should be direct sown, because transplanting disturbs the taproot and gives you forked, stunted vegetables. Cucumbers, squash, melons and pumpkins can go either way, but they grow fast and resent root disturbance, so many gardeners direct sow them at or just after the last frost and skip the indoor start entirely.

The crops that reward an early start are the slow, warmth-loving ones: tomatoes, peppers, eggplant, onions and celery. They need the head start to produce before the season ends. Fast cool-weather crops like lettuce, spinach and radishes can be direct sown three to six weeks before the last frost and will be ready sooner than a transplanted start. Match the method to the crop, and the calendar takes care of itself.

Seed packets print a range, like start indoors 6 to 8 weeks before the last frost, and the range is there because your climate is not the same as the next gardener's. If you garden in a cool, short-season area, lean toward the longer end so plants are ready early. In a hot, long-season area, lean shorter, because an early start means weeks of holding leggy seedlings indoors before it is safe to plant out. The packet also lists days to maturity, which is the count from transplant to harvest for most vegetables. Add that to your transplant date and you get a rough harvest window, which is how you decide whether a crop has time to finish before your first fall frost.