When should you start your seeds?

What the two dates mean

Every crop here has two dates. The first is when to sow seeds indoors; the second is when it is safe to set the seedlings, or the seed itself, in the garden. Both are measured from the average date of the last spring frost, which you enter. The tomato is the usual example: it is sown indoors, spends weeks growing in a pot, and goes out only after the danger of frost is gone. A cabbage also starts indoors, but it goes out weeks before the last frost, because it tolerates cold. So two crops with the same number of weeks in the pot get sown at very different times.

How the dates are worked out

The table comes from one publication, a fact sheet by the University of New Hampshire Cooperative Extension (April 2016). For each crop it gives the time from seed to transplant in weeks, and the safe time to set it out, in weeks before or after the last frost. The sheet's own method is to mark the planting date on a calendar and count back the weeks the seed takes to grow. The calculator does the same from the first safe day of the set-out window: it sows from the maximum weeks before that day to the minimum weeks before it. A later set-out date inside the window moves the sowing date later by the same amount. Dates are calendar arithmetic in UTC, seven days to a week, so your time zone and daylight saving time cannot move a date by a day. Crops that the sheet says are direct-seeded show the sowing window or the soil temperature instead of an indoor date.

Worked example: last frost on May 15

Take an average last frost of May 15, 2027. UNH says tomatoes are safe to set out 1 to 2 weeks after the last frost, so from May 22 to May 29, and take 6 to 8 weeks from seed to transplant. Counting back from the first safe day, May 22, gives sowing from Mar 27 to Apr 10. Peppers go out 2 weeks after the frost (May 29) and need 8 to 10 weeks, so they are sown from Mar 20 to Apr 3, about two weeks before the tomatoes. Broccoli is set out 2 to 4 weeks before the last frost (Apr 17 – May 1) after 4 to 6 weeks in the pot, so it is sown from Mar 6 to Mar 20. Cucumbers and squash need just 3 to 4 weeks in the pot and go out 1 to 2 weeks (cucumber) or 2 weeks (squash) after the frost, so they are sown from Apr 24 to May 1 (cucumbers) and from May 1 to May 8 (squash). Celery is the long one: 10 to 12 weeks, set out after the last frost, which puts the sowing from Feb 20 to Mar 6.

Where two extension sources disagree

Purdue Extension's guide to starting seeds indoors (HO-14-W, Rosie Lerner, November 2020) prints one number per crop: how many weeks before the last frost to sow. For the ten crops that both publications cover, UNH's numbers work out to a range of weeks before the last frost, and Purdue's number falls inside that range for 5 of 10:

The UNH range runs from the sowing date for the earliest safe set-out to the date for the fewest weeks in the pot. Purdue lists cantaloupe and a single “Squash” row, which are compared here with UNH's melon and summer squash, and head lettuce, compared with lettuce.

Common mistakes and limits

The first mistake is counting back from the last frost date for every crop. That is right only for a crop that is set out on the frost date. A tomato goes out 1 to 2 weeks after it, a cabbage 4 weeks before, so the sowing dates of the two differ by more than their weeks in the pot suggest. The second is trusting the average date as a guarantee: UNH tells readers not to seed or transplant frost-sensitive plants before the last frost date, and calls Memorial Day a conservative estimate for central New Hampshire, so a later date is the cautious choice. Third, skipping hardening. Purdue says plants grown indoors cannot go straight into the garden, advises starting to harden them 2 weeks before planting, and not putting tender seedlings outdoors on windy days or below 45°F; even cabbage is hurt by freezing temperatures until it has been hardened. Fourth, using the spring dates for fall plantings: Purdue's own footnote has broccoli and cabbage sown 5 to 7 weeks and cauliflower 7 to 9 weeks before the fall planting-out date. Finally, the two tables were written by extension services in New Hampshire and Indiana, so treat a date as a planning aid and compare it with your local extension office's calendar.

When should I start tomato seeds indoors?

Sow tomatoes 6 to 8 weeks before the day you want to set them out, and set them out 1 to 2 weeks after the average last frost (University of New Hampshire Cooperative Extension). With a last frost on May 15 that means sowing from Mar 27 to Apr 10. Purdue Extension prints 7 weeks before the last frost for tomatoes, Mar 27 in this example.

When should I start pepper seeds?

Peppers need 8 to 10 weeks from seed to transplant and are set out 2 weeks after the last frost (UNH), which puts the sowing from Mar 20 to Apr 3 for a May 15 frost. Purdue gives 7 weeks before the last frost, Mar 27, which falls within the UNH range.

How do I find my average last frost date?

NOAA's National Centers for Environmental Information publishes a map of the average last spring freeze (32°F) built from the 1991–2020 U.S. Climate Normals; the link is below the calculator. Your state or county extension office can also tell you the date for your area. Enter the date as a calendar date; the calculator does not guess it from a ZIP code.

What does weeks before last frost mean?

It means the date that many weeks, seven days each, before the last frost date you entered. Ten weeks before May 15 is March 6. A negative number, such as 2 weeks after the last frost, means the date is later than the frost date: May 29.

Which vegetables should be sown directly in the garden?

The UNH table lists bean, beet, carrot, corn, parsnip, pea, radish, rutabaga, turnip as direct-seeded only, with the soil temperature or the number of weeks before the last frost to do it. Potatoes are planted as seed potatoes and sweet potatoes as slips rather than from seed. Several crops, among them cucumber, pumpkin and the squashes, can be started indoors or sown in the soil once it warms to 70°F.

Why do two extension publications give different weeks?

They measure different things. UNH gives the weeks a seedling takes to reach transplant size and a separate window for setting it out, and the sowing date follows from both. Purdue prints one number, the weeks before the last frost, with no window. They also come from different climates and years, and each rounds in its own way. For cucumbers, melons and squash Purdue is 1–2 weeks earlier than the UNH numbers work out to, while for tomatoes, peppers and eggplant the two agree.

What if my last frost is in winter or early spring?

The calculator works with any date. If sowing would fall in the year before the frost date, as in the south where the last frost may come in January, the dates in that year show the year. A last frost on January 20, 2027, for example, puts tomato sowing in December 2026.

Sources: University of New Hampshire Cooperative Extension, “Growing Vegetables: When to Plant your Vegetable Garden” (April 2016, Karen Downing and Olivia Saunders); Purdue Extension, HO-14-W, Starting Seeds Indoors (November 2020, Rosie Lerner (originally M. N. Dana and Allen E. Boger)), Table 1 and the section on hardening; NOAA National Centers for Environmental Information, “When to Expect Your Last Spring Freeze”; all read October 27, 2026. The dataset is in one file with its source and check date, and the week arithmetic is ours. Next steps: plant spacing calculator, raised bed soil calculator, lawn watering calculator, tree height calculator and the calendar. How we check our calculators.