When to Start Seeds Indoors, Based on Your Last Frost Date

The Short Answer

Start seeds indoors by counting back from your average last spring frost. Sow onions and leeks 12 to 14 weeks before that date, cabbage 8 to 10 weeks, peppers 6 to 8 weeks, tomatoes 4 to 7 weeks and cucumbers 1 to 3 weeks. Those windows come from combining two facts for each crop: how long it takes to reach transplant size, and how soon around the frost date it can safely go outside.

The last frost date itself is an average. NOAA’s maps of the last spring freeze show when temperatures typically dip to 32°F or below for the final time, based on the 1991 to 2020 climate normals. NOAA cautions that any one year can vary widely from the average, so treat the date as a planning anchor and watch the forecast as it approaches.

Weeks Before Last Frost, Crop by Crop

University of New Hampshire Extension publishes both numbers needed for the calculation. The last column below is the result.

Each bar is a sowing window counted back from the last frost. The crops fall into three waves: alliums first, then cabbage-family crops and peppers, then tomatoes and cucumbers.

Crop	Seed to transplant	Safe to set out	Sow indoors
Onion, leek	8 to 10 weeks	4 weeks before last frost	12 to 14 weeks before
Cabbage	4 to 6 weeks	4 weeks before	8 to 10 weeks before
Broccoli	4 to 6 weeks	2 to 4 weeks before	6 to 10 weeks before
Lettuce	4 to 5 weeks	3 to 4 weeks before	7 to 9 weeks before
Pepper	8 to 10 weeks	2 weeks after	6 to 8 weeks before
Eggplant	8 to 10 weeks	2 to 3 weeks after	5 to 8 weeks before
Tomato	6 to 8 weeks	1 to 2 weeks after	4 to 7 weeks before
Basil	6 to 8 weeks	2 weeks after	4 to 6 weeks before
Cucumber	3 to 4 weeks	1 to 2 weeks after	1 to 3 weeks before
Summer squash, melon	3 to 4 weeks	2 weeks after	1 to 2 weeks before

Sources differ by a week or two on growing time, and it helps to know where. University of Maryland Extension gives tomatoes 5 to 6 weeks and leafy greens 2 to 4 weeks. South Dakota State University Extension gives peppers and eggplant 6 to 8 weeks and squash only 2 to 3. If your seedlings were oversized at planting time last year, use the shorter figure.

Seed packets often state a simpler rule that ignores the set-out delay. A University of New Hampshire Extension article notes that a tomato packet will direct you to start the seeds six to eight weeks before the last frost, which is 1 to 2 weeks earlier than the chart’s result. The difference is the 1 to 2 weeks a tomato waits after the frost before it goes out.

Some crops skip this step entirely. New Hampshire’s table lists beans, beets, carrots, corn, peas and radishes for sowing directly in the garden. Cucumbers and squash can go either way. They may be started indoors, or direct-seeded once the soil warms to 70°F.

Work the Dates Backward

Fix the day the seedling goes outside first, then subtract the weeks it needs indoors. South Dakota State’s guidance is the same in brief: check the weeks listed on the seed packet and count back from the last frost date in your area.

The same four steps work for any crop in the chart once you swap in its own set-out timing and weeks indoors.

Here is the arithmetic with a last frost date of May 15, which South Dakota State gives as the approximate date for much of South Dakota.

Tomatoes go out 1 to 2 weeks after frost, between May 22 and May 29. Subtract 6 to 8 weeks and the sowing window is March 27 to April 17.
Peppers go out 2 weeks after frost, on May 29. Subtract 8 to 10 weeks and you sow between March 20 and April 3.
Onions go out 4 weeks before frost, on April 17. Subtract 8 to 10 weeks and you sow between February 6 and February 20.

Gardeners with a later frost shift everything accordingly. University of New Hampshire Extension notes that in Zone 5 parts of that state the last frost typically falls in late May, which pushes every window in the example later by a week or two. The week-by-week guide to growing tomatoes from seed follows that crop from sowing to set-out.

What Seedlings Need While They Wait

Timing only works if the seedlings grow at a normal pace, and that depends on warmth and light.

For germination, University of Maryland Extension says a potting mix temperature of 65° to 75°F is good for most vegetable seeds. South Dakota State adds that seeds sprout across a wide range of temperatures and that the ideal ones simply give the quickest germination.

Light matters more once the seedlings are up. Both sources call for 14 to 16 hours a day under fluorescent or LED fixtures, set on an outlet timer. Maryland places the fixture 2 to 6 inches above the plants and South Dakota State says 4 to 6 inches. Raise it as the seedlings grow. The guide to grow lights for seed starting covers fixture types, and the one on seed starting trays covers cell sizes.

Feeding can wait. Most potting mixes contain enough fertilizer to carry a seedling for at least 6 weeks, according to Maryland.

Harden Off Before Transplanting

Seedlings raised indoors need a gradual introduction to sun and wind. University of Maryland Extension’s method starts 1 to 2 weeks before the set-out day:

Put the plants outside in a shaded, protected spot on warm days and bring them in at night.
Give them more sunlight each day.
Water less often to slow growth, but do not let the plants wilt.
Keep tender seedlings indoors on windy days and whenever temperatures are below 45°F.

Cucumbers and melons are the touchy ones. Maryland warns that they stop growing if hardened too severely, and says they can stay out overnight only when the temperature will not drop below 50°F. Cold-hardy crops need the process too, because even they are hurt by freezing temperatures before they have been hardened. The full day-by-day process is in the guide to hardening off seedlings.

New Hampshire’s fact sheet gives one firm rule for the final step: do not seed or transplant frost-sensitive plants before the frost-free date.

Why Starting Too Early Backfires

All three extension services warn against the same mistake. South Dakota State calls starting too early the most common one and says it produces tall, weak, spindly or root-bound plants. Maryland describes tall, root-bound plants that do not perform well in the garden. New Hampshire puts it plainly: plants started too early will probably be of poor health and quality by planting time.

The chart shows why onions can be sown in February while a tomato sown the same week cannot. An onion needs 8 to 10 weeks and goes out 4 weeks before the frost. A tomato needs 6 to 8 weeks and cannot go out until 1 to 2 weeks after it, so a tomato sown in mid-February for a May 15 frost would spend more than 12 weeks indoors against the 6 to 8 it needs.

Keep a record of the day you sowed, the day you transplanted and how the seedlings looked when they went out. Those notes show whether your own setup runs toward the short or the long end of each range.