When to Start Seeds Indoors

Your last spring frost date is the anchor. Every crop has a head-start window measured backward from that date. Get the math right and you will have strong transplants ready the day the ground warms.

The one date you need first

Everything revolves around your average last spring frost date. Find it before you buy a single seed. Look it up by ZIP on the USDA map or your local extension service. Write it at the top of your seed list.

How to count back from last frost

Every seed packet lists a "start indoors" window like "6 to 8 weeks before last frost" or a "direct sow" instruction like "after last frost." The number is not a suggestion. Starting too early gives you leggy, pot-bound seedlings that sulk after transplant. Starting too late gives you plants that are still small when the summer heat hits.

Take your last frost date and count backward in whole weeks for each crop.
If the packet says "6 to 8 weeks," aim for the middle (7 weeks) on your first try. Adjust based on how your seedlings behave.
Round to the nearest Sunday so your sowing day is easy to remember.
Write the sowing date on each seed packet with a permanent marker.

Last Frost Planting Calculator Tool

Seed Starting Calendar Tool

Which crops to start indoors

Start indoors the slow, warm-season crops that need a running start to produce before frost returns. Each has a typical head-start window measured backward from last frost.

Tomatoes: 6 to 8 weeks before last frost. Sow 7 weeks out for most varieties.
Peppers (sweet and hot): 8 to 10 weeks. They germinate slowly and grow slowly. Treat them as a 9-week crop.
Eggplant: 8 to 10 weeks. Same warmth-loving behavior as peppers.
Broccoli, cabbage, cauliflower, kale: 4 to 6 weeks. Cool-season brassicas that go out before last frost.
Onions (from seed): 10 to 12 weeks. Longest lead time. Most beginners buy sets or plants instead.
Celery, leeks, parsley: 10 to 12 weeks. Slow germination.
Basil: 4 to 6 weeks. Do not put it outside until nights are reliably above 50F.
Lettuce, chard (for a head start): 4 to 6 weeks. These can also be direct-sown.

Which crops to direct-sow instead

Some crops either resent transplanting or grow so quickly that an indoor start is wasted effort. Put these straight in the ground on the right date.

Direct-sow 2 to 4 weeks before last frost: Peas, spinach, radishes, arugula, early carrots, beets, turnips. They tolerate cold soil.
Direct-sow on last frost date: Lettuce, chard, a second round of carrots and beets, potatoes.
Direct-sow 1 to 2 weeks after last frost: Bush beans, pole beans, corn. They rot in cold wet soil.
Direct-sow 2 to 4 weeks after last frost: Cucumbers, squash, zucchini, pumpkins, melons. These hate transplant shock. Only start indoors if your season is short (under 100 days).

What you need to start seeds indoors

Seed-starting mix, not potting soil: A fine, light, sterile medium. Potting soil is too dense and holds too much water for young roots.
Cell trays or small pots with drainage: 72-cell trays work for most crops. Peppers and tomatoes do better in 4-cell or 2-inch pots from week 3 on.
A light source: A sunny south window is rarely enough, even in warm climates. A basic $30 LED grow light on a timer (14 to 16 hours a day) is the single best investment for strong seedlings.
Bottom heat (for warm-season crops): A $20 heat mat keeps soil at 75 to 85F, which doubles germination speed on tomatoes, peppers, and eggplant. Remove the mat once seedlings emerge.
A fan or gentle airflow: A few hours a day of air movement builds stem strength and prevents damping-off fungus.
Labels: Write the variety and the sow date on every cell. You will not remember which tomato is which by May.

How to harden off before transplanting

A seedling grown indoors is soft. Its leaves are thin, its stems are weak, and it has never felt wind or direct UV. Putting it straight into the garden will shock it, and a shocked plant can lose two weeks of growth or die outright. The fix is hardening off: seven to ten days of gradual outdoor exposure before transplant day.

Day 1 to 2: One hour outside in dappled shade. No direct sun. No wind.
Day 3 to 4: Two to three hours in filtered sun. Bring them in if the wind picks up.
Day 5 to 6: Four to six hours including some direct morning sun.
Day 7 to 8: Full day outside. Bring in at night if the temperature drops below 50F.
Day 9 to 10: Overnight outside if night temperatures hold above 50F. Transplant on day 10 or 11.
Cut water during hardening off. Let the soil dry between waterings so the plants toughen up. Never let them wilt hard.

Red flags that seedlings are struggling

Tall, pale, leaning toward the window: Not enough light. Add a grow light 3 to 4 inches above the leaves.
Yellow bottom leaves: Usually overwatering. Let the soil surface dry between drinks.
Stems collapsing at soil line (damping off): Fungal. Remove affected cells, improve airflow, reduce water, and do not reuse the mix.
Roots visible at the drainage holes or circling the cell: Pot-bound. Bump up to a larger pot or they will stall.
Purple-tinged leaves: Cold soil or phosphorus stress. Warm up the root zone and start a half-strength liquid feed once true leaves appear.

FAQ

How do I find my last frost date?

The USDA and NOAA both publish frost date maps by ZIP code. Your local cooperative extension office will have more accurate local data, including a 10% and 50% risk date. For planning, use the 50% date (the "average" last frost). For tender crops, wait for the 10% date (the "safe" last frost) to be sure.

Can I start seeds on a sunny windowsill alone?

You can germinate seeds there, but most seedlings get leggy without a supplemental light. A south-facing window in winter delivers only a few hours of usable sun. If windowsill plants are tall, pale, and falling over, a basic LED grow light on a timer fixes it almost overnight.

What is "damping off" and how do I stop it?

Damping off is a fungal collapse at the soil line that kills seedlings within hours. The fix is prevention: use sterile seed-starting mix, do not reuse old mix, keep airflow moving with a small fan, water from the bottom, and let the surface dry between waterings.

Can I skip hardening off if I'm in a hurry?

No. A skipped hardening-off is the single most common reason transplants fail. Even three or four days is better than zero. Plants that skip it usually look scorched or wind-torn within 48 hours of going outside.

When should I pot up seedlings to a bigger container?

When roots are circling the bottom of the cell, or when the plant's leaves are three to four times wider than the cell. For tomatoes and peppers, that is usually around week 3 or 4 after germination. Pot up one size at a time (from cell to 3-inch, then 3-inch to 4-inch if needed).

Bottom line

Write down your last frost date, look up each crop's head-start window, and sow on schedule. Give seedlings strong light, steady warmth, airflow, and honest water. Harden them off for a full week before transplant. Do those four things and a window or a $100 shelf can grow hundreds of dollars of healthy transplants.