Seed starting is simple chemistry and patience: warm, moist mix to wake the seed, strong light to build a sturdy plant, and a slow introduction to the outdoors. The details matter because seedlings are fragile. A week of too little light or too much water shows up as a stretched, pale, or collapsed plant that never quite catches up.
This guide walks through the whole process, from choosing what to start to setting plants out. When you are ready to set dates, the seed starting calculator turns your last frost date into a sowing schedule.
The short answer
- Start the crops that need a head start: tomatoes, peppers, eggplant, and cabbage-family crops. Sow beans, peas, and root crops directly in the garden.
- Count back from the day each crop can go outside, not forward from when you feel like starting. Too early is as bad as too late.
- Use clean containers and a sterile seed-starting mix, not garden soil.
- Keep the mix at 70–75 °F until seeds sprout, then move seedlings under lights.
- Give them 14–16 hours of strong light a day, with the lights close. A windowsill is rarely enough.
- Water from below, keep air moving, and harden plants off for one to two weeks before planting out.
What to start indoors, and what to sow outside
Indoor sowing pays off for crops that need a long season before they produce, and for crops whose transplants are expensive or hard to find in the varieties you want.
- Start indoors: tomatoes, peppers, eggplant, broccoli, cabbage, kale, and cauliflower.
- Optional: cucumbers, squash, and melons grow fast, needing only three to four weeks indoors. Starting them inside gives a small head start in short seasons, but they resent root disturbance, so sow them in individual pots.
- Sow directly in the garden: beans, peas, carrots, radishes, beets, and other root crops, which grow quickly from seed or transplant poorly. The March guide covers direct sowing.
When to sow
Every crop needs a certain number of weeks from sowing to a transplant ready for the garden. Work backward from the day that crop can safely go outside.
Iowa State Extension gives these sow-to-transplant times, plus a week for hardening off:
| Weeks indoors | Crops | Goes outside |
|---|---|---|
| 6–8 | Peppers, eggplant | After the last frost, in warm soil |
| 5–7 | Tomatoes | After the last frost, in warm soil |
| 4–5 | Broccoli, cabbage, cauliflower, kale | Weeks before the last frost, once hardened off |
| 3–4 | Cucumbers, squash, melons | After the last frost, in warm soil |
Your last frost date sets the anchor. Tender crops go out after it, once the soil is warm. University of Maryland Extension notes that tomatoes, peppers, and eggplant do best planted into soil above 60 °F. Hardy cabbage-family crops go out weeks earlier. The seasonal planner explains how to find and read your frost dates, and the seed starting calculator does the counting for you.
Resist the urge to start early. Illinois Extension cautions that transplants held indoors too long become overgrown and root-bound, and they establish more slowly than younger, stockier plants set out on time.
Containers and seed-starting mix
Containers. Cell packs, small pots, soil blocks, or recycled containers with drainage holes all work. Reused pots and trays should be washed and disinfected, since pathogens that cause damping off survive on dirty containers.
Mix. Use a commercial seed-starting mix or another fine, well-drained, soilless mix. Garden soil compacts in small containers, drains poorly, and can carry weed seeds and the organisms that cause damping off. Moisten the mix before filling containers, so it is damp throughout but not dripping.
Sow at the right depth and temperature
Depth. A common rule of thumb is to sow a seed about twice as deep as it is wide. Very fine seeds, and seeds that need light to germinate, go on the surface and are pressed lightly into the mix. Check the packet; it will say if a variety needs light.
Temperature. Most vegetable seeds germinate best when the mix is 70 to 75 °F. A cool room slows germination badly, especially for peppers and eggplant. A seedling heat mat under the tray is the easiest way to keep the mix warm. Cover trays with a clear lid or plastic to hold moisture until the seeds sprout.
Once seedlings emerge, remove the cover, take the tray off the heat mat, and move it under lights straight away. Seedlings grown warm and dim stretch within a day or two.
Light: the part windowsills get wrong
Too little light is the most common cause of tall, spindly, pale seedlings. A south-facing windowsill looks bright to us, but late-winter days are short and the light through glass is far weaker than outdoors. Seedlings there lean toward the glass and stretch.
Grow lights solve this:
- Duration. Give seedlings about 14 to 16 hours of light a day. Plug the lights into a timer so you never forget. Seedlings also need some darkness each night.
- Distance. Light intensity falls off steeply as a light gets farther from the plants. Keep fluorescent tubes just a few inches, about 2 to 4, above the leaves. LED fixtures vary widely, so follow the maker's recommended height for seedlings. University of New Hampshire Extension gives an example of how much height matters: one LED bar needed 8 hours a day when hung 8 inches above the plants, but 16 hours at 20 inches.
- Raise the lights as seedlings grow. Keep the gap constant rather than leaving the fixture where it started.
Inexpensive shop lights with full-spectrum LED or fluorescent tubes are enough for vegetable seedlings. You do not need a specialized fixture.
Water, air, and damping off
Damping off is the collapse of young seedlings at the soil line, caused by soil-borne fungi and water molds. Seedlings that looked fine the night before topple over with a pinched, dark stem. Once it starts in a tray, affected seedlings cannot be saved, so prevention is everything:
- Start with clean containers and a fresh, sterile mix.
- Keep the mix moist, not soggy. Water from below by setting trays in a shallow tray of water for a few minutes, then let them drain.
- Keep air moving. A small fan on low near the seedlings dries the surface and also makes stems sturdier.
- Keep germination temperatures warm; seeds sitting in cold, wet mix are the most vulnerable.
If fungus gnats appear, the mix is staying too wet. Let the surface dry between waterings.
Thin, feed, and pot up
Thin early. When seedlings have their first true leaves (the leaves after the first pair), keep the strongest seedling in each cell and snip the others at the soil line. Pulling them disturbs the roots of the one you keep.
Feed lightly. Seed-starting mixes carry little fertilizer. University of Maryland Extension recommends starting once seedlings have true leaves, with a water-soluble fertilizer at half the label rate, about once a week. Too much fertilizer damages young roots, especially in dry mix.
Pot up if needed. Seedlings that fill their cells well before planting time, especially tomatoes, do better moved into larger pots. Tomatoes can be set deeper when potted up, because they root along buried stem.
Harden off before planting out
Seedlings raised indoors have never felt wind, full sun, or cool nights. Moving them straight into the garden scorches and stunts them. Harden them off over one to two weeks instead:
- Start with an hour or two outdoors in a sheltered, shaded spot.
- Each day, lengthen the time and increase the sun and wind.
- Water as needed; pots dry quickly outside.
- Bring them in on cold nights, especially tender crops.
- After one to two weeks, plants can stay out day and night and are ready to plant.
The February guide and the vegetable garden guide cover hardening off and planting in more detail.
Troubleshooting
| Problem | Usual cause | Fix |
|---|---|---|
| Tall, thin, pale seedlings leaning to one side | Too little light; sometimes too warm | Lower the lights, lengthen hours, move off the heat mat |
| Seedlings collapse at the soil line | Damping off | Discard affected cells; clean containers, fresh mix, less water, more airflow |
| Seeds never came up | Mix too cold or too dry, seed sown too deep, or old seed | Check mix temperature, depth, and moisture; test old seed on a damp paper towel |
| Yellowing lower leaves on older seedlings | Mix staying wet, or nutrients used up | Let the surface dry between waterings; begin light feeding |
| White fuzz or green film on the mix surface | The surface is staying constantly wet | Water from below, let the surface dry between waterings, and increase airflow |
The February guide's troubleshooting section covers more seedling problems.
Sources and further reading
- Iowa State University Extension: Guide to starting seed indoors — sow-to-transplant times and the extra week for hardening off.
- Iowa State University Extension: How to successfully start seed indoors — containers, mix, surface-sown seeds, and transplanting at true leaves.
- Iowa State University Extension: How do I know if I am providing the right amount of light for my seedlings?
- University of Minnesota Extension: Starting seeds indoors.
- University of Minnesota Extension: Lighting for indoor plants and starting seeds.
- UNH Extension: Growing seedlings under lights.
- UNH Extension: Optimizing plant growth with indoor lighting — how light intensity falls with distance.
- Illinois Extension: Supplemental lighting for indoor seed starting.
- Illinois Extension: What's wrong with my seedlings?
- Illinois Extension: Don't get started too early on planting seed.
- Utah State University Extension: Starting vegetable seeds indoors, germination — germination temperatures.
- UC Statewide IPM Program: Damping-off diseases in the garden.
- University of Maryland Extension: Care of vegetable seedlings — thinning and feeding at half strength.
- University of Maryland Extension: Planting vegetable transplants — warm soil for tomatoes, peppers, and eggplant.