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Starting seeds indoors

Starting seeds indoors gives slow crops a head start and lets you grow varieties you will never find as transplants. The work is not complicated, but timing and light make most of the difference.

Which crops are worth starting indoors

Crops that are slow to reach planting size, or that need a long warm season to ripen, benefit most. Peppers, eggplant, tomatoes, onions, leeks and the cabbage family are the classics. Crops that grow fast or dislike root disturbance are usually better sown directly in the garden. That includes beans, peas, corn, carrots, radishes, beets and most squash. Cucumbers and squash can be started indoors, but only a few weeks ahead and in individual pots.

From our crop table, the indoor start windows, counted back from your last spring frost:

CropStart indoorsPlant out
Onion (from seed)12–10 wk before last frost4–2 wk before
Leek12–10 wk before last frost2 wk before to at last frost
Pepper10–8 wk before last frost2–4 wk after
Eggplant10–8 wk before last frost2–4 wk after
Parsley10–8 wk before last frost2 wk before to 1 wk after
Tomato8–6 wk before last frost1–3 wk after
Broccoli8–6 wk before last frost4–2 wk before
Cabbage8–6 wk before last frost4–2 wk before
Cauliflower8–6 wk before last frost2 wk before to at last frost
Brussels sprouts8–6 wk before last frost2 wk before to at last frost
Kale8–6 wk before last frost4–2 wk before
Collards8–6 wk before last frost4–2 wk before
Marigold8–6 wk before last frostat last frost to 2 wk after
Lettuce6–4 wk before last frost2 wk before to 2 wk after
Swiss chard6–4 wk before last frostsee crop guide
Okra6–4 wk before last frost3–5 wk after
Basil6–4 wk before last frost1–3 wk after
Zinnia6–4 wk before last frost1–2 wk after
Cosmos6–4 wk before last frostsee crop guide
Cucumber4–3 wk before last frost1–3 wk after
Summer squash4–3 wk before last frost1–3 wk after
Winter squash4–3 wk before last frost1–3 wk after
Melon4–3 wk before last frost2–4 wk after
Watermelon4–3 wk before last frost2–4 wk after

The seed-starting calculator turns these into calendar dates for your ZIP code.

Timing: the most common mistake is starting too early

It is tempting to sow tomatoes in February. But seedlings that sit too long indoors get leggy, root-bound and stressed, and often do worse than younger plants once they are planted out. Count back from the planting date, not forward from the first warm day. If your last frost is around May 10 and tomatoes go out one to three weeks after it, a start in late March or early April is plenty.

What you need

  • Containers with drainage: cell trays, small pots or recycled containers with holes. Wash reused trays with soapy water.
  • Seed-starting mix: a fine, light, soilless mix. Garden soil is too heavy and can carry disease.
  • Light: see below. This is where most indoor seedlings fall short.
  • Warmth for germination: a heat mat helps for peppers, eggplant and other warm-season crops. Our table notes that peppers germinate best with bottom heat around 75–85°F.
  • Labels: seedlings of different tomato or pepper varieties look identical.

Light: the key to sturdy seedlings

A sunny windowsill is rarely enough. Even a south-facing window gives far less light than outdoors, and seedlings lean and stretch toward it. Leggy, pale seedlings with long, thin stems are almost always a sign of too little light.

  • Use grow lights or ordinary LED or fluorescent shop lights. Seedlings don’t need special colours.
  • Keep lights close: typically a few inches above the tops of seedlings for fluorescent and many LED shop lights. Raise them as plants grow. Strong LED fixtures may need more distance, so follow the manufacturer’s guidance.
  • Run them 14–16 hours a day on a timer. Plants also need a dark period.
  • Move air: a small fan on low for part of the day strengthens stems and reduces disease.

Sowing

  1. Moisten the mix before filling containers, so it is damp but not dripping.
  2. Sow two or three seeds per cell, at the depth on the seed packet. A common rule is two to three times the seed’s diameter. Some seeds, such as many lettuces, need light to germinate and should only be pressed onto the surface.
  3. Cover trays with a clear lid or plastic to hold humidity until seedlings emerge, then remove it.
  4. Once seedlings are up, thin to the strongest one per cell by snipping the others at soil level. Pulling them disturbs the roots of the keeper.

Watering and feeding

  • Keep the mix evenly moist, not wet. Overwatering is a common cause of seedling death.
  • Bottom watering, pouring water into the tray and letting cells soak it up for 20–30 minutes before draining the excess, keeps the surface drier and reduces damping-off.
  • Feed lightly once seedlings have their first true leaves, which come after the first pair of seed leaves. Use a diluted liquid fertilizer, following the label for seedlings. Seed-starting mixes contain few nutrients.

Damping-off

Damping-off is a group of soil fungi that attack seeds and young seedlings. Seedlings suddenly collapse at the soil line, as if pinched. Once it starts, affected seedlings cannot be saved. Prevention:

  • Use fresh, sterile seed-starting mix and clean containers.
  • Don’t overwater, and water from below.
  • Give good air movement and remove humidity lids once seeds sprout.
  • Don’t sow too thickly.

Potting up

Fast-growing seedlings such as tomatoes often outgrow small cells before it is time to plant out. When roots fill the cell, or the plant is two to three times as tall as the cell is wide, move it into a larger pot. Tomatoes can be buried deeper when potted up, which gives a sturdier plant.

Before they go out

Seedlings need a week or so of gradual exposure to sun, wind and cool nights before transplanting. See hardening off seedlings. Then check your frost date and soil temperature before planting.

Related

Written for SowByZip. Last reviewed 2026-10-20. See about this site for how pages are made.

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