Starting Seeds Indoors

When should I start seeds indoors?

Work backward from your average last spring frost date. Each seed packet or extension planting guide gives a start window, usually stated as a number of weeks before the last frost. Starting too early produces tall, stressed plants that outgrow their pots before the weather allows planting out.

Slow crops such as peppers and some herbs need a longer head start, while fast ones such as squash and cucumbers need only a few weeks. Some crops, such as beans, peas and root vegetables, do better sown directly outdoors, so skip starting them inside unless you have a reason.

Write the dates on a calendar and adjust each year after you see how the weather behaved. Your county extension office can give you local frost dates and a seed-starting calendar.

What light do seedlings need?

Light is the most common limit. A windowsill rarely gives enough, especially in winter, and seedlings stretch toward the glass and fall over. Grow lights placed close above the plants and left on for long hours each day produce short, sturdy growth.

Keep the lights a short distance above the leaves and raise them as plants grow. Use a timer so the schedule stays steady. Read the light manufacturer's instructions, and follow electrical safety guidance: keep cords and plugs away from water and use equipment rated for the conditions.

Reflective surfaces such as a white wall or foil-free white board beside the tray bounce light back to the plants, and rotating trays each day keeps growth even.

• Lights close above seedlings, raised as they grow
• A timer for regular day length
• Sturdy stems mean enough light
• Tall, pale, floppy plants mean too little

How warm should the seed tray be?

Most vegetable seeds sprout best in warm soil, warmer than comfortable room air in many homes. Seed packets and extension guides list ideal soil temperatures by crop. Heat mats made for seed starting can help, and a thermometer in the mix tells you what is actually happening.

Once seedlings are up, most prefer slightly cooler air, which keeps growth compact. Remove heat sources when seeds sprout unless the crop calls for continued warmth.

How do I sow and care for the seeds?

Sow at the depth listed for the crop, since too deep and weak seeds never reach the surface, and too shallow and they dry out. Moisten the mix before sowing, label every pot or cell with the variety and date, and cover trays with a clear lid or wrap until sprouts show.

Once seeds sprout, remove the cover to improve air flow. Water from below or with a gentle spray, and let the surface dry slightly between waterings. Constantly soggy mix invites damping-off, a fungal problem that kills seedlings at the base.

Thin to one plant per cell by snipping the extras at the soil line instead of pulling, which disturbs the roots of the keeper.

Do seedlings need feeding?

Most seed starting mixes have little nutrition. Once true leaves appear, a diluted fertilizer labeled for seedlings may help, but follow the label and never exceed it. Over-fertilizing can burn roots and produce weak, soft growth.

If seedlings are pale or stunted, check light, temperature and watering first, since those are more common causes than hunger.

A spare label with the crop name and a quick note on what you fed and when makes it easy to repeat what worked next year.

What are the common mistakes?

Starting too early, too little light, overwatering, reused dirty containers, crowded seedlings and skipping the step of hardening off before planting out are the usual ones. Another is sowing crops that resent transplanting.

Air movement helps prevent disease. A small fan on low, running part of the day, strengthens stems and dries the surface. Keep the area clean and remove any seedlings that collapse.

Pot up crops that stay indoors longer into slightly larger containers once they have true leaves and crowded roots, handling them by the leaves.

What should I do about disease and pests indoors?

Damping-off and fungus gnats both come from wet, poorly drained mix. Let the surface dry between waterings, use clean containers and fresh mix, and avoid overcrowding. Remove affected seedlings promptly.

If you see persistent insect problems, ask your extension office what is safe to use indoors, and read and follow any label exactly. Keep the seed starting area away from stored chemicals and out of reach of children and pets.

Last reviewed: November 12, 2026. Written by the Homestead & Small Farm Editorial Team. See our editorial policy.