Starting seeds indoors: from seed tray to garden

When to start: count back from the last frost

Every timing decision starts with one date: your area's average last spring frost. Count back from it by the number of weeks each crop needs indoors. Our Sowing calendar tool does this count for your location.

• Peppers and eggplant: 8–10 weeks before the last frost; they germinate and grow slowly.
• Tomatoes: 6–8 weeks before.
• Basil: about 6 weeks before.
• Lettuce: 4–6 weeks before; it can go out a little before the last frost.
• Cucumber and zucchini: only 3–4 weeks before; they grow fast and dislike root disturbance.

Starting too early is a common trap: seedlings outgrow their pots, stretch and get stressed before it is warm enough outside. When in doubt, start a week later, not earlier.

What you need

• Seed trays or cell trays: Plug trays with individual cells make transplanting easy. Clean, reused pots and yogurt cups work too, as long as they have drainage holes.
• Seed-starting mix: A fine, light, sterile mix. Do not use garden soil; it compacts and can carry disease.
• Labels and a pencil: Write the variety and sowing date. Many seedlings look identical.
• Light: A bright south-facing window, or better, an LED grow light or shop light.
• A clear lid or plastic wrap: Keeps humidity high until seeds germinate.
• A spray bottle and a tray without holes for bottom watering.
• Optional: A seedling heat mat with a thermostat for peppers and eggplant.

Sowing depth and germination temperature

A rule of thumb: sow a seed about two to three times as deep as it is wide. Tiny seeds are just pressed onto the surface. The values below are starting points (estimated); always check the seed packet.

Step-by-step sowing

The whole job takes less than an hour for a few trays:

1. Moisten the mix: Put the seed-starting mix in a bowl and add warm water until it feels like a wrung-out sponge, damp but not dripping.
2. Fill the cells: Fill each cell to the top, tap the tray to settle the mix and press lightly. Do not pack it hard.
3. Sow: Make a hole at the right depth and drop in 2–3 seeds per cell. Large seeds like cucumber and zucchini: 1–2 per cell.
4. Cover and label: Cover the seeds with mix (or press tiny seeds onto the surface) and add a label with the variety and date.
5. Keep warm and humid: Cover the tray with a clear lid and place it somewhere warm. Seeds do not need light to germinate, but check them daily.
6. Uncover at first sprouts: As soon as the first seedlings appear, remove the lid and move the tray under bright light the same day.

Light: window or lamp?

Seedlings that lack light grow tall, thin and pale, leaning toward the window. This stretching (legginess) cannot be reversed, so good light from day one is the most important factor after germination.

Watering from below

Seedlings need evenly moist, never soggy, mix. The easiest way is bottom watering: pour about ½ in of water into a solid tray under the cells and let the mix soak it up for 15–20 minutes, then pour away what is left (estimated). This keeps the surface drier, which helps prevent damping off.

• Check daily by lifting the tray; a light tray needs water.
• The surface may look dry while the cells are still damp below.
• Use room-temperature water.
• A gentle fan on low for a few hours a day dries the surface and makes stems stronger.

⚠️ Constantly wet mix and stagnant air are the main causes of damping off. Water only when needed and never leave the tray standing in water.

Thinning and potting on

If you sowed more than one seed per cell, thin to the strongest seedling once the first true leaves appear. Snip the others at soil level with scissors rather than pulling them, which would disturb the roots.

Seedlings sown in an open tray, or outgrowing small cells, need potting on (pricking out) into individual pots about 3–4 in wide:

• Water an hour beforehand.
• Ease each seedling out with a pencil or plant label, keeping as much root as possible.
• Always hold seedlings by a leaf, never by the stem; a damaged leaf regrows, a crushed stem does not.
• Tomatoes can be planted deeper, up to the first leaves; they root along the buried stem.

Feeding seedlings

Seed-starting mix holds few nutrients. Once seedlings have one or two true leaves, they benefit from a light feed.

• Use a balanced liquid fertilizer at half the strength recommended for adult plants, about every 1–2 weeks (estimated). Follow the label of your product.
• Always feed into moist mix, never into bone-dry cells.
• Pale, yellowish seedlings in good light often just need feeding or potting on.
• More is not better: too much fertilizer burns roots and makes soft, sappy growth.

Hardening off and planting out

Indoor seedlings have never felt sun, wind or cold nights. Hardening off gets them used to the outdoors over 7–14 days (estimated):

• Days 1–3: 1–2 hours outside in shade, sheltered from wind.
• Days 4–7: Lengthen the time and add morning sun.
• Days 8–14: Full days outside; leave them out overnight if nights are mild.

Planting out: Wait until the frost risk has passed and the soil has warmed. Tomatoes, peppers, eggplant, basil, cucumber and zucchini want nights above about 50 °F (estimated). Plant on a cloudy day or in the evening, water well and keep the soil moist for the first week.

Common problems and fixes

Frequently asked questions

Crops to start from seed

TomatoPepperEggplantBasilCucumberLettuceZucchini