How to start seeds indoors, from sowing to planting out

Starting seeds indoors gives slow, frost-tender plants a head start. Most failures come from two things: seedlings that stretch because the light is too weak, and seedlings that collapse from damping off, a disease that thrives in crowded, damp, still conditions (RHS). The steps below are built to avoid both.

How to start seeds indoors

1. Count back from your last frost date.

   

   Colorado State University Extension says most seeds should be started 4 to 8 weeks before the last killing frost. Look up the usual last frost date for where you live, then check each packet: fast growers need less time indoors than slow ones.

   You should see: a sowing date for each packet, so seedlings are ready to go outside soon after the frost risk has passed.

2. Fill a clean tray with fresh compost and firm it.

   

   Fill to just below the rim, including the corners, then press it level with another tray or a piece of wood to about 1 cm below the rim (RHS). Use pots or modules for big seeds and trays for small ones (RHS).

   You should see: a flat, evenly firm surface with no hollows, about 1 cm below the rim.

3. Sow thinly, then cover the seeds lightly.

   

   Scatter a small pinch thinly and evenly; sow big seeds one at a time (RHS). Thin sowing matters: the RHS says crowding raises the risk of damping off.

   Cover with sieved compost. The RHS says usually no deeper than the seed itself; Utah State gives a rule of 2 to 3 times the seed's diameter. Both say the packet decides.

   You should see: seeds spaced apart, not touching, under a thin, even layer of compost.

4. Water gently, from below or with a fine rose.

   

   Water with an upside-down rose so you don't wash the seeds about, or stand the tray in water and lift it out once the compost is damp throughout (RHS). Colorado State suggests misting with a fine hand sprayer. The RHS suggests mains water where possible for seedlings.

   You should see: compost that has turned dark all over, with no puddles and no seeds washed into a corner.

5. Cover the tray and keep it warm until shoots appear.

   

   Put a propagator lid, glass or clear plastic over the tray to hold in moisture (RHS). The RHS says most seeds need about 18 °C to germinate; Colorado State says most germinate best at 70 to 75 °F (about 21 to 24 °C), and tomatoes and peppers better with the compost close to 80 °F (about 27 °C). Check the packet, and look every day.

   You should see: the first shoots. How long that takes varies by seed; the packet usually says.

6. As soon as shoots appear, take the cover off and give strong light.

   

   Remove the lid as soon as seedlings show (RHS). Then light is everything. Without a bright window, Colorado State suggests a fluorescent light 3 to 4 inches (about 7.5 to 10 cm) above the seedlings for 12 to 16 hours a day; Utah State suggests 2 to 3 inches (5 to 7.5 cm) above and at least 12 to 14 hours, raising the light as they grow.

   On a windowsill, the RHS suggests a south- or west-facing one. Good airflow helps too: the RHS says to keep seedlings well ventilated to reduce humidity.

   You should see: short, sturdy seedlings standing upright, not pale, thin stems leaning towards the light.

7. Water when the compost starts to dry, and feed once true leaves appear.

   

   Never let the compost dry out completely, and never keep it saturated (RHS). Utah State says moist compost looks dark brown to nearly black and dry compost light brown, and a tray that feels noticeably lighter needs water.

   The first two leaves are seed leaves. Once the true leaves (the next ones, shaped like the adult plant's) appear, Colorado State suggests a water-soluble fertiliser at a quarter to half strength.

   You should see: compost that's moist but not wet, and seedlings with dark green true leaves.

8. Move the seedlings into their own pots once you can handle them.

   

   This is called pricking out. The RHS says to move seedlings into individual pots once they're large enough to handle, before they crowd each other; Utah State says to do it when their leaves start to overlap. Hold each seedling by a leaf, not the stem, and plant it at the same depth it was growing (Utah State).

   You should see: one seedling per pot, upright, watered in, with the compost at the same level on its stem as before.

9. Careful: frost-tender plants such as tomatoes can't go outside until the risk of frost has passed (RHS). Bring them in on cold nights: Utah State says to keep warm-season crops above 45 to 50 °F (about 7 to 10 °C).

   Harden them off for about two weeks, then plant out.

   

   Hardening off gets indoor plants used to wind, sun and cooler air. Colorado State and Utah State say to start about two weeks before planting out; the RHS says it takes about two to three weeks. Utah State says to begin with a few hours outside in the shade, add time each day, and gradually move them into full sun.

   Plant out in the morning or on a cloudy day, and water straight after (Utah State).

   You should see: plants that stay upright outside all day. Some yellowing or browning of leaves during hardening off is normal (Utah State).

Light and warmth at a glance

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