Tomato Seedlings – From Sowing to Planting Out
A tomato seedling is nine-tenths light management. Get the light close and the timing right, and the plant looks after itself until planting out.
TL;DR
- Sow six to eight weeks before planting out, at about 1 cm deep, in a seed starting mix.
- Light two to four inches above the leaves, 14-16 hours a day, from the day the first leaves open.
- Pot on once, burying the stem to the seed leaves; plant out after the frost date and after hardening off.
Sowing and the first month
Sow into damp seed starting mix, one or two seeds per cell, covered with about a centimetre of mix. Tomatoes germinate at ordinary room temperature — a heat mat makes them faster but is not required. Keep the surface damp until the seedlings are up, then take the dome off immediately and put light on them.
The month after germination is where most trays are won or lost. A windowsill in February is not bright enough, so the plant stretches: long pale stem, small leaves, and a lean toward the glass. If you cannot give them a light, sow later instead — a three-week-old seedling under a proper light will overtake a six-week-old one that has been stretching at a window.
Potting on without breaking anything
When roots show at the drainage holes, or the plant has three or four true leaves, move it into a larger pot. Water the tray an hour beforehand so the root ball holds together, then support the seedling by a leaf — never by the stem — and turn the cell out.
Set it deeper than it was growing, up to just below the seed leaves, and strip any leaf that would end up touching the mix. Tomatoes root along buried stem, so this is the one transplant where going deep is the right answer rather than a risk. Water it in, keep it out of direct sun for a day, and it will not notice the move.
The three problems you will actually hit
Stretched stems. Light, not feed. Move the lamp closer, lengthen the day, and when you pot on, bury the stretch. A leggy tomato can be saved; a leggy tomato in a dark room cannot.
Purple undersides. Cold roots. Below roughly 55F the plant struggles to move phosphorus and the leaf goes violet. Warm the room or lift the tray off a cold windowsill; the newest leaves come back green.
Wilting that does not respond to water. Check the roots before you water again — a plant that is wet and wilting is usually drowning, or has lost roots to a cold, soggy mix. Roots should be white; brown ones mean the mix stayed wet too long.
Tools for this job
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Tomato seed starter kit
Tray, dome and cells sized for a windowsill of six to twelve plants.
Deep propagation pots
For potting on without burying half the stem in a shallow tray.
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Frequently asked questions
How many weeks before planting out do I start tomato seeds?
Six to eight weeks. Earlier than that and the plant spends March in a pot it has outgrown, which costs you more than it gains. The seed starting calendar counts the window back from your own frost date.
Why are my tomato seedlings tall and thin?
Not enough light, almost always. A windowsill in February gives a fraction of the light a seedling wants, so the stem stretches looking for more. Move the light to two to four inches above the leaves, run it 14 to 16 hours a day, and raise it as the plant grows.
Why are the leaves turning purple underneath?
Usually cold, not disease. Tomato seedlings held below about 55F at the roots cannot take up phosphorus properly and the underside of the leaf goes violet. Warm the tray up and the colour returns on new growth.
Should I bury the stem when I pot on?
Yes. Tomatoes grow roots along any buried stem, so potting on deeper — up to the seed leaves — builds a stronger root system and shortens a leggy plant. Strip the lowest leaves first so none sit in the mix.
When can tomato seedlings go outside?
After the last frost date for your area, and only after a week to ten days of hardening off. Soil temperature matters as much as air: below about 50F the roots stop working and the plant sits there looking sulky for a fortnight.
Do I need to feed tomato seedlings?
Not until they have true leaves and have been potted on once. A seed starting mix has little nutrition on purpose; after potting on into a richer mix, a weak liquid feed every two weeks is enough until planting out.