Seed Starting Basics – Trays, Mix, Water and Light
Starting seeds indoors is five decisions: what to fill, how deep, how warm, how wet, how much light. Get the order right and the rest is waiting.
TL;DR
- Seed starting mix, never garden soil. Sown at two to three times the seed's own depth.
- Warmth and a dome until the first leaves open, then the dome comes off for good.
- Light the day they sprout, a few inches above the tray, 14-16 hours a day.
Seed starting, step by step
1. Fill the tray with seed starting mix
Use a seed starting mix, not garden soil and not potting compost straight from the bag. It is light, holds moisture evenly and has enough drainage that a beginner cannot easily drown the roots. Fill to just below the rim, firm it gently, then water the tray from the bottom before sowing.
2. Sow at the right depth, not deeper
The rule is two to three times the seed diameter. Tomato and pepper seeds sit about 1 cm deep; lettuce and basil barely need covering. Deeper sowing is the most common beginner failure — the seed runs out of stored energy before the shoot reaches the surface.
3. Keep the tray warm and covered until sprouts show
Germination needs warmth more than light. A clear dome holds humidity; a heat mat takes soil to roughly 70-75F, which is what peppers and eggplant need to come up at all. Remove the dome the moment the first seedlings open their leaves, or damping off moves in.
4. Water from below and let the surface dry between
Bottom watering keeps the foliage dry and stops the surface from growing mould. Add water to the tray, let it soak up, then pour off what is left. A seedling tray should feel like a wrung sponge, never a puddle.
5. Put light on them the day they sprout
Once the first leaves open, the clock starts: without strong light the stems stretch thin and pale within days. A shop light or LED panel a few inches above the tray, run 14 to 16 hours a day, keeps them compact. Raise the light as they grow; keep it close as they can take.
What actually decides success
Light and water pull in opposite directions. Weak light makes seedlings stretch; too much water makes them rot at the base. The way through is to keep the light close and the water low — dry surface, moist root zone — rather than splitting the difference on both.
Temperature is the third lever and the one beginners skip. Germination happens on a heat mat or above a radiator; growing on happens cooler, at ordinary room temperature. A tray kept at germination heat after sprouting grows fast and soft, which is exactly the plant that collapses in its first cold night outdoors.
Space is the fourth. A 72-cell tray fills a windowsill; twenty tomato plants do not. Decide how many plants you will actually put in the ground before you sow, then add 20 percent for the ones that fail — the tray guide covers cell counts, and the calendar tells you the dates.
Tools for this job
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Seed starting mix
Light, free-draining and sterile — the one input worth buying rather than improvising.
72-cell seed tray with dome
The standard size for a windowsill or a single shelf of lights.
Related guides
Frequently asked questions
When should I start seeds indoors?
Use the seed starting calendar: it counts back from your last frost date by crop. Tomatoes take six to eight weeks, peppers eight to ten, cucumbers three to four. Starting earlier than that does not give you an earlier harvest, it gives you an overcrowded tray in March.
Do I need a heat mat?
Only for warm-season crops. Tomatoes germinate at room temperature; peppers, eggplant and okra often sit for weeks without extra bottom heat. Once the seedlings are up, the mat comes off — warm roots plus weak light is what makes leggy plants.
Can I use potting soil instead of seed starting mix?
It works, but it is heavier and holds more water, which makes overwatering easier and can crust over the surface. If potting soil is all you have, mix it half and half with perlite and water more carefully.
Why did my seedlings die right after sprouting?
Usually damping off: a fungal collapse at the soil line, caused by a wet surface, stale air and a dome left on too long. Take the cover off at first sprout, water from below and let air move around the tray.
How many seeds should I sow per cell?
Two, then thin to one when both are up — but only if your germination rate is uncertain or the seed is old. Fresh seed sown one per cell avoids the thinning step and the root damage that comes with it.
When do seedlings go outside?
Not straight from the windowsill. They need hardening off: an hour outside in shade on day one, building to full days over seven to ten days, and only after the frost date for that crop. The calendar gives you the date; the process takes a week.