A sunny windowsill is nowhere near enough: a realistic seed-starting setup

Seed catalogs arrive in January, which is the cruellest possible timing. By the first week of February most of us have sown something, and by the middle of March we are looking after a windowsill of pale, floppy, root-bound plants that still have six weeks to wait before they can go anywhere.

Sowing indoors works well. It just runs on arithmetic rather than enthusiasm, and the arithmetic starts at the other end of the process: the date it is safe to plant out, minus the time each crop needs to reach transplant size.

Count backward, and be honest about the date

Find the average last frost date for where you live, then work backward from it. The word to notice is average: it is a midpoint, which means that in roughly half of all years the last frost comes later than that. If a crop is expensive in time or you only get one shot at it, add a week of margin rather than betting on the average.

Onions and leeks from seed	10 to 12 weeks. The earliest thing worth starting
Peppers and eggplant	8 to 10 weeks. Slow to germinate and slow after that
Tomatoes	6 to 8 weeks. Earlier just gives you a bigger plant to look after
Brassicas, lettuce, most annual flowers	4 to 6 weeks
Cucumbers, squash, melons	2 to 3 weeks and no more. They grow fast and resent root disturbance
Carrots, parsnips, beets, radishes, beans, peas	Do not start indoors. Sow where they are to grow

The root crops are on that last line for a physical reason: a taproot that gets bent or broken during transplanting produces a forked, stunted carrot, and no amount of care afterward straightens it out. Cucumbers and squash are on it for a different reason, which is that three weeks is genuinely all they need. A squash sown in March is a sprawling, yellowing thing by May, and a squash sown in early May overtakes it within a fortnight of both going in the ground.

Warmth germinates, light grows

These are two separate requirements that happen in two different places, and treating them as one thing is behind a lot of poor results. Germination is mostly about soil temperature. Peppers are sluggish and erratic below about 70 degrees Fahrenheit and much happier nearer 80, which is why a heat mat under the tray transforms them and does very little for lettuce, which germinates happily in a cool room.

The moment green shows above the surface, both the heat mat and the humidity dome have done their job and should come off. From that point the seedling needs light more than anything else, and a warm dark shelf produces exactly the stretched, weak growth that people blame on bad seed.

The windowsill problem

A south-facing window feels bright because your eyes adjust across an enormous range. Compared with open sky it is dim, and it delivers what it has from one side only. Seedlings respond to both facts by doing the same thing: they stretch, reaching for the light, producing a long thin stem carrying a small pair of leaves. That plant will never recover its proportions, and it usually falls over first.

The fix is a light bar hung a few inches above the leaves and raised as the plants grow, run fourteen to sixteen hours a day on a timer. Distance matters more than the number on the box, because intensity drops away sharply as you pull a fixture back, and two inches above the canopy with a modest light beats a foot above it with a bright one. There is one more trick that costs nothing: brush your palm across the tops of the seedlings once a day, or point a small fan at them on low. Plants that get moved around grow shorter and thicker stems, and the airflow suppresses disease at the same time.

Damping off, and why it is a prevention problem

One morning a tray of healthy seedlings is lying flat, each stem pinched and darkened right at the soil line. That is damping off, caused by soil-borne fungi that thrive in cool, wet, still, crowded conditions. There is no treatment worth attempting once it has taken hold in a tray. Everything useful happens beforehand.

• Wash and rinse reused pots and trays properly. The organisms overwinter in the crust of last year's mix.
• Use fresh seed-starting mix rather than garden soil or the open bag that has been sitting in the shed since August.
• Sow thinly. A dense mat of seedlings holds moisture between the stems and never dries out.
• Water from below, letting the tray take up what it needs and pouring off the rest, and let the surface dry between waterings.
• Keep air moving. A fan on the lowest setting for a few hours a day is enough.

Depth, and moving up a pot

The workable rule for sowing depth is to cover a seed with about twice its own thickness of mix, which for most vegetable seed means a quarter of an inch and for something dust-fine means practically nothing. A few things need light to germinate and must not be buried at all: lettuce, snapdragons and poppies among them. Press those onto the surface so they make contact, and leave them uncovered.

The first pair of leaves to appear are the seed leaves, and they usually look nothing like the plant. Wait for the first true leaves, and pot on when roots start showing at the bottom of the cell. Tomatoes are the one crop you can plant deeper each time, right up to the lowest leaves, because they grow new roots directly from the buried stem. Do not do this with peppers or brassicas, which simply rot at the point you buried them.

Hardening off is not the optional bit

A plant raised indoors has thin leaves, no wind conditioning and no defense against ultraviolet light. Put it straight into open ground on a bright breezy day and it will bleach, scorch and sit motionless for a fortnight, by which time a direct-sown seedling has caught it up. Give it seven to ten days instead: an hour outside in shade and shelter on day one, building up to a full day, brought back in for the first few nights. Wind does as much of the toughening as sun does, so a spot with some air movement is better than a windless corner, and the whole exercise is over in the time it takes to lose one plant to impatience.