You have the biology, the setup, the light and the mix. Now the actual act: putting seed into medium. It looks like the simplest step in the whole process, and in a sense it is - but a handful of small decisions made here quietly determine whether you get even germination and easy transplants or a patchy, tangled, mistimed mess. Sowing well is not hard; it is just deliberate. Let us make each decision on purpose.
Depth: the one rule that covers almost everything
Beginners agonise over how deep to plant each kind of seed. There is a single rule that handles the vast majority of them: sow a seed about two to three times as deep as it is wide. That is it. A big fat bean, a centimetre across, goes a couple of centimetres down. A tomato seed, a millimetre or two across, goes a few millimetres down - barely covered. A dust-fine seed like lettuce, basil or many flowers goes on the surface or under the thinnest scatter of fine mix or vermiculite.
The rule works because of the packed lunch. A seed can only push its shoot up as far as its stored energy allows before it must reach the light and start feeding itself. A big seed has a big lunch and can afford to climb from depth; a tiny seed has almost nothing and must be near the surface or it exhausts itself in the dark and dies before emerging - the classic cause of “my seeds never came up.” When in doubt, err shallow: a seed sown too shallow usually still makes it, while one sown too deep often does not.
Remember too the light-germinators from the first guide - lettuce, many herbs and small flowers that need light to sprout. Sow-by-size handles them automatically: being tiny, they end up on top where the light is. You do not need a separate mental list. Just size the depth to the seed and you will be right almost every time.
Spacing and how many per cell
How many seeds go in each cell or pot depends on how confident you are in the seed and what the crop wants.
For most crops, the aim is one strong seedling per cell. But germination is never 100%, so sowing exactly one seed per cell means gaps. The common practice is to sow two or three seeds per cell and then, once they are up, keep the strongest and remove the rest. Remove, not transplant apart: snip the extras off at soil level with scissors rather than pulling them, so you do not tear the roots of the keeper you are trying to save. This “sow a few, thin to one” approach guarantees a full tray of vigorous seedlings and is worth the tiny extra seed.
A few crops actively like company and are multi-sown on purpose - a small cluster grown and transplanted as one clump. Onions, leeks, scallions, beets, and many quick salad and herb crops do well multi-sown: four to eight onion seeds in a cell become a clump that goes out together and simply pushes each other apart as they swell, saving enormous time and space versus single plants. It is a genuine technique, not a shortcut, and once you know it you will use it every year for the alliums especially.
Some crops do not want to be started in cells at all. Root crops that resent disturbance - carrots, parsnips, most direct-sow roots - are usually sown straight into the garden, because transplanting deforms the root. Big, fast crops like beans, squash and cucumbers are often direct-sown too once the soil is warm, since they resent root disturbance and grow so fast that the indoor head start is small. Know which crops earn a tray (the slow, frost-tender, transplant-tolerant ones - tomatoes, peppers, brassicas, lettuce, alliums, many herbs and flowers) and which are better sown where they will grow.
Timing: count back from your last frost
This is the decision that separates a smooth spring from a crisis. Start too early and your seedlings outgrow their pots, go leggy and root-bound, and languish indoors for weeks waiting for a garden that is not ready - peaking in your living room instead of in the ground. Start too late and you lose the head start entirely. The fix is to count backwards from your last spring frost date, not forwards from your impatience in January.
Every transplant crop has a recommended number of weeks to start indoors before the last frost - typically found on the seed packet and in our seed chart. Peppers and eggplant, slow to grow and very frost-tender, might be eight to ten weeks before last frost. Tomatoes, faster, around six weeks. Brassicas four to six. Fast crops like cucumbers and squash just two to three weeks, because they grow so quickly that starting earlier only produces a pot-bound plant. Find your region’s last frost date, mark it on a calendar, and count back the right number of weeks for each crop to get its sowing date. Our frost-date sowing calendar does this arithmetic for you if you give it your frost date, and the seed chart lists the lead times crop by crop.
The discipline this imposes is worth stating plainly: the urge to sow everything on the first exciting weekend of the year is the enemy. Different crops have different sowing dates spread across many weeks. Sowing them all at once because you are keen guarantees that half of them are the wrong age at transplant time. Trust the count-back. Sow each crop on its date, not on your mood.
Labels: the five-second habit that saves the season
Label every tray, every cell block, the instant you sow it. This sounds trivial and is the advice most often ignored and most often regretted. Seedlings of different varieties - three kinds of tomato, four of pepper - look identical for weeks. Un-labelled, you will lose track by the second week and spend the season guessing which plant is which, discovering only at harvest that you have no idea what you grew or where to plant it.
Write the variety and the sowing date on a waterproof label in pencil or permanent marker (many “permanent” markers fade under grow lights, so pencil on plastic or wood is safest). The sowing date is not decoration: it lets you judge whether germination is slow or normal, know when to expect true leaves, and plan the transplant date. A photograph of the labelled tray on your phone is a cheap backup. Thirty seconds of labelling at sowing prevents weeks of confusion later.
Keep a record - next year’s gardener will thank you
The most underrated tool in seed starting is a simple notebook or spreadsheet. Each time you sow, jot the date, crop, variety, how many cells, and later the germination result and transplant date. It feels like busywork the first year. By the second year it is gold: you know that your peppers really need those extra two weeks, that a particular variety germinates poorly and should be sown thicker, that you always start tomatoes a fortnight too early and pay for it. Gardening is an annual feedback loop with a very slow clock - one attempt per year - and the only way to learn fast from it is to write down what you did so next spring’s you is not starting from memory and optimism.
None of this is difficult. Sow to size for depth, a few seeds per cell thinned to the best, the right crops in trays and the rest direct, on dates counted back from your frost, labelled and logged. Do it deliberately and you have set up an even, well-timed, traceable batch of seedlings. Now comes the phase where more of them live or die than any other: the first week after sowing, which is all about water and heat.