Seed Tray Calculator

How many seed trays do you need?

Seed starting fails most often before a seed is even sown, when a gardener buys too few trays, discovers halfway through sowing that the bench is full, and ends up cramming seedlings into pots they have outgrown. The arithmetic is forgivingly simple: trays multiplied by cells per tray gives total growing spaces, and total spaces plus a spare allowance gives the seeds to buy. The calculator on this page does that maths, but the useful part is deciding how many plants you actually want, because that number is almost always smaller than the optimistic version written in January.

Start from the garden, not the catalogue. Count the planting spaces you genuinely have for each crop, allow for the plants that will be eaten, shaded out, or simply forgotten, and then add a modest margin rather than doubling everything. A family that eats tomatoes twice a week needs far fewer plants than the twenty seedlings that germinate so enthusiastically on a windowsill, and every extra seedling still needs water, light, and potting on, whether or not it ever reaches the garden.

Choosing a cell size

Cell size is a trade-off between bench space and root room, and getting it wrong costs weeks of growth. Small cells, such as seventy-two or one hundred and four cell trays, suit fast crops like lettuce, brassicas, and flowers that will be planted out young, because they pack many plants onto a small heated bench. Large cells, such as twenty-four or forty cell trays, suit tomatoes, peppers, squash, and anything that stays indoors for six weeks or more, because a cramped root system checks growth in a way that extra feeding never fully repairs.

Deep cells, sometimes called root trainers, are worth their extra bench space for sweet peas, beans, and other plants with long tap roots, which resent being transplanted from a shallow cell. Whatever size you choose, check that the tray drains freely, because seedlings standing in water rot faster than seedlings that are slightly dry, and a tray without drainage holes is a nursery for damping off. Standard tray footprints also matter: trays that match your propagator, heat mat, and bench make watering and moving plants far less fiddly.

How many seeds per cell?

One seed per cell is the efficient choice for fresh, reliable seed of crops that germinate well, such as beans, squash, and most brassicas, because it avoids thinning and wastes nothing. Two seeds per cell is the sensible insurance for old seed, slow germinators like parsley and peppers, and tiny seed that is hard to handle, with the weaker seedling snipped out once both are up. Sowing three or more per cell is almost never good economy, because the seedlings compete from day one and the thinning damages the roots of the one you wanted to keep.

Germination rates printed on packets are measured in laboratory conditions, not on a draughty windowsill, so treat them as a ceiling rather than a promise. Seed saved at home or kept for several years germinates less reliably, and some species, notably parsnips, need genuinely fresh seed every year. The calculator's ten percent spare allowance covers normal losses on fresh seed; if your seed is old or the crop is slow, sow a few extra cells rather than trusting the allowance to save a variety you only have ten seeds of.

Sowing depth and compost

The old rule still works: sow a seed about twice as deep as its own size, which means fine seed like lettuce is barely covered at all, while beans go in about an inch deep. Seed compost should be fine, free-draining, and low in nutrients, because rich compost encourages soft, leggy growth and can scorch delicate roots, and the plants will be fed properly once they are potted on. Sieve the top layer for tiny seed, water from below or with a fine rose so the seed is not washed into corners, and label every tray immediately, because unlabelled seedlings all look identical until they flower.

Warmth matters more than light until germination, and most vegetables germinate fastest between eighteen and twenty-four degrees, which is why a heated propagator or the top of a warm cupboard beats a cold windowsill in early spring. Once seedlings emerge, light becomes the priority, and a bright windowsill supplemented by a simple grow light on dull days prevents the pale, stretched stems that never recover. Turn trays daily if light comes from one side, so seedlings grow straight instead of leaning permanently toward the glass.

Watering and damping off

Damping off, the fungal disease that fells seedlings at soil level overnight, is the most common cause of tray failure, and it thrives in wet, still, overcrowded conditions. Water in the morning so the surface dries during the day, water from below where possible, and give seedlings air movement, even a gently open window or a small fan, because stagnant humid air is exactly what the fungus wants. Thin promptly, remove any collapsed seedling at once, and never reuse compost from a failed tray for new seed.

Potting on is a timing judgement, not a calendar date. Move seedlings when roots fill the cell but before they circle tightly around the bottom, because a root-bound plant sulks for weeks after planting out. Water the tray an hour before potting on so the root ball holds together, handle seedlings by a leaf rather than the stem, since a crushed stem never heals, and plant them slightly deeper than they were, which most vegetables tolerate and tomatoes actively enjoy.

Hardening off before planting out

Seedlings raised indoors are soft in a way that garden plants are not, and moving them straight outside on a bright, windy day can scorch leaves and check growth for weeks. Harden them off over seven to ten days, starting with a couple of sheltered hours outside and building up to full days and eventually a night outdoors, bringing them in if frost threatens. A cold frame makes this easy, but a sheltered corner by the house wall works too, provided the trays are not forgotten during a sudden cold snap.

Plant out on a dull, calm day or in the evening, water the seedlings well beforehand and again after planting, and protect young plants from slugs, which can erase a tray of brassicas in a single night. A ring of grit, a beer trap, or a simple cloche buys the plants time to establish, and established plants withstand pests far better than fresh transplants. Keep the labels in the garden too, because by midsummer every green plant looks alike, and the label is the only honest record of which variety earned its place.

Seed starting checklist

• Count real planting spaces before ordering seed, not hopeful ones.
• Match cell size to how long the crop stays indoors.
• Sow one seed per cell for fresh seed, two for old or slow seed.
• Use free-draining seed compost and label every tray at sowing.
• Give warmth before germination and strong light after it.
• Water in the morning, thin promptly, and harden off for a week.

Good seed starting is mostly planning and patience: the right number of trays, the right cell size, fresh seed, and a steady routine of light, water, and air. The calculator gives you the numbers; the daily check on the windowsill gives you the plants.