Seed Count, Germination & Transplant Planner

How it works

Seedlings = plants × (1 + margin); seeds = seedlings ÷ (germination × survival); cells = seedlings ÷ [1 − (1 − germination)^(seeds per cell)]; sow date = transplant date − weeks; row = plants × spacing

Seed packets are cheap and seedlings are not, so growers sow more than they need and thin: the count starts from the plants wanted, adds a margin for culling weak or damaged seedlings, then divides by the germination rate (printed on the packet, or found by sprouting ten seeds on a damp towel) and by the share of germinated seedlings that survive damping-off and transplanting. Sowing two or three seeds per cell and thinning to one makes almost every cell productive even at modest germination rates, at the price of a few extra seeds. Counting back from the last-frost or transplant date by the crop's indoor growing period gives the sowing date, and the plant spacing tells you whether the garden actually has room for what you are about to raise.

Worked example

For 24 tomato plants with a 15% culling margin you want 28 seedlings; at 85% germination and 90% survival that is 37 seeds, but sowing 2 per cell means 29 cells (98% will have a seedling) and 58 seeds — one 72-cell tray and two 40-seed packets (0.19 g). Sow six weeks before the transplant date; the plants fill 14.4 m of row at 60 cm spacing.

Frequently asked questions