Seed-Starting Calculator

How to use the seed-starting calculator

Starting seeds indoors is how gardeners buy extra weeks of season. A tomato started indoors in March is already a sturdy young plant when the frosts end in May, while a tomato sown outdoors in May is still a seedling — the head start is worth a month or more of harvest. This calculator turns that idea into dates: give it your average last frost date and a crop, and it counts backwards by the standard number of weeks for that crop to give you an indoor sowing window, and forwards to give you a transplant window.

Where the weeks come from

The weeks are standard extension guidance, and they follow the crop's nature. Tender crops that need a long warm season — tomatoes, peppers, eggplant — are started six to eight weeks before the last frost, because they grow slowly indoors and need size before they face the outdoors. Hardy and frost-tolerant crops started indoors — brassicas, lettuce, onions — need only four to six weeks, because they grow fast and transplant young without complaint. Crops that are direct-sown only, such as carrots, beans, peas and corn, are left out of indoor starting altogether: their roots dislike disturbance, or they germinate so fast outdoors that indoor starting gains nothing. The calculator is honest about that instead of giving you a date that would do more harm than good.

Your last frost date is the anchor, so it is worth getting right. Use the mid-point of your zone's published range as a starting point, then adjust from experience: if your garden sits in a frost pocket, add a week; if it is a sheltered, south-facing plot, you may get away with a few days earlier. The date you enter is an average, and in any given year the real last frost can fall two weeks either side of it — which is why the calculator gives you a window of dates rather than a single day, and why hardening off and frost protection matter more than hitting an exact date.

Starting seeds well

Dates are only half the job; conditions are the other half. Seeds need warmth to germinate and light to grow on — a heat mat or the top of a fridge helps germination, and a bright windowsill or a simple grow light prevents the pale, stretched stems that doom so many indoor seedlings. Use fresh seed-starting mix, not garden soil, which is too heavy and can carry damping-off disease. Sow thinly, water from below or with a fine mist, and give seedlings air movement — a small fan on low, or a daily brush of the hand over the tops, builds sturdier stems. Feed lightly once the first true leaves appear, with a half-strength liquid feed.

The most important step is the last one: hardening off. A week before transplanting, put seedlings outside for a few hours a day, increasing the time and exposure, and bring them in at night. A seedling moved straight from a warm room to full sun and wind will wilt, scorch or stall for weeks. Transplant on a cloudy day or in the evening, water well, and keep row cover handy for the first cold nights. Done right, a transplanted seedling barely notices the move; done wrong, it can take a month to recover — which wipes out the whole point of starting early.

A final note on how many plants to start: always sow more than you need. Not every seed germinates, not every seedling survives transplanting, and the extras are your insurance. Start a third more than the bed requires, keep the strongest, and give the spares away or pot them on — a neighbour will always take a spare tomato plant in May. Label everything as you sow it, with the variety and the date: six weeks later, when a tray of green seedlings all looks the same, you will be glad you did. And keep the seed packets — they carry the germination and maturity figures this calculator uses, and they are the first thing to check when a sowing behaves differently from the plan.

Related tools: find your frost date with the Frost Date Calculator, check how long seeds take to sprout with the Germination Time Calculator, and work out when your crop will be ready with the Harvest Date Calculator. For month-by-month sowing lists, see the planting calendar hub.