Free seed starting calculator: enter your last frost date and crop to get your indoor sowing window, transplant date, and hardening-off start.
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Enter your average last frost date, which is the spring date after which frost is unlikely in an average year, and pick a crop. The calculator counts back from that date by the crop's indoor lead time to give a sowing window, not a single day, because seed packets give ranges such as 6 to 8 weeks. It then applies the crop's transplant offset, before the last frost for hardy crops such as broccoli and cabbage, after it for tender crops such as tomatoes and peppers. The hardening-off start is set 7 days before transplanting, so seedlings can adjust to outdoor sun and wind gradually. Results are planning estimates: always check the short-range forecast before moving tender plants outside, and remember that the average last frost date is a 50 percent date, meaning frost arrives later than that date in about half of years.
The count-back formula is simple: sowing date equals last frost date minus weeks before last frost multiplied by 7 days. For tomatoes at 6 to 8 weeks before a 15 May last frost, the window is 20 March to 3 April. For peppers at 8 to 10 weeks, the window is 6 March to 20 March. Using a window instead of a single midpoint respects the range printed on the packet and gives you room to sow in two batches a week apart as insurance. If you sow at the early end of the window, seedlings may need potting on before transplanting; if you sow at the late end, plants are smaller but sturdier at transplant time.
Step 1: find your average last frost date from your national weather service, a planting calendar, or your local extension service, or by entering your zip code into a frost date lookup. Step 2: pick your crop from the list. Step 3: read the four dates as one schedule: sow inside between the two sowing dates, start hardening off on the hardening date, and transplant on the transplant date. Step 4: if a late frost is forecast after you planned to transplant tender crops, hold them indoors or cover them overnight. Keep a notebook of the dates you actually used, because your garden's microclimate, a frost hollow, a north-facing slope, or city warmth, shifts the real date by days or weeks.
Tomatoes need 6 to 8 weeks indoors and transplant 1 to 2 weeks after the last frost. Peppers need 8 to 10 weeks and transplant 2 weeks after. Eggplant needs 8 to 10 weeks and transplants 2 to 3 weeks after. Broccoli, cabbage, and cauliflower need 4 to 6 weeks and transplant 2 to 4 weeks before the last frost, since they tolerate light frost once hardened. Celery needs 10 to 12 weeks and transplants 1 week after. Onions and leeks need 8 to 10 weeks and transplant 2 to 4 weeks before. Lettuce needs 4 to 5 weeks and transplants about 3 weeks before, or can be direct sown. Cucumbers and squash need only 3 to 4 weeks and transplant 1 to 2 weeks after, and are often direct sown because they dislike root disturbance. Basil needs 6 weeks and transplants 1 week after. Marigolds need 4 to 6 weeks and transplant 1 week after. Parsley needs 6 to 8 weeks and transplants 1 week after.
Use this seed starting chart as your planting calendar for the season, and copy the dates for each crop you grow into a single schedule.
Some seeds should never be started indoors: beans, carrots, beets, radishes, corn, and peas all grow better sown directly where they will grow, because their roots resent transplanting or they germinate fast in warm soil. Other crops repay indoor starting because they need a long warm season: tomatoes, peppers, eggplant, celery, and onions all benefit from weeks of protected growth. Between the two groups sit crops that work either way, such as lettuce, kale, and squash, where indoor starting buys earliness and direct sowing saves effort. When in doubt, read the packet: it usually says start indoors, direct sow, or both, with the weeks to use.
Hardening off is the 7 to 10 day process of moving seedlings outside gradually so sun, wind, and cool nights do not shock them. Start with a sheltered, shady spot for an hour or two, then increase sun and time each day until plants stay out all day and, if frost-free, overnight. This calculator sets the hardening start 7 days before transplanting as a practical default; in windy or very sunny gardens, allow the full 10 days. Water slightly less and hold back fertiliser during hardening, handle seedlings by their leaves rather than their stems, and transplant on a cloudy day or in the early morning when possible.
Seeds sprout by soil temperature, not by the calendar. Most vegetable seeds germinate in 5 to 14 days at a steady 18 to 24 degrees Celsius, while peppers prefer warmer soil at 24 to 29 degrees and can take 7 to 21 days, and parsley can take 14 to 28 days. Use a seed starting mix, not garden soil, keep it moist but never waterlogged, and give seedlings 14 to 16 hours of bright light a day, with lamps 2 to 6 inches above the plants, moving the lamp up as they grow. Leggy, pale seedlings usually mean too little light, and starting far too early produces tall, weak plants that struggle after transplanting, so the sowing window matters as much as the date itself.
Germination is fastest when soil temperature stays steady day and night, so a heat mat helps peppers and other warmth lovers, while lettuce germinates better in cooler soil.
A USDA hardiness zone describes the average coldest winter temperature, not the spring frost end, so a zone number alone cannot give your sowing date; use a frost date lookup for your town or state instead. Weather services also publish 10 percent and 90 percent frost dates: the 10 percent date is the safer, later date to use for tender crops if a single late frost would ruin your planting. In frost-free climates, the same count-back logic does not apply; gardeners there plant by season, such as the Kharif, Rabi, and Zaid seasons in South Asia, or by the local rainy season. For a longer harvest, practise succession planting: sow small batches of fast crops such as lettuce and radish every 2 to 3 weeks rather than everything at once.
Count back from your average last frost date by the weeks on the packet: tomatoes 6 to 8 weeks, peppers 8 to 10 weeks, broccoli 4 to 6 weeks, cucumbers 3 to 4 weeks. This calculator gives you that window plus the transplant and hardening dates.
Use the formula: sowing date equals last frost date minus weeks multiplied by 7 days. For a 15 May last frost and tomatoes at 6 to 8 weeks, the window is 20 March to 3 April.
Use your national weather service frost date tables, a planting calendar, or your local extension service, or search a frost date lookup by zip code or town. The average date is the 50 percent date, so frost arrives later in about half of years.
A hardiness zone describes average winter minimum temperature, not the spring frost end. It helps you choose perennial plants that survive winter, but it cannot replace a local last frost date for seed starting.
Start tomatoes 6 to 8 weeks before your last frost date, and transplant them 1 to 2 weeks after it, once nights are reliably warm. For a 15 May last frost, sow between 20 March and 3 April.
Start peppers 8 to 10 weeks before your last frost date, and transplant them about 2 weeks after it. Peppers need warm soil to germinate, ideally 24 to 29 degrees Celsius, and can take 7 to 21 days to sprout.
Hardy crops such as broccoli, cabbage, lettuce, and onions can go out before the last frost once hardened. Tender crops such as tomatoes, peppers, and cucumbers should wait 1 to 2 weeks after it, and always check the forecast for a late frost.
Yes for many crops: beans, carrots, beets, radishes, corn, and peas should always be direct sown, and lettuce, squash, and kale work either way. Tomatoes, peppers, eggplant, and celery repay indoor starting with a much earlier harvest.
Beans, carrots, beets, radishes, corn, and peas are best direct sown, because transplanting disturbs their roots or they germinate quickly in warm soil. Root crops in particular should never be moved once sown.
Over 7 to 10 days, move seedlings outside for increasing periods, starting with an hour or two in sheltered shade and building to a full day in sun and wind. This calculator starts hardening 7 days before your transplant date.
Ordinary full-spectrum LED or fluorescent shop lights work well: run them 14 to 16 hours a day, 2 to 6 inches above the seedlings, and raise them as plants grow. Never leave lights on around the clock; plants need a dark period too.
Seedlings started far too early grow leggy and root-bound indoors and transplant poorly. Sow inside the window this calculator gives, pot on if plants outgrow their pots, and protect tender transplants from any late frost with covers overnight.