Enter your last frost date and pick a crop to get the exact indoor seed-starting date and the safe transplant date, computed week by week.
Your results will appear here.
Enter your values and press Calculate.
Every seed-starting schedule is anchored to your average last spring frost date. Each crop needs a known number of weeks growing indoors before it is sturdy enough to transplant, so the calculator subtracts those weeks (times 7 days) from your frost date to find the sowing day. Because frost dates are averages and spring weather varies, you can add a safety buffer of 1 to 2 weeks for insurance. The calculator also returns a transplant date for each crop: hardy crops like broccoli and kale can go out about 2 weeks before the last frost, while tender crops like tomatoes and peppers should wait about 2 weeks after it, once the soil has warmed.
The last frost date is an average, not a promise. It is the calendar date by which spring frost has usually ended in your area, so in roughly half of all years frost arrives later. Look yours up with the Old Farmer's Almanac frost-date tool (search by ZIP code) or ask your county cooperative extension office for the local figure. If you are outside the US, your national weather service or a local gardening club can supply the equivalent spring-frost date.
Enter that date into the calculator as YYYY-MM-DD, for example 2026-04-15. A wrong or guessed frost date shifts every result by the same number of days, so spending two minutes on an accurate date is the highest-value step on this page.
The calculator multiplies the crop's recommended indoor growing time by 7 to get days, subtracts that span (plus any safety buffer you add) from your last frost date, and returns the sow date. Many gardeners ask how many weeks before the last frost to start tomato seeds: the standard answer is 6 to 8, and you can simply count back from your last frost date with the calculator above. The transplant date is then set relative to the same frost date: hardy crops go out about two weeks before it, tender crops about two weeks after.
Worked example: tomatoes need 7 weeks indoors. With a last frost date of April 15, 2026 and a 1-week safety buffer, the calculator subtracts 8 weeks (56 days) and returns February 18, 2026 as the sow date, with a transplant date of April 29, 2026.
Sowing too early is the most common beginner mistake, and every competitor warns about it. Seedlings started weeks ahead of schedule become leggy, pale, and root-bound while they wait for warm soil, and they usually perform worse than seeds sown on time.
The safety-buffer input exists for the opposite worry: because the last frost date is an average, gardeners in exposed or high-elevation spots often add 1 to 2 weeks of margin and sow slightly earlier so seedlings are ready even if spring runs late. Use 0 for the textbook date or add a buffer for insurance, but do not stack a buffer on top of an already-early sow date.
Not every vegetable belongs in a seed tray. Use this chart as a quick reference for the crops in the calculator, then read the direct-sow section below.
| Crop | Weeks to sow before last frost | Transplant timing | |---|---|---| | Onion, Leek | 9 | 2 weeks before last frost | | Celery | 11 | 1 week after last frost | | Pepper (sweet/hot), Eggplant | 9 | 2 weeks after last frost | | Artichoke | 9 | 1 week after last frost | | Tomato | 7 | 2 weeks after last frost | | Broccoli, Cabbage, Cauliflower, Brussels sprouts | 7 | 2 weeks before last frost | | Kale | 6 | 2 weeks before last frost | | Tomatillo, Parsley | 7 | 2 weeks after last frost | | Basil | 5 | 2 weeks after last frost | | Lettuce, Spinach, Swiss chard | 5 | 2 weeks before last frost | | Cucumber, Squash, Pumpkin, Watermelon | 3 | 2 weeks after last frost |
These ranges match what extension services and the Old Farmer's Almanac publish; always cross-check your seed packet, since varieties differ.
Some crops resent transplanting and should be sown directly where they will grow: carrots, beets, radishes, parsnips, beans, peas, and corn are the classic examples. Their roots are disturbed by moving, and they germinate fast once soil is warm.
Squash-family crops (cucumber, squash, pumpkin, watermelon) need only about 3 weeks indoors. Sow them earlier and they stall in the pot, so they are in the calculator but with the shortest lead time. If a crop you want is not listed, pick the closest relative in the chart and confirm the packet's recommended weeks.
1. **Know your frost date.** Everything counts back from it; a guessed date corrupts every result. 2. **Use a sterile seed-starting mix**, not garden soil, to avoid damping-off disease, and use clean containers with drainage. 3. **Give warmth first, light second.** Most seeds germinate best with bottom heat (a heat mat helps); once sprouted, seedlings need 14 to 16 hours of strong light a day. A sunny windowsill alone usually produces leggy plants, so a simple grow light is the single best upgrade. 4. **Harden off before transplanting.** Over 7 to 10 days, move seedlings outdoors for lengthening periods so they adjust to sun, wind, and temperature swings. Skipping this step causes transplant shock even when the calendar date is right. 5. **Watch soil temperature, not just the calendar.** Warm-season crops want soil above 60F (peppers and cucumbers above 65F). A soil thermometer at 2 to 4 inches deep is cheap insurance.
The same math works in reverse for autumn. Take your first fall frost date, subtract the crop's days to maturity plus about 2 weeks for slower fall growth, and you get your last safe sowing date. The calculator above targets spring starts, so do that subtraction by hand for fall crops like spinach, kale, and lettuce.
Microclimates matter too: a south-facing slope, an urban heat island, or a nearby lake can shift your effective frost date by up to two weeks either way. Treat the computed dates as a starting plan, then fine-tune with a soil thermometer and keep row cover handy for surprise late frosts. Gardeners in frost-free areas (USDA hardiness zone 9 to 11 and similar climates) should plan around heat instead, sowing to dodge peak summer.
The average calendar date of the last spring frost in your area, the point after which frost is unlikely. Find it with the Old Farmer's Almanac frost-date lookup by ZIP code, your state cooperative extension, or your national weather service. The calculator counts every date backward or forward from it.
They grow leggy, pale, and root-bound while waiting for warm soil, and they usually transplant worse than seeds sown on time. If spring runs late, use the calculator's safety-buffer input (1 to 2 weeks) instead of sowing a month early.
Carrots, beets, radishes, parsnips, beans, peas, and corn. Their roots resent disturbance, and they germinate quickly once soil warms. Sow them directly where they will grow instead of starting them indoors.
For germination, most seeds need warmth (a heat mat helps) more than light. Once sprouted, seedlings need 14 to 16 hours of strong light daily, and a sunny windowsill alone usually is not enough. A basic grow light is the most reliable way to prevent leggy seedlings.
Hardening off is the 7 to 10 day process of gradually exposing indoor seedlings to outdoor sun, wind, and temperature swings before transplanting. It prevents transplant shock. Move trays outside for a few hours daily, increasing the time each day, before planting them out on the calculator's transplant date.
Yes, with the same backward-counting logic. Take your first fall frost date and subtract the crop's days to maturity plus about 2 weeks for slower fall growth. That gives your last safe sowing date for fall crops like spinach, kale, and lettuce.
Soil temperature decides germination, not air temperature. Most warm-season crops need soil at or above 60F, and peppers and cucumbers want 65F or more. Check with a soil thermometer at 2 to 4 inches deep before transplanting, even if the calendar date has arrived.
Cover tender transplants overnight with row cover, frost cloth, buckets, or milk jugs, and water the soil first since moist soil holds heat. Remove covers in the morning so plants do not overheat. This is also why the calculator offers a safety buffer: the last frost date is an average, and late frosts happen.
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