Seed Starting & Planting Date Calculator
Sow indoors, transplant and direct sow dates for 37 crops, counted from your last frost.
Every crop has its own arithmetic: peppers want nine weeks indoors, tomatoes seven, and beans want no indoor time at all. This calculator holds the per-crop numbers and counts them from your zone’s average last frost, returning the three dates that run a spring: when to sow indoors, when to transplant or sow outside, and when the harvest should start.
Pick the crop and your zone. If your garden runs ahead of or behind its zone (a walled city plot, a cold valley), shift the zone a half step and the dates move about five days with it. Your anchor date itself comes from the Last Frost Date Calculator.
How this is worked out
Transplant date = average last frost plus the crop’s offset in weeks (negative for hardy crops that go out early, positive for tender ones waiting on warm soil). Sow indoors = transplant date minus the crop’s indoor weeks, the time it needs from sowing to a sturdy transplant. Direct sow date = last frost plus the crop’s direct offset. First harvest = the start date plus the packet days to harvest for the method used. All per-crop figures are the standard extension planning values shown in the reference table below.
Assumptions:
- Indoor weeks assume a bright setup at room temperature with lights close overhead. Leggy windowsill seedlings need the dates, but also better light.
- Days to harvest run from transplant for transplanted crops and from sowing for direct crops, matching how packets print them.
- Your zone average anchors everything; a personal frost offset moves every date by the same amount.
Worked examples
1. Tomatoes, Zone 6b. Last frost Apr 13; transplant one week after, about Apr 20; sow indoors seven weeks before that, around Mar 2; first harvest about Jun 29.
2. Peppers, Zone 5b. Last frost about Apr 23; transplant two weeks after, May 7; sow indoors nine weeks ahead, around Mar 5. Peppers are the earliest sowing in most northern gardens.
3. Bush beans, Zone 7a. No indoor start: direct sow about a week after the last frost in mid April, harvest from mid June, and keep sowing every two weeks for a continuous supply.
Reference: indoor weeks and offsets
| Crop | Weeks indoors | Outdoor timing vs last frost | Days to harvest |
|---|---|---|---|
| Tomato | 7 | +1 weeks | 70 |
| Pepper (bell) | 9 | +2 weeks | 75 |
| Eggplant | 9 | +2 weeks | 75 |
| Potato | direct only | -2 weeks | 90 |
| Cucumber | 3 | +1 weeks | 55 |
| Zucchini | 3 | +1 weeks | 48 |
| Winter Squash | 3 | +1 weeks | 95 |
| Pumpkin | 3 | +1 weeks | 105 |
| Melon | 4 | +2 weeks | 80 |
| Watermelon | 4 | +2 weeks | 85 |
| Lettuce | 4 | -3 weeks | 45 |
| Spinach | direct only | -5 weeks | 42 |
| Kale | 5 | -3 weeks | 55 |
| Swiss Chard | 5 | -2 weeks | 55 |
| Cabbage | 6 | -4 weeks | 70 |
| Broccoli | 6 | -3 weeks | 65 |
| Cauliflower | 6 | -1 weeks | 70 |
| Brussels Sprouts | 6 | -1 weeks | 100 |
| Carrot | direct only | -3 weeks | 70 |
| Beet | direct only | -3 weeks | 58 |
| Radish | direct only | -4 weeks | 30 |
| Turnip | direct only | -4 weeks | 50 |
| Onion | 10 | -4 weeks | 105 |
| Garlic | direct only | -28 weeks | 240 |
| Leek | 10 | -2 weeks | 110 |
| Peas | direct only | -5 weeks | 62 |
| Beans (bush) | direct only | +1 weeks | 55 |
| Sweet Corn | direct only | +1 weeks | 75 |
| Sweet Potato | 0 | +3 weeks | 100 |
| Okra | 5 | +2 weeks | 60 |
| Celery | 11 | -1 weeks | 110 |
| Asparagus | direct only | -3 weeks | 730 |
| Strawberry | 0 | -2 weeks | 60 |
| Basil | 6 | +2 weeks | 45 |
| Cilantro | direct only | -2 weeks | 50 |
| Parsley | 9 | -2 weeks | 75 |
| Dill | direct only | 0 weeks | 55 |
Offsets are weeks relative to the average last frost; negative means the crop goes out before it. The calculator applies the transplant offset for transplanted crops and the direct offset for direct-sown ones.
Common mistakes
- Starting everything on the same day. The whole point of per-crop weeks is that peppers sown with the beans are nine weeks of windowsill for nothing; the beans should not be indoors at all.
- Sowing to the packet’s enthusiasm. Packets print the earliest viable dates. A tomato transplanted at the first legal moment into cold soil sits still for two weeks and is overtaken by one planted later into warmth.
- Skipping hardening off. Indoor-grown plants moved straight to full sun and wind scorch and stall. The week of gradual exposure in the results is part of the schedule, not a garnish.
- One giant sowing of succession crops. Lettuce, radish, beans and cilantro want small repeats; the Succession Planting Planner exists for exactly them.
Putting the result to work
Write the three dates for each crop on the seed packet itself with a marker; packets survive the season better than notebooks. Check off sowings as you make them, because the gap between plan and soil is where gardens are lost. When the transplant date is set, the Transplant Date Calculator works the same problem forwards from a sowing you have already made.
How do I get my sowing dates?
- Pick the crop; its indoor weeks and outdoor offset are built in.
- Pick your USDA zone to set the last frost anchor.
- Read the sow indoors, transplant or direct sow, and first harvest dates.
- Shift everything by your garden’s personal offset if you know it runs early or late.
- Write the dates on the seed packet and sow to the packet, not to memory.
When should I start tomato seeds indoors?
About 6 to 8 weeks before the transplant date, which is itself about a week after your average last frost. In Zone 6b that is sowing around the start of March for a late April transplant. Earlier is not better: an overgrown, root-bound tomato transplants worse than a compact young one.
Which vegetables should not be started indoors?
Beans, peas, corn, carrots, beets, radish and the squash family all resent transplanting or gain nothing from it; they are direct sown. Cucumbers and squash can be started indoors three to four weeks ahead in pots they can leave without root disturbance, which is the exception this calculator allows.
What if I miss the sow indoors date?
Sow anyway, up to about two weeks late, and expect the harvest to shift by roughly the same amount. Past that, buy transplants for slow crops like peppers and celery, and direct sow or skip fast ones. The season forgives a late start far more readily than a frost-killed early one.
Why do my dates differ from the seed packet?
Packets often print dates for a reference region or from their trial ground, and ranges (‘sow March to May’) span several zones. This calculator personalises the same per-crop rules to your zone’s frost anchor, so a difference of one to three weeks from a generic packet range is normal.
How do I adjust for a garden that runs cold or warm?
Move the zone a half step: a cold valley garden in Zone 6b plans as 6a, a walled city garden as 7a. Every date shifts about five days. After one season of noting your actual last frost, that personal offset is the most accurate setting you own.
Per-crop indoor weeks, outdoor offsets and days to harvest are standard US extension planning values. Frost anchors are zone averages consistent with the 2023 USDA Plant Hardiness Zone Map climate basis.
