Seed Starting Date Calculator
Indoor sow and transplant dates from last frost and crop.
What it does
Indoor sowing buys the season weeks the outdoor calendar cannot. Tomatoes started 6 weeks before the last frost transplant as stocky plants while direct sown seed still waits on warm soil. The math is one subtraction: last frost minus crop weeks.
The tool above runs that subtraction in UTC with crop weeks built in. Pick the crop and enter the last frost date; it prints the sow date, the transplant date a week later, and days indoors. May 10 frost with tomatoes gives Mar 29 sowing and May 17 transplant.
Extension charts set the weeks. Clemson tomato guidance says 6 to 8 weeks before last frost, Clemson pepper says 6 to 8 with Penn State stretching peppers to 8 to 10, and UNH frames 4 to 12 weeks by species with brassicas near 5. UGA adds hardening about two weeks before the move.
Light decides success after dates. Windowsills stretch seedlings; strong light or 14 to 16 hour grow lights keep them compact. Harden trays with lengthening outdoor visits, and hold tender crops until nights stay reliably mild.
How to use
- Enter the last spring frost date.
- Pick the crop for its indoor weeks.
- Read the indoor sow date first.
- Read the transplant date near a week later.
- Harden seedlings 1 to 2 weeks before moving.
- Sow only what fits under your lights.
Formula
- last frost
- Median last 32 F spring date.
- weeks
- Indoor weeks for the crop, 3 to 8.
- transplant
- Move-out target about a week past frost.
Week bands follow Clemson (tomato 6-8, pepper 6-8), Penn State (pepper 8-10), and UNH (4-12 range, brassicas 4-6). UTC dates throughout.
Worked example
Example: tomatoes, frost May 10
Defaults: last frost May 10, 2026, tomatoes.
- Inputs: last frost May 10, 2026, tomatoes at 6 weeks.
- Indoors: 6 x 7 = 42 days.
- Sow: May 10 minus 42 days = Mar 29, 2026.
- Transplant: May 10 plus 7 days = May 17.
- Tool shows Sun Mar 29 2026, Sun May 17 2026, 42 days indoors.
Reference table
| Crop | Weeks | Transplant |
|---|---|---|
| Tomato | 6-8 | 1 week past frost |
| Pepper | 8-10 | Warm soil, past frost |
| Broccoli / cabbage | 4-6 | Tolerates light frost |
| Lettuce | 4-5 | Early spring |
| Cucumber / squash | 2-4 | Or direct sow |
Limits
- Median frost only; late freezes still threaten.
- Covers timing, not light, heat mats, or mix.
- One crop per run; succession needs repeats.
- Variety vigor shifts ideal weeks within bands.
Common mistakes
- Sowing by the packet alone without the frost date.
- Starting too early under weak window light.
- Skipping hardening and sun burning transplants.
- Moving tenders out on the frost date exactly.
- Sowing more cells than the lights cover.
Try it yourself
Quick checks. Answers sit tucked under each question.
Peppers, last frost Apr 15?
8 x 7 = 56 days. Day 105 minus 56 = day 49 = Feb 18. Transplant Apr 22.
Broccoli, last frost May 1?
5 x 7 = 35 days. Day 121 minus 35 = day 86 = Mar 27. Transplant May 8.
Questions
How is the sow date figured?
Last frost minus crop weeks times 7. May 10 minus 42 days is Mar 29 for 6-week tomatoes.
How many weeks indoors per crop?
Tomatoes 6 to 8, peppers 8 to 10, brassicas 4 to 6, lettuce 4 to 5, cucurbits 2 to 4 weeks.
When do transplants move out?
About a week after last frost, once soil warms. The tool prints last frost plus 7 days.
What is hardening off?
Harden 1 to 2 weeks before moving out: short outdoor visits growing longer daily.
Can I direct sow instead?
Yes. Cucumbers, squash, and melons resent root disturbance; many gardeners direct sow them instead.
Why are seedlings leggy?
Leggy, pale growth means weak light. Move trays to strong light or run grow lights 14 to 16 hours.
Sources
- Clemson HGIC 1323: Tomato (6-8 weeks before last frost)
- Clemson HGIC 1316: Pepper (6-8 weeks before frost)
- UNH Extension: Starting Plants From Seed (4-12 weeks, hardening)