About
Most warm-season crops need 6-10 weeks indoors before last frost. Cool-season started earlier (10-12 weeks for onion, leek). Hardening off: 7-10 days before transplant, gradually expose to outdoor conditions. Local extension offices have detailed calendars.
Formula
Frequently asked questions
How many weeks before the last frost should I start seeds indoors?
It depends on the crop. Most warm-season vegetables want 6 to 8 weeks indoors before the last frost: tomatoes, peppers, broccoli, and cabbage all fall in that band. Slow growers like onions and leeks need 10 to 12 weeks, while fast cucurbits such as cucumber and squash need only 3 to 4 weeks. The calculator subtracts the crop's weeks from your last frost date to give a sow-indoors date.
How do I find my last frost date?
Your average last spring frost date is a local climate figure. In the United States, look it up by ZIP code through the National Centers for Environmental Information or your state Cooperative Extension service; the Old Farmer's Almanac frost-date tool uses the same station data. It is an average, not a guarantee, so a late cold snap is always possible.
What does hardening off mean?
Hardening off is the 7 to 10 day process of gradually exposing indoor-raised seedlings to outdoor conditions before transplanting. Start with an hour or two of shade and wind, then lengthen the time and increase sun each day. It lets the plants toughen their cuticles and adjust to temperature swings, which prevents the transplant shock that hits seedlings moved straight from a windowsill to a garden bed.
Can I transplant seedlings outside before the last frost?
Generally not warm-season crops. Tomatoes, peppers, and squash are frost-tender and a single freezing night can kill them, so wait until after the last frost and once soil has warmed. Cool-season crops such as broccoli, cabbage, and lettuce tolerate light frost and can go out a couple of weeks earlier. Always check the soil temperature, not just the calendar.
What happens if I start seeds too early?
Seedlings started too early outgrow their containers before it is safe to transplant. They become leggy, root-bound, and stressed, and often transplant worse than younger, sturdier plants started on time. If a cold spring delays your transplant date, it is usually better to have sown a week or two later than to nurse oversized seedlings indoors for an extra month.
About the seedling start date calculator
Starting seeds indoors gives warm-season crops a head start so they are ready to fruit when summer arrives. The catch is timing. Sow too early and the seedlings outgrow their pots before it is safe to plant out; sow too late and the plants never reach full size before frost returns. This calculator removes the guesswork: you enter your average last spring frost date and pick a crop, and it counts back the right number of weeks to give you a sow-indoors date.
The whole schedule pivots on one local figure: your average last frost date. Every gardening calendar is built backward from it, because the last frost marks the point when frost-tender seedlings can safely move outdoors. Once you know that date, the lead time for each crop is well documented by seed companies and extension services, so the start date is just subtraction.
How the start date is calculated
Start indoors = Last frost date - (Weeks before transplant x 7 days) Hardening off = Last frost date - 10 days (begin acclimatising) Transplant = on or just after the last frost date
- Last frost date is your local average last spring frost, the anchor for the whole schedule.
- Weeks before transplant is the crop's indoor lead time: 6 to 8 weeks for tomatoes and peppers, 10 to 12 for onions.
- Hardening off begins about 7 to 10 days before transplant to acclimatise seedlings to the outdoors.
- Transplant happens after the frost risk has passed and the soil has warmed.
Worked example
Say your last frost date is 15 April and you are growing tomatoes, which want 8 weeks indoors.
- Convert weeks to days: 8 weeks x 7 = 56 days.
- Count back from the frost date: 15 April minus 56 days lands around 18 February.
- Mark hardening off: 10 days before 15 April, so begin around 5 April.
- Plan the transplant: on or shortly after 15 April, once nights stay above freezing.
Indoor lead time by crop
| Crop | Weeks before last frost | Season |
|---|---|---|
| Onion, leek | 10 to 12 weeks | Cool |
| Tomato, pepper | 6 to 8 weeks | Warm |
| Broccoli, cabbage | 6 to 8 weeks | Cool |
| Eggplant | 6 to 8 weeks | Warm |
| Lettuce | 4 to 6 weeks | Cool |
| Cucumber, squash | 3 to 4 weeks | Warm |
Common pitfalls
- Treating the frost date as a hard guarantee. It is a long-term average; a late freeze can still arrive, so watch the forecast near transplant time.
- Starting everything at once. Onions need months more lead time than cucumbers. Stagger sowings by crop.
- Skipping hardening off. Moving seedlings straight from a windowsill to full sun and wind causes transplant shock and can kill them.
- Planting tender crops too early. Tomatoes and peppers die in a frost; wait until the soil warms, not just until the calendar date.
- Ignoring soil temperature. Even after the last frost, cold soil stalls warm-season growth. Check soil temp, not only air temp.