Grow
Seed Starting Schedule Calculator
Enter your last frost date and crop to get exact sowing, hardening-off, and transplant dates.
Your seed-starting dates
Dates assume average conditions — adjust a week either way based on your indoor light setup and the current forecast.
Why seed-starting timing matters so much
Starting seeds indoors gives slow-growing or frost-tender crops a head start on the growing season, but the timing window is narrower than it looks. Start too early and seedlings outgrow their trays, becoming rootbound and leggy well before it's safe to plant outside. Start too late and you lose weeks of potential harvest, or the plant never matures before your growing season ends. The right sowing date works backward from your last frost date, accounting for how many weeks each crop needs to reach a sturdy, transplant-ready size.
Typical weeks-before-frost by crop
| Crop | Weeks before last frost to sow | Transplant timing |
|---|---|---|
| Onions / leeks | 8-10 weeks | 2 weeks before frost |
| Peppers | 8-10 weeks | 1-2 weeks after frost |
| Tomatoes | 6-8 weeks | 1-2 weeks after frost |
| Broccoli / cabbage family | 5-7 weeks | 1-2 weeks before frost |
| Lettuce | 4-6 weeks | 2-3 weeks before frost |
| Cucumbers / squash | 3-4 weeks | 1 week after frost |
The hardening-off process
Seedlings grown under grow lights or on a windowsill develop thin leaf cuticles and soft cell walls because they've never experienced real wind, temperature swings, or full sun intensity. Moving them directly outdoors often causes sunscald, wind damage, or transplant shock that can stunt growth for weeks even if the plant survives. Hardening off gradually toughens seedlings by exposing them to outdoor conditions in increasing doses — starting with an hour or two in a shaded, sheltered spot and building up to a full day and, eventually, an overnight outdoors — typically over 7 to 10 days before the final transplant date.
- Days 1-2: 1-2 hours in dappled shade, sheltered from wind.
- Days 3-4: 3-4 hours including some direct morning sun.
- Days 5-6: Half a day outdoors, introducing afternoon sun gradually.
- Days 7-8: Full day outdoors, bring in at night if frost risk remains.
- Days 9-10: Leave out overnight if temperatures are safely above the crop's minimum, then transplant.
Reading your last frost date correctly
A published 'last frost date' for your area is typically the date with a 50% probability of frost — meaning roughly half of past years still saw frost after that date. Treat it as a planning anchor rather than a guarantee, and watch your local extended forecast closely in the two weeks surrounding your planned transplant date, especially for tender crops like tomatoes and peppers that can be set back significantly by even a light frost.
Common seed-starting mistakes to avoid
- Sowing everything on the same day regardless of crop — stagger sowing based on each crop's own timeline.
- Skipping hardening off entirely because seedlings 'look fine' indoors.
- Using trays without drainage, leading to damping-off disease in wet soil.
- Starting warm-season crops (squash, cucumbers, melons) too many weeks early — they resent being rootbound far more than tomatoes do.
- Ignoring your specific indoor light levels — insufficient light makes any schedule run leggy regardless of timing.
Why do different crops start at different times before frost?
Crops vary in how long they take to grow a strong root system and a few true leaves indoors. Slow growers like peppers and onions need 8-10 weeks to reach transplant size, while fast growers like cucumbers and squash need only 3-4 weeks and resent being started too early, since they get rootbound and stressed in small trays.
What is hardening off and why can't I skip it?
Hardening off is gradually exposing indoor-started seedlings to outdoor sun, wind, and temperature swings over 7-10 days before transplanting. Seedlings grown entirely indoors have thin cell walls and no wind toughening, so moving them straight outside often causes sunscald, wind snap, or transplant shock that stunts or kills them.
What if my last frost date changes from year to year?
Your local average last frost date is a 50% probability estimate, not a guarantee — some years it's earlier, some later. Use it as a planning anchor, but watch your local extended forecast in the two weeks around your planned transplant date before moving tender crops outside.
Can I start seeds too early?
Yes, and it's one of the most common seed-starting mistakes. Seedlings started too early outgrow their trays before it's safe to transplant, becoming leggy, rootbound, and stressed, which often produces weaker plants than seeds started at the right time or even direct-sown outdoors.
Last reviewed February 2026