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How long before the last frost should you start seeds indoors?
Most vegetables are started indoors 4–10 weeks before your area's average last frost date: tomatoes 6–8 weeks, peppers and eggplant 8–10, broccoli and cabbage 4–6, onions and leeks 10–12. Fast growers like squash and cucumbers need only 2–4 weeks — or direct sowing.
The full answer
The system: count backwards from your last frost date
Indoor seed starting is a backwards-counting exercise. Find your area's average last spring frost date, then subtract each crop's recommended head start. Start too early and you get leggy, root-bound transplants that stall after planting; too late and you lose weeks of harvest.
Your last frost date comes from your local cooperative extension service or a frost-date lookup (they interpolate NOAA climate-normal data by zip code). Remember it's an average — a roughly 50/50 gamble — so tender crops go out a week or two after it, not on it.
The head-start table (university extension consensus)
| Crop | Start indoors before last frost | Transplant outside |
|---|---|---|
| Onions, leeks | 10–12 weeks | 2–4 weeks BEFORE last frost (hardy) |
| Peppers, eggplant | 8–10 weeks | 1–2 weeks AFTER last frost |
| Tomatoes | 6–8 weeks | 1–2 weeks after last frost |
| Broccoli, cabbage, cauliflower, kale | 4–6 weeks | 2–4 weeks before last frost (hardy) |
| Lettuce | 4–6 weeks | around last frost, or direct sow |
| Basil | ~6 weeks | after all frost danger |
| Squash, cucumbers, melons | 2–4 weeks | 1–2 weeks after last frost — or direct sow |
Skip the indoor start entirely for: beans, peas, corn, carrots, beets, radishes, turnips. These either dislike root disturbance or grow so fast that direct sowing wins.
Why the ranges differ so much
Slow, heat-loving crops (peppers, eggplant) germinate slowly and grow slowly — they need the long runway. Cucurbits (squash, cucumbers) do the opposite: they germinate in days, grow explosively, and sulk when their roots are disturbed, so a big transplant is a WORSE transplant. Onions from seed are simply slow to reach plantable size.
Germination temperature is the hidden variable
Seeds don't count days; they integrate soil temperature:
- Most vegetables germinate well at 65–75°F (18–24°C) soil temperature
- Peppers and tomatoes prefer 75–85°F (24–29°C) — on a cool windowsill, pepper germination can stretch from 8 days to 3+ weeks
- A seedling heat mat under the tray solves this for the warm-lovers; remove it after germination
Light: the step most beginners under-do
A sunny window is usually not enough in late winter — seedlings stretch toward the glass and go leggy. The extension-service standard: bright light 12–16 hours/day, positioned a few inches above the canopy. This is the single biggest difference between stocky transplants and floppy ones.
Hardening off: the mandatory final week
Indoor seedlings have never met wind or direct sun. Harden off over 7–14 days: an hour or two of sheltered outdoor time, increasing daily, before transplanting. Skipping this step sunburns and wind-whips transplants — the classic "my seedlings died the week I planted them" cause.
A worked example
Average last frost May 10:
- Feb 15–Mar 1 → start onions/leeks
- Mar 8–22 → start peppers/eggplant (on a heat mat)
- Mar 22–Apr 5 → start tomatoes
- Apr 5–12 → start broccoli/cabbage; transplant them out ~Apr 19–26
- Apr 26 → start squash/cucumbers in pots (or direct sow May 17–24)
- May 17–24 → transplant tomatoes, peppers, squash
Time ranges by condition
| Condition | Duration | Note |
|---|---|---|
| Onions, leeks | 10–12 weeks before last frost | — |
| Peppers, eggplant | 8–10 weeks before | — |
| Tomatoes | 6–8 weeks before | — |
| Broccoli, cabbage, kale, lettuce | 4–6 weeks before | — |
| Squash, cucumbers, melons | 2–4 weeks before (or direct sow) | — |
| Beans, peas, corn, root vegetables | Direct sow — no indoor start | — |
What changes the time
- Your last frost date. The anchor for every count-back. It is an AVERAGE — half of years frost later. Tender transplants go out 1–2 weeks after it, hardy brassicas 2–4 weeks before it
- Soil temperature at germination. Peppers at 60°F soil: 3 weeks to germinate. At 80°F: about a week. A thermostat heat mat compresses the calendar for warm-season crops
- Light quantity. 12–16 hours of close, bright light makes stocky transplants; a window alone in Feb–Mar usually makes leggy ones that transplant poorly
- Container size. Seedlings held too long in small cells go root-bound and stall for weeks after transplant — the reason "earlier is better" is false
- Hardening off. The 7–14 day gradual outdoor transition. Skipping it costs more plants than any timing mistake
Common questions
What does my USDA zone tell me about seed starting?
Less than people think — zones rate average winter minimum temperatures, useful for choosing perennials and varieties. Your seed-starting calendar hangs on the last FROST date, a different dataset. Use zone for what to grow, frost date for when.
Can I just start everything earlier to get a head start?
No — oversized indoor seedlings go root-bound, leggy, and stall after transplant. Field trials repeatedly show right-sized transplants catch up to and outproduce overgrown ones.
Do I really need a grow light?
For February–March starts at most latitudes, yes — window light is too weak and too angled. Bright light 12–16 h/day kept a few inches above seedlings is the standard for stocky transplants.
What if a frost is forecast after I transplant?
Cover transplants overnight with frost cloth (row cover), buckets, or cloches — a few degrees of protection is usually enough for a radiation frost. Uncover in the morning.
Sources
We cite primary research, expert practice, and authoritative reference. Higher-tier sources weighted heavier. See methodology.
- T1University of Minnesota Extension — Starting seeds indoors — Extension-service head-start table, germination temperatures, hardening-off protocol
- T2Old Farmer's Almanac — Frost dates lookup (NOAA climate normals) — Zip-code average first/last frost dates interpolated from NOAA station data
- T1USDA Plant Hardiness Zone Map — Zone context for variety selection (zones rate winter lows, not frost dates — use both)
Gear that helps with this
If you're shopping for the kit this question is about, here are solid places to start on Amazon.
- Seed starting trays with humidity domeSee options on Amazon
- Full-spectrum LED grow lightSee options on Amazon
- Seedling heat mat with thermostatSee options on Amazon
- Frost cloth / floating row coverSee options on Amazon
As an Amazon Associate, AskedWell earns from qualifying purchases at no extra cost to you. These are general category suggestions to help you shop, not a specific endorsement. See our disclosure.
Cite this page
de Vries, P. (2026). How long before the last frost should you start seeds indoors?. AskedWell. Retrieved 2026-07-16, from https://askedwell.com/pages/how-long-does/start-seeds-before-last-frost
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