When to Start Seeds Indoors: A Zone-by-Zone Guide
Seed starting is mostly backwards counting: find your average last frost, count back the weeks on the packet, and sow. This guide gives you the frost dates, the week counts for 20 crops, and the mistakes to avoid.
Look up your USDA zone’s average last spring frost in the table below, then count backwards: peppers and eggplant 8–10 weeks, tomatoes 6–8 weeks, brassicas 6–8 weeks, lettuce and chard 4–6 weeks. Transplant warm-season crops 1–2 weeks after the frost date; set out hardy crops 2–4 weeks before it. Frost dates are zone averages — confirm with your local extension service.
The one date that matters: last frost
Almost every sowing decision in spring hangs off a single date: the average last spring frost — the typical date of the last 0°C (32°F) frost, averaged over about 30 years of weather records. Seed packets say “sow 6 weeks before last frost” because the packet writer does not know your garden; the frost date is the anchor that turns that advice into a calendar date.
Two things to keep in mind. First, “average” means roughly half of years see frost after that date, so tender transplants always want frost cloth handy. Second, your garden is not the zone average: a low spot can run a week later, a sheltered wall a week earlier.
Average last frost by zone
Representative long-term averages for USDA zones 3–10. Use these for planning; verify against your county extension’s station data before you commit.
| USDA zone | Typical last spring frost |
|---|---|
| Zone 3 | 15 May |
| Zone 4 | 10 May |
| Zone 5 | 30 Apr |
| Zone 6 | 20 Apr |
| Zone 7 | 10 Apr |
| Zone 8 | 20 Mar |
| Zone 9 | 15 Feb |
| Zone 10 | 30 Jan* |
*Many zone 10 gardens rarely frost at all; treat the date as a cool-season planning anchor.
How many weeks before last frost?
The table below gives the standard indoor-sowing window and the transplant or direct-sow timing for 20 popular crops, all relative to your last frost date. Positive offsets are after the frost; negative offsets are before it.
| Crop | Start indoors | Transplant / sow outdoors |
|---|---|---|
| Tomatoes | 6–8 weeks before | Transplant 1–2 weeks after |
| Peppers | 8–10 weeks before | Transplant 2 weeks after |
| Eggplant | 8–10 weeks before | Transplant 2–3 weeks after |
| Cucumbers | 3–4 weeks before (or direct) | 1–2 weeks after |
| Squash | 3–4 weeks before (or direct) | 1–2 weeks after |
| Broccoli | 6–8 weeks before | Transplant 2–4 weeks before |
| Cabbage | 6–8 weeks before | Transplant 2–4 weeks before |
| Cauliflower | 6–8 weeks before | Transplant 2–4 weeks before |
| Lettuce | 4–6 weeks before | 2–4 weeks before |
| Spinach | Direct sow | Sow 4–6 weeks before |
| Peas | Direct sow | Sow 4–6 weeks before |
| Beans | Direct sow | Sow at / just after frost |
| Carrots | Direct sow | Sow 2–3 weeks before |
| Radish | Direct sow | Sow 4–6 weeks before |
| Kale | 6–8 weeks before | Transplant 2–4 weeks before |
| Chard | 4–6 weeks before (or direct) | Around frost |
| Beets | Direct sow | Sow 2–4 weeks before |
| Onions | 10–12 weeks before | Transplant 2–4 weeks before |
| Corn | Direct sow | Sow 1–2 weeks after |
| Basil | 6–8 weeks before | Transplant 1–2 weeks after |
A worked example
Say you are in zone 6 (last frost 20 April) and growing tomatoes:
Start indoors = 20 Apr − 6–8 weeks = 24 Feb – 9 MarTransplant = 20 Apr + 1–2 weeks = 27 Apr – 4 MayHarvest = transplant + 60–80 days = roughly late June – late July
That is the whole method. Our Seed Starting Calendar Generator does this arithmetic for all 20 crops at once.
Skip the arithmetic — get your dates
Pick your zone and get a personalised sowing calendar for all 20 crops: start-indoors dates, transplant dates and harvest windows.
Why these dates are averages
USDA zones describe winter cold, not spring frost timing, and each zone spans a huge area. The frost dates above are representative mid-points compiled from published frost-date tables and extension 30-year normals. Your actual date depends on elevation, distance from water, urban heat and plain microclimate luck. County extension services publish frost dates for specific weather stations — use the nearest one as your real anchor and treat the zone table as a starting point.
Hardening off before transplanting
Indoor seedlings are soft: still air, no wind, no direct sun. Moving them straight outside shocks them. Harden them off over 7–10 days — a few hours of sheltered outdoor time at first, building to full days and finally a night or two out — before they go in the ground. Skip this and even perfectly timed seedlings stall for weeks.
Frequently asked questions
What if my last frost has already passed this year?
Count forward instead: work out what you can still direct-sow (beans, squash, cucumbers) and what needs a transplant you buy rather than raise. For autumn crops, count backwards from your first autumn frost.
Do I need grow lights?
For most seedlings, yes — a sunny windowsill is rarely enough and produces leggy plants. Inexpensive LED shop lights hung a few inches above the seedlings, raised as they grow, outperform any windowsill.
Why do my seedlings get leggy?
Almost always too little light, often combined with too much warmth. Give them strong light close overhead and slightly cool conditions (15–18°C / 60–65°F for most crops once germinated).
Can I start seeds in a cold greenhouse instead of indoors?
For hardy crops (brassicas, lettuce, onions), yes — an unheated greenhouse a few weeks before last frost works well. For warm-season crops (tomatoes, peppers), the greenhouse is usually still too cold at sowing time; start them where it is warm.
Related guides
How to Read a Seed Packet
Sow dates, depth and spacing decoded — what every line on the packet actually means.
How Much Soil Do I Need for a Raised Bed?
The box maths: volume, bag counts and the 60/30/10 soil mix.
Seed Starting Calendar Generator
Your personalised dates for all 20 crops, computed from your zone.
Sources: USDA Plant Hardiness Zone Map; cooperative extension planting calendars (including Cornell and University of Georgia); Old Farmer’s Almanac frost-date methodology. Frost dates are long-term averages — verify with your local extension service.
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