When should I start tomato seeds indoors?

About 6 weeks before your last spring frost date, then transplant about 2 weeks after last frost once soil has warmed. Enter your frost date in the seed starting calculator for exact calendar days.

When should I start peppers and eggplant?

Both want about 8 weeks indoors before last frost, with transplant about 2 weeks after frost. They sulk in cold soil — patience beats leggy early starts.

What if I live in a frost-free zone?

Zones 11–13 are treated as effectively frost-free in our data. Use local month planting windows instead of frost offsets, and still check microclimate guidance on each zone page.

Can I start seeds too early?

Yes. Oversized seedlings get leggy and root-bound. A stocky transplant started on the offset usually outperforms a 10-week monster under lights. Follow the weeks in the crop table, not enthusiasm.

When to Start Seeds Indoors: A Zone-by-Zone Guide

Seed starting is calendar math, not vibes. Pick a last spring frost date, subtract each crop’s indoor weeks, and you get a sow day. Add the transplant offset (positive weeks mean after frost; negative means before). That is the method behind our seed starting date calculator and every crop row in our data.

How the offset method works

Find your average last spring frost (extension office, NOAA, or our zone averages below).
For indoor crops: sow date = frost − (indoor weeks × 7 days).
For transplant: transplant date = frost + (transplant weeks × 7 days) — if transplant weeks are negative, you set plants out before frost.
Direct-sow-only crops skip the indoor step and use their direct-sow offset instead.

Example using our tomato offsets (6 weeks indoors, transplant +2 weeks after frost) and a Zone 6 representative frost of April 15:

Indoor sow ≈ March 4 (15 Apr − 42 days)
Transplant ≈ April 29 (15 Apr + 14 days)

Peppers and eggplant use 8 weeks indoors and the same +2 transplant offset — start them earlier than tomatoes.

Representative frost dates by zone

These are regional averages from our zone data — always verify locally. Frost-free guidance applies to zones 11–13.

Zone	Last spring frost (avg)	First fall frost (avg)	Zone page
1	Jun 1	Aug 20	Zone 1
2	May 20	Sep 1	Zone 2
3	May 15	Sep 15	Zone 3
4	May 10	Oct 1	Zone 4
5	Apr 30	Oct 13	Zone 5
6	Apr 15	Oct 22	Zone 6
7	Apr 1	Nov 5	Zone 7
8	Mar 15	Nov 15	Zone 8
9	Feb 15	Dec 10	Zone 9
10	Jan 15	Dec 31*	Zone 10
11	Frost-free	—	Zone 11
12	Frost-free	—	Zone 12
13	Frost-free	—	Zone 13

*Zone 10’s first-fall date is a rare-frost placeholder in our data.

Zone 1 guidance notes a very short season (~80 frost-free days); Zone 2 about 104. Those short windows punish late indoor starts and reward sticking to the offsets.

Worked offsets for common crops

Numbers below come from our crop seed-start fields — not hand-waved weeks.

Crop	Indoor weeks before frost	Transplant vs frost	Notes
Tomatoes	6	+2 weeks	Warm soil; bury stem deep
Peppers	8	+2 weeks	Slow starters
Eggplant	8	+2 weeks	Hates cold soil
Broccoli / cabbage / cauliflower	6	−1 week	Tolerate light frost
Onions (from seed)	10	−4 weeks	Long lead time
Head / leaf lettuce	4	−2 weeks	Also direct-sow at −4 weeks
Winter squash	3	+2 weeks	Prefer peat pots
Peas, beans, carrots, radish	—	direct sow	See crop offsets; carrots are direct-only

Hardy crops that transplant before frost (negative weeks) still need hardening off. Tender crops that wait until after frost still die in a late cold snap — average frost dates are coin-flip dates, not guarantees.

A second worked example (Zone 5 peppers)

Zone 5’s representative last frost is April 30. Peppers need 8 weeks indoors:

Indoor sow ≈ March 5 (30 Apr − 56 days)
Transplant ≈ May 14 (30 Apr + 14 days)

Same frost date, tomatoes at 6 weeks sow nearer March 19. That two-week gap is why pepper trays fill the shelf first. If you only remember one rule: start the slow warm crops before the tomatoes, then follow the printed offsets.

Building a zone calendar without guessing

Open your zone page, note the representative last frost, then paste that date into the seed calculator to generate the full multi-crop table. Pair spacing later with the garden spacing calculator so you do not start forty tomato seedlings for a bed that only holds eight.

Fall gardens reverse the logic: count backward from first fall frost using days to maturity plus a short-day buffer. Our zone pages list first-fall averages for zones that frost; use them as a planning scaffold, then tighten with local history.

Hardening off and the classic early-start mistake

Seven to ten days of gradual outdoor time turns indoor seedlings into field plants (hardening-off window from common transplant practice — not a separate constant in our seed-start offsets). Skip it and you sunburn weeks of work. The transplant dates in our tool assume you harden off.

Starting too early is the other classic error. A tomato sown ten weeks before frost under weak light becomes a vine that stalls after planting. Six weeks of stocky growth beats ten weeks of stretch. If lights or windowsill space are limited, prioritize peppers and eggplant (8-week crops) and keep tomatoes on their 6-week mark.

Cover crops and empty beds

When a bed will sit idle before cash crops, the cover crop seed calculator sizes clover, rye, vetch, and friends. Cover crops have their own timing relative to frost (rye later than most fall sowings) — they are not a substitute for vegetable seed-start offsets, but they keep soil busy while seedlings finish indoors.

Bottom line

Frost date ± (weeks × 7) is the whole game. Tomatoes: 6 weeks in, +2 out. Peppers and eggplant: 8 weeks in, +2 out. Broccoli-family: 6 weeks in, about 1 week before frost. Use the zone pages for representative dates, then lock the calendar with the seed starting calculator. Estimates only — confirm local frost history with your extension office.