Step 3 - Seed Starting
When to Start Seeds Indoors
Everything counts backwards from one date. Get that date right and the rest is arithmetic.
Janet H. - the hands-on owner behind Janet Media Group.
Published - last updated
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Find your average last frost date, then count backwards by the number of weeks each crop needs. Tomatoes go in six to eight weeks before, peppers eight to ten, brassicas four to six, and cucumbers and squash only three to four.
Weeks before last frost, by crop
| Crop | Weeks before last frost | Notes |
|---|---|---|
| Peppers, eggplants | 8 to 10 | Slowest to germinate; a heat mat makes a real difference |
| Tomatoes | 6 to 8 | Fast growers -- earlier is worse, not better |
| Onions from seed | 8 to 10 | Or plant sets outdoors and skip this entirely |
| Brassicas: broccoli, cabbage, kale | 4 to 6 | Can go out before the last frost -- they are hardy |
| Lettuce and leafy greens | 3 to 4 | Also sows direct happily once soil is workable |
| Cucumbers, squash, melons | 3 to 4 | Dislike root disturbance -- use pellets or large cells |
| Basil and tender herbs | 6 to 8 | Wait for warm soil before planting out |
What not to start indoors
Carrots, parsnips, beets, radishes and turnips all form a taproot that transplanting damages, so they go straight into the bed. Beans and peas germinate fast and resent disturbance; direct sowing is simpler and works better.
Sweetcorn is usually direct sown as well. It transplants adequately but germinates readily in warm soil, so the indoor step buys very little.
Why earlier is worse
A seedling started too early runs out of room. Roots circle the cell, growth stalls, and the plant sits stressed for weeks waiting for the weather. A stressed transplant is set back further at planting than a younger, smaller one that went out at the right moment -- and a later-sown plant frequently overtakes it within a month.
Sowing early is the most common first-year mistake. In February, with seed packets on the table and nothing else to do, it is a difficult one to resist.
Hardening off
A plant raised indoors has thin cuticles, soft stems and no experience of wind or direct sun. Put it straight into a bed and it scorches or collapses.
- 1
Day 1 to 2
One to two hours outdoors in a sheltered, shaded spot. Bring it back in.
- 2
Day 3 to 5
Extend to three or four hours and introduce some morning sun.
- 3
Day 6 to 8
Most of the day outside, including a few hours of direct sun. Watch for wilting.
- 4
Day 9 to 10
Leave them out overnight if no frost is forecast. Then plant, ideally on an overcast day or in the evening.
A cold frame makes this considerably easier -- open the lid by day and close it at night and the schedule largely takes care of itself.
Sowing for autumn
The same arithmetic runs in reverse for a fall crop, counting back from the first frost date instead. See fall seed starting, which is the half of the year most first-time gardeners never use.
What to buy
The products that fit the advice above, chosen by the process on our how we choose page. We have not hands-on tested them; specifications are the manufacturers’ own.
Quick picks
| # | Product | Best for | Score | Price |
|---|---|---|---|---|
| 1 |  Top pickBurpee Self-Watering Seed Starting KitCell tray, dome and watering mat in one box | First-time seed starting | 8.4 | Check price on Amazon |
| 2 |  Espoma Organic Seed Starter MixFine, low-nutrient starting medium | Most seed trays | 8.9 | Check price on Amazon |
| 3 |  VIVOSUN Seedling Heat MatWaterproof mat sized for 1020 trays | Peppers, tomatoes and slow germinators | 8.8 | Check price on Amazon |
#ad We may earn a commission - prices are shown by Amazon, not by us
Scores are our own editorial judgment, formed from published specifications and independent sources. They are not aggregated customer ratings, and they are not the result of hands-on testing, which we do not currently do. See how we choose.
Common questions
How many weeks before last frost should I start tomato seeds?
Six to eight weeks. Tomatoes grow quickly indoors, so starting earlier produces a root-bound, stressed seedling that transplants worse than a younger one. Peppers and eggplants are the slow ones and need eight to ten weeks.
What vegetables should not be started indoors?
Carrots, parsnips, beets, radishes and turnips, because transplanting damages the taproot they form. Beans, peas and sweetcorn germinate fast and resent disturbance, so direct sowing is simpler and gives better results.
Can you start seeds too early?
Yes, and it is the most common first-year mistake. A seedling with nowhere to go becomes root-bound and stressed, and a stressed transplant is set back more at planting than a smaller, later-sown one -- which usually overtakes it within a month.
How do you harden off seedlings?
Over about a week to ten days, starting with one to two hours in a sheltered shaded spot and building up to a full day and then a night outside. A cold frame makes it much easier -- open the lid by day, close it at night, and the schedule looks after itself.
Sources
Factual claims on this page are backed by the published sources below, and product specifications are the manufacturers’ own. Where a figure is not published, we say so rather than estimating it.