Photo: Frederick Depuydt · CC BY-SA 4.0, via Wikimedia Commons.
Starting seeds indoors
Which crops actually need a head start, how many weeks before your last frost to sow them, and the honest list of what you should just sow straight in the ground instead.
What you need
Your last frost date - everything counts backward from it
A seed-starting mix and small containers or trays with drainage
A bright light - a grow light, or the sunniest window you have
Steady warmth for germination, and somewhere to harden the seedlings off later
Starting seeds indoors is one of those skills that sounds advanced and mostly isn't - it's a warm, bright windowsill and a calendar. But before the how, the more useful question is which crops actually need it, because the honest answer is: not most of them. You start seeds indoors for one reason - to buy weeks a warm crop needs and your season doesn't have - and for everything else it's extra work for no gain.
Why some crops need the head start
A long-season, frost-tender crop can't finish if you only sow it outdoors after the last frost - there aren't enough warm weeks left. So you start it under cover weeks early and set it out once frost is safely past:
Promising
Long-season warm crops are started indoors several weeks before the last frost and set out after it — tomato and tomatillo about six weeks ahead, pepper and eggplant about eight. Basil, six.
suboptimal if ignored → advise
Mechanism: A frost-tender crop with a long days-to-maturity cannot mature if it is only direct-sown after the last frost — the frost-free window is too short. A head start under cover buys the weeks the season lacks; the seedling is set out only once frost has passed — the transplant date is still frost-gated.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
University of Minnesota Extension, 'Starting seeds indoors', extension.umn.edu/planting-and-growing-guides/starting-seeds-indoors: seed-starting chart gives tomato '5-6 weeks' and pepper and eggplant '9 weeks' before the outdoor planting date (warm crops set out after the last frost). Read 2026-07-19 (LLM-summarised); read by the maintainer 2026-07-25, which is the read that promoted this rule. Note what the corpus's per-crop values do with the sources: tomato 6 sits at the top of Minnesota's '5-6 weeks' and inside Illinois's '6-8'; pepper and eggplant 8 sit BELOW Minnesota's '9 weeks' and at the top of Illinois's '6-8'. They are a reconciliation of two published ranges, not a figure either source prints.
Those leads are counted from your last frost date: a tomato started about six weeks ahead, a pepper or eggplant about eight. Eggplant carries the longest lead precisely because it's the slowest to get going. The frost dates guide explains how to find that date; everything here counts backward from it.
The transplant date is still frost-gated
The head start indoors does not move the day the plant is allowed outside. That day is set by frost, not by how big your seedling has gotten:
Well established
Tender species must not be scheduled before the site's last frost date.
fatal if ignored → block
Mechanism: Ice nucleation in leaf tissue.
Evidence: settled without a trial - climatology.
remedy
Sow or set out after the last frost date, or start indoors and transplant once it has passed.
So the schedule is two dates, not one: sow indoors at last-frost-minus-the-lead, and set out on or after the last frost. Starting too early is a real mistake - a tomato sown ten weeks ahead becomes a leggy, root-bound plant stuck waiting for a frost date that hasn't moved. Count the weeks; don't just start in January because you're eager.
What you should NOT start indoors
This is the part most seed-starting guides skip. Two whole groups of crops are better off never seeing your windowsill:
Promising
Cold-hardy annuals can be sown or transplanted before the last spring frost — hardy crops about three weeks before, half-hardy about two weeks before. Tender crops wait for the frost itself.
suboptimal if ignored → advise
Mechanism: Cold hardiness. Hardy crops tolerate hard frost and half-hardy crops tolerate light frost, so they can occupy the ground weeks before frost-tender crops are allowed in. The offset is measured from the last spring frost.
Evidence: confirmed against 4 sources in the research-based literature.
the 4 sources
University of Illinois Extension, 'Time Your Vegetable Plantings by Cold Hardiness' (2018-03-26), extension.illinois.edu/blogs/ilriverhort/2018-03-26-time-your-vegetable-plantings-cold-hardiness: very hardy vegetables 'withstand freezing temperatures and hard frosts without injury' (planted earliest in spring); frost-tolerant vegetables 'can withstand light frosts, but not freezing temperatures' (planted a couple of weeks later). LLM-summarised from a read of the page; it establishes the CATEGORIES but states no week offsets.
Colorado State Univ. Extension, GardenNotes #720, 'Vegetable Planting Guide', extension.colostate.edu/resource/vegetable-planting-guide (read 2026-07-25): hardy vegetables go in 'As early as 2-4 weeks before the date of the average last spring frost'; semi-hardy 'As early as 0-2 weeks before the date of the average last spring frost'; tender from seed 'around the date of the average last spring frost'; very tender 'Typically 2+ weeks after'. This is the source that states the offsets in weeks.
Where this rule sits in that range: hardy at 3 weeks is mid-range of CSU's 2-4. Half-hardy at 2 weeks is the EARLIEST EDGE of CSU's 0-2 - the most aggressive planting CSU sanctions, not the middle of it. The rule is inside published guidance at both ends, but it leans early on the half-hardy side and a gardener following it in a cold spring is taking that boundary.
Do not import an offset from another publication's hardiness scheme without checking the category names: CSU has no 'very hardy' tier, so a '4-6 weeks before frost' figure written against a four-tier scheme does not describe CSU's 'hardy'. Categories are not comparable across sources.
Cold-hardy crops - lettuce, peas, spinach, radishes, many brassicas - tolerate frost and can go straight into the ground weeks before the last frost, so starting them indoors just adds a transplant step they didn't need. And large-seeded, fast crops - beans, squash, cucumbers, corn, carrots, beets - resent transplanting and germinate fast in warm soil, so they're sown directly where they'll grow. Reserve the indoor effort for the long-season warm crops that actually can't finish without it.
The short version of the how
For the crops that do earn a start: sow into a light seed-starting mix, keep it warm until they germinate, then give them strong light - a bright window works for a few weeks, but seedlings reaching and leaning are telling you it isn't enough, and a cheap grow light fixes it. Before they go outside, harden them off: over a week or so, give them a few hours of outdoor conditions a day, building up, so the move doesn't shock them. Then set them out on the frost-gated schedule above - the first bed walkthrough puts the whole sequence together.
One thing the packet compresses into a number: how deep to sow. The rule is roughly two to three times the seed's own width, so a big bean goes down a good inch while a fine lettuce seed is barely covered - sow it too deep and it exhausts itself before it reaches the light.
How deep each seed goes, from the corpus — a soil cross-section to one scale. The rule of thumb every seed packet compresses: the bigger the seed, the deeper the hole.Source: corpus sowing_depth_cm.
