When do I start my seeds?

Method

How this decides

Extension services publish spring timing as an offset, not a date: start peppers indoors eight weeks before your last frost, set tomatoes out a week after it, sow spinach six weeks before. That form is the only one that travels — an absolute date is true for one latitude and wrong everywhere else. Attach it to your own last-freeze date and it becomes a calendar.

Your last freeze comes from NOAA's 1991–2020 normals at the station nearest your ZIP code, and it is published as a probability rather than an average. The median is the default. The safe option uses the late tail, which the last freeze beats in roughly nine years out of ten — it pushes every date back, which is the right trade for anything frost will kill outright.

Why the dates count forwards, never backwards

For most of the year a spring calendar is a list of dates that have already gone by. Read naively that looks broken, so nothing here is ever shown as past — every job is counted forward to the next time it comes round. In August your tomatoes are not "overdue since March", they are about six months out, and that is the useful way to say it.

Two places where a spring calendar stops meaning what it says

Where there is no freeze at all. A few hundred stations record no freeze at any probability. There is no anchor to count from, so this tool shows no dates rather than inventing them — heat and day length set your calendar instead, and the other two tools are the useful ones there.

Where the freeze window crosses New Year. At about 140 stations the last spring freeze lands in early January and the first fall freeze just before it. The arithmetic still holds and the dates are still right — Phoenix genuinely does sow peppers under cover in November and set tomatoes out in January — but calling that window "spring" would mislead. The tool says so where it applies.

The week offsets are a judgement — conventional extension guidance, and they vary by variety and by how much protection you can give a young plant. The freeze date they are counted from is measured, and every result names the station it came from and how far away it is.

The rest of the year

This tool covers the opening of the season. For the closing of it — how many growing days are left and what still has time to mature — see what can I still plant? For the things that have to live through the winter rather than be sown each year, see what survives my winter?

Gear

If you want to act on this

Each link opens an Amazon search rather than a product we picked. We have not tested these and do not rank them — the search is the honest version, and you choose.

• Seedling heat matsMost of the dates above are indoor sowings, and germination is driven by soil temperature rather than by the calendar.
• Grow lightsStarts sown this early are indoors for weeks before your last freeze, and a windowsill in late winter is not enough light.
• Seed trays and domesThe schedule above has several crops going under cover in the same week.

As an Amazon Associate we earn from qualifying purchases. That costs you nothing extra, and it buys no influence over anything on this site: the dates come from NOAA normals and the zones from the USDA, and no product affects either.