A planting calendar built from your own frost dates
Most planting calendars begin by asking where you live and then quietly decide a great deal on your behalf. There is a better input, and it is one you can go and find: your own two frost dates.
The two numbers the whole calendar hangs on
Your average last spring frost, and your average first autumn frost. Everything else is arithmetic on those two dates. That is what lets a single method work in any climate and in either hemisphere — nothing has to be assumed about where you are, because your location is not the input.
Get the dates from a real source. National meteorological services and agricultural extension services publish average first and last frost dates by location. A nearby allotment site, a garden club, or a neighbour who has grown on that soil for thirty years is often better still. Treat both as averages rather than promises: an average is a middle, not a floor, and a fair share of years run later than it. That is the whole reason tender crops go out last.
The method: every rule is an offset in weeks
Conventional growing guidance — the sort printed on seed packets and in national growing guides — is almost always expressed relative to frost rather than as a date. Start tomatoes under cover about six weeks before the last frost. Put them out about a week after it. Sow peas about six weeks before it, because they hate a hot summer. Sow carrots where they are to grow about three weeks before it, because they resent being moved.
So the calculation is always the same: take your date, shift it by the crop's offset in weeks, and you have a real calendar date. Then add the crop's approximate days to a first picking to get a harvest window.
A worked example
These are example inputs, not a recommendation for anywhere: a last frost of 2026-04-15 and a first frost of 2026-10-20, which leaves a frost-free season of 188 days between them.
Tomato, conventionally started under cover and transplanted:
- Start seed indoors 6 weeks before the last frost:
2026-04-15 − 42 days = 2026-03-04. - Plant out 1 week after the last frost:
2026-04-15 + 7 days = 2026-04-22. - First picking, about 60 days from planting out:
2026-04-22 + 60 days = 2026-06-21.
Carrot, sown where it is to grow:
- Direct sow 3 weeks before the last frost:
2026-04-15 − 21 days = 2026-03-25. - First pulling, about 75 days from sowing:
2026-03-25 + 75 days = 2026-06-08.
Count from the right event
A days-to-harvest figure is useless unless you know what it counts from. For a crop raised in modules and transplanted, the count conventionally starts at planting out. For a crop sown where it grows, it starts at sowing. Mixing the two is the most common way a home-made calendar drifts by a month without anybody noticing.
There is a related correction worth making. If you direct sow something normally started under cover, you give up the head start the module gave it, so adding about a fortnight to the days figure is fairer than quoting the transplant-day number as though nothing had changed. The same tomato direct sown on 2026-04-22 at 60 + 14 = 74 days lands on 2026-07-05 instead of 2026-06-21.
Check every harvest against the far end of the season
The last-frost date sets the start. The first-frost date is the deadline. If a projected first picking lands after your first autumn frost, the plan does not work as written — start earlier, choose an early variety, or grow it under cover. That single comparison catches more disappointments than any other check you can make on paper in March.
What none of this can tell you
The offsets are conventional rules of thumb rounded to whole weeks, and days-to-harvest figures are approximate and vary by variety. Nothing here is authoritative for your garden: timings move with variety, microclimate, soil, aspect and season, and no calendar promises a crop. Where a seed packet disagrees with any calendar, follow the packet — it was written for that variety, and it is the most reliable source you already own.
Build yours
The free Garden Planting Calendar takes your two frost dates and a list of crops and prints the dates with the frost-relative rule beside each one, so you can see the reasoning rather than trust a number. It runs in your browser, and it has no idea where you are.