A frost date is a median, not a deadline
The single most misunderstood number in gardening is the last frost date. It is not the date after which frost cannot happen. It is the date by which frost has finished in half of all years — which means that in the other half, it has not.
That distinction matters because of what people do with the number. Planting tomatoes on the last frost date is planting them with a coin-flip chance of losing them. The date is a centre point in a distribution with a long tail on both sides, and the tail is what kills seedlings.
This is why the calculator gives you two dates. The median is the statistical answer. The safe date, two weeks later, is the practical one: by then the probability of a damaging frost has fallen to roughly one year in ten, which is a risk most people are willing to take with a tray of plants they raised themselves.
Why the zone system is only half the story
USDA hardiness zones describe average annual minimum winter temperature. They are excellent for answering "will this perennial survive my winter" and much weaker for answering "how long is my growing season", because two places in the same zone can have very different summers.
The clearest example is maritime versus continental climate. The UK, Ireland and the Pacific Northwest sit in zones 8 and 9 on winter minimums, alongside places like Georgia and Texas. But their summers are far cooler. A zone-8 continental gardener ripens melons outdoors; a zone-8 maritime gardener struggles to ripen an outdoor tomato. Same zone, different world.
That is why this calculator carries a separate maritime entry rather than pretending the zone number covers it. If you garden somewhere mild and damp with cool summers, use that setting and treat heat-loving crops as needing protection regardless of what the frost dates say.
| Zone | Median last frost | Median first frost | Season |
|---|---|---|---|
| 3 | May 15 | Sep 15 | 123 days |
| 4 | May 10 | Sep 25 | 138 days |
| 5 | Apr 30 | Oct 10 | 163 days |
| 6 | Apr 21 | Oct 17 | 179 days |
| 7 | Apr 8 | Nov 1 | 207 days |
| 8 | Mar 22 | Nov 12 | 235 days |
| 9 | Feb 20 | Dec 5 | 288 days |
| UK / maritime | May 5 | Oct 28 | 176 days |
Your garden is not the weather station
Published frost dates come from official weather stations, which sit in open ground at a standard height. Your garden almost certainly does not match that, and the difference can be a fortnight in either direction.
Cold air behaves like water: it flows downhill and pools at the bottom. A garden in a dip, or one with a solid fence or hedge across the slope below it, collects that cold air and frosts earlier and later than the surrounding area. A garden on a slope with an open bottom drains cold air away and gets a longer season than the official figure.
Walls and paving work the other way. A south-facing wall absorbs heat all day and releases it overnight, and a bed at its foot can run two or three weeks ahead of the same garden twenty metres away. Urban gardens generally frost less than rural ones for the same reason at a larger scale.
The adjustment field exists for this. Keep notes for one season — the date you actually saw the last frost, and the first — and shift the dates by the difference. After two years of notes you will have something better than any published table, because it describes your garden rather than a region.
Common questions
What is the last frost date?
It is the date by which the last spring frost has occurred in half of all recorded years. Frost after that date happens roughly one year in two, which is why tender plants should go out about two weeks later rather than on the date itself.
How do I find my hardiness zone?
US gardeners can look it up from a postal code on the USDA plant hardiness zone map. Elsewhere, match on your average coldest winter temperature — or use the maritime setting if you have mild winters and cool summers.
Can I trust these dates exactly?
Treat them as a regional starting point, not a local forecast. Elevation, slope, walls and urban heat can all shift your own dates by two weeks, so use the adjustment field once you have kept notes for a season.