Vegetable Planting Calendar by Zone: When Each Crop Goes In

Every date on the board comes from the record behind the site's frost-date lookup: 5,363 NOAA stations, each with the 1991–2020 climate normals computed for its own coordinates and the USDA 2023 half-zone it sits in. A half-zone's spring date is the median of the stations inside it — the date half of them freezes on or after — rather than the average of all of them. The distinction matters: freeze dates cluster with a long tail of late years, and an average is dragged out into that tail by a handful of cold springs.

A median is a fifty-fifty date, and the Missouri Extension guide to frost probabilities is the clearest statement of what that means: at a station whose median last freeze is April 20, half the years freeze after April 20. The colder-tenth date beside it is the same calculation one decile further into the tail, and it is the one to plan a transplant around when the crop cannot afford a setback.

A half-zone is a band on a map, and the stations inside it disagree. Elevation, distance from water, slope, soil texture, wind and shelter each move a freeze date, and a garden at the bottom of a valley can freeze a fortnight after one on the ridge above it. That spread is why the zones are drawn in five-degree bands rather than as a single number per region, and why a board of half-zone medians still leaves room for two neighbours to plant at different times.

The sharper reading is not the band. The lookup takes one station's coordinates, and that station's median across its own winters anchors a window better than the half-zone it happens to sit in: the zone narrows the season, the station dates the window. Two gardens with the same zone number but different elevation, water and shelter are genuinely on different calendars, which is how one zone chart is right in one yard and a fortnight early in the next.

The last spring freeze is a risk that arrives once and is over; the first autumn freeze is a deadline that closes in all season. A window counted forward from the spring date is waiting for safety, and a window counted back from the autumn date is racing a clock. That asymmetry is why a late spring freeze costs an early sowing without shortening the autumn at all.

It is also why the fall sowings here count backwards from the first autumn freeze, and why a half-zone with a short season loses its fall windows rather than getting them moved earlier: twelve weeks before a late-August freeze is ground that is still frozen in June. Where a crop cannot fit a second sowing, its row says so instead of printing a date that will not work.

A window is a range of dates that are usually safe; the soil decides when the crop actually roots. Lettuce, carrots and potatoes work from about 45°F, tomatoes and beans from about 60°F, peppers and cucumbers from about 65°F — which is how the same crop goes into two gardens three weeks apart inside a single half-zone. Those floors come from the crop record the live planting calendar works from, so the window beside a crop and the recommendation in the calendar cannot tell two different stories about it.

Where the probe goes decides the answer: the same bed that reads 60°F at noon can read 52°F at dawn, and the dawn reading is the one the roots meet. A raised bed against a south wall reaches it a week or two before the low ground beside it, which is why one half-zone holds both a March sowing and an April one.

The National Weather Service issues a freeze warning when the growing-season low is forecast at 32°F or below, and a frost advisory from 33°F to 36°F on a clear, calm night. The two products describe two different nights: an advisory is a light ground frost that marks tender growth, a warning is a killing temperature at plant level. Neither of them appears on a zone chart, and either can arrive in the fortnight after a window opens.

So the order is zone, then window, then the week ahead — and a cold forecast after the window has opened does not close it. A crop held in its pots through a cold snap is usually ahead of one planted into cold soil, and row cover, a cloche or a warm wall buys back most of a fortnight. A half-zone's date is the date to start paying attention rather than the date to stop thinking.