Frost date finder
Frost date finder
How this works
A frost date is not a date. It is a probability, and reading it as a deadline is the most common way gardeners lose a crop. The figures this tool shows come from the NOAA National Centers for Environmental Information 1991-2020 U.S. Climate Normals: for every weather station, thirty years of daily minimum temperatures are sorted, and the software reports the day by which a freeze at 32 °F had occurred in 10 per cent, 50 per cent and 90 per cent of those years. The 50 per cent date is a coin toss. Half the years in the record had their last spring freeze before it, and half after it. That is why three dates appear instead of one. The 90 per cent last-spring date is the conservative one: in nine years out of ten, the last freeze had already happened by then, so a tomato set out on that date survives nine seasons in ten. The 10 per cent date is the optimistic one, useful for hardy greens that shrug off a light freeze and worthless for anything tender. In autumn the arithmetic runs the other way: the 10 per cent first-fall date is the early, cautious one, and the 90 per cent date is the late, optimistic one. The number of days between the two 50 per cent dates is the average growing season, which is what a seed packet means when it prints "days to maturity" and expects you to do the subtraction. The dates are measured at a weather station, not in your garden, so the tool names the station and how far away it is. A station twenty kilometres away on the other side of a ridge can be a week apart from you. Cold air drains downhill and pools in hollows, so a low garden frosts earlier and later than the station; a town centre, a south-facing slope or a spot beside a large body of water runs warmer. Some stations publish no frost date at all, because freezing is rare enough there that thirty years produced too few events to build a normal from. That is a real answer, and the tool shows it as one rather than inventing a date. Two more limits are worth stating plainly. These are 32 °F freeze dates, which is not the same as frost: frost can form on a clear still night while the air a metre up reads a degree or two above freezing, and on a windy night the air can drop below freezing with no frost visible at all. And the normals cover 1991 to 2020, so they describe the recent past, not the season you are about to have. Nothing here replaces a local forecast in the week you plan to plant.
Using the result
Work backwards from the percentile that matches what you are risking. For tender crops that a single freeze destroys — tomato, pepper, basil, squash — gardeners commonly use the 90 per cent last-spring date, or the 50 per cent date with fleece or a cloche on hand. For hardy crops that tolerate a light freeze — peas, spinach, kale, onions — the 50 per cent date or earlier is normal practice. For starting seeds indoors, count back from your chosen transplant date by the weeks the packet names. In autumn, reverse it: count back from the first-fall date at the percentile you are comfortable with to find the last sensible sowing date for a crop, and remember that growth slows sharply as days shorten, so a crop needs more days in September than the same crop needed in June. Frost dates and hardiness zones answer different questions, and most planting decisions need both. Every plant page on this site lists its hardiness zones alongside the source.