July 21, 2026 · Updated September 12, 2026

Your last-frost date is probably wrong for your yard

The last-frost date you look up is an average built from stations spread across a wide region, and your yard sits somewhere off that average. The date marks the day after which a spring frost grows unlikely across that region. It says little about the cold pocket behind your fence. To garden by your own frost risk, you start from the published date and then adjust it to what your ground records.

Where the published date comes from

The published last-frost date is a probability line. Agencies take decades of readings from official stations and find the date after which the chance of a 32°F night drops below a set level. Those stations sit in open fields, at airports, and on rooftops. Your bed sits among fences, walls, slopes, and pavement that each hold or shed heat differently. The date fits the region and blurs the single yard.

Why your yard runs colder or warmer

Once the sun goes down, the ground radiates its stored heat upward toward open sky. On a calm, cloudless night the plant surfaces shed heat faster than the air cools, so leaves can drop below air temperature. Cold air weighs more than warm air, so it slides downhill and gathers against anything that blocks its path. A low corner, the foot of a slope, or a spot behind a solid fence traps that cold and reads colder than the regional station.

• Low ground. Cold air flows downhill and settles, so a dip or valley floor frosts first and clears last.
• Open sky. A bed with nothing overhead radiates heat straight up and cools faster than one under a tree or an eave.
• Hard surfaces. Pavement, walls, and stone soak up daytime heat and release it at night, which warms the beds beside them.
• Wind and water. A breeze mixes warm air down and blunts frost, and a large pond or the coast slows the swing.

What our own garden reads

In our own garden the published last-frost date is April 28. Our connected station runs about 7°F colder than the forecast on the nights that matter, so the date we plant by moves to May 20. That is more than three weeks later than the printed date. The last spring frost we recorded landed on April 30, two days past the published date and inside our adjusted window. We garden in zone 7a, and the map tells a warmer story than our thermometer does. You can check whether your own yard freezes on a given night in our guide to a freeze warning and your yard.

How to find your yard's date

There are two paths. The free path is a manual offset. You watch a few cold nights, see how your yard's low compares to the forecast, and enter that gap in Settings. Siltwell then subtracts your offset before it warns you. The Pro path connects a supported station and learns the offset from your own readings. It compares your station low to the forecast each night and trusts the learned number after about 14 overlapping nights. See how to connect a station if you want the learned version.

Without a station of your own, the public microclimate checker fills the gap. It weighs the forecast low against readings from nearby home stations over recent nights and reports the typical gap for your area. The result carries a station count and a confidence figure, so you gauge the size of your own offset before you spend anything.

Start logging your overnight low against the forecast this fall, and match your planting to your adjusted date next spring. Once you know your real low, you can choose plants that survive it with our guide to frost-resistant plants and your real low.