As a Cincinnati native, I find it very surprising that so many people that live in the Tri-State – including many natives – that don’t expect the temperature to drop near freezing in late April or early May. Hope springs eternal? Perhaps. While many cheer for warmth all spring, flowers almost always start blooming before the last 32° or 36° temperature of the cold season comes.
While a temperature to or below 32° is scored as a freeze, frost can develop at various temperatures. The temperature at eye-level can be 40°, but there can still be frost at your feet. At local airports (where the meteorological standard is used), the temperature is measured 2 meters (or roughly 6.5 feet) above the ground:
Because relatively cold air sinks and relatively warm air rises, the temperature of the air below the temperature sensor’s height is likely below the temperature of the air at the temperature sensor’s height. If a plant was planted below the temperature sensor at an airport, it may be damaged or killed on a morning where the temperature sensor above it recorded a low of 35° to 40°. This is why Frost Advisories are issued when the temperature drops into the mid and upper 30s.
When does history suggest you’ll be able to plant your flowers and completely forget (well, virtually) about a killing frost or freeze? A sign at Natorp’s Nursery Outlet in Mason says:
…but historical records suggest otherwise!
The latest freeze of the spring in Cincinnati typically comes in April, but we have gone the entire month of April without a freeze and had to wait until late May to get the last freeze of the season:
A freeze (hitting 32° or below) will kill plants very sensitive to the cold and severely damage plants sensitive to the cold. A frost, however, will damage – in full or part – plants sensitive to cold. Using a temperature of 36°, Cincinnati’s last frost of the spring typically comes in late April; historically, it has come as early as late March and as late as late May:
Mother’s Day is usually a save bet for planting flowers; more specifically, looking at the forecast on Mother’s Day to make sure the temperature won’t drop below 40° is a save bet for planting flowers. Since 1871 (not including 1872 because those spring records are missing), the historical odds of the last spring frost (using a temperature of 36°) occurring during May in the Queen City is 32.8%. In other words, the last frost of the year comes before May begins two out of every 3 years.
So…back to the big question…when does history suggest you’ll be able to plant your flowers and completely forget about a killing frost or freeze? Based on records back to 1871, you’ll have to wait to late May to completely avoid a freeze and June 1st to avoid frost:
Happy planting! But seriously, wait until Mother’s Day.