Over the 19th and 20th centuries, frosts in the month of May were pretty common in many western and southern Minnesota counties, where farmers were accustomed to waiting until midmonth to do most of their planting.

A look at the historical climate data for many Minnesota counties — Chippewa, Yellow Medicine, Brown and Cottonwood among them — shows that minimum temperatures of 32 degrees F or colder occurred with an average frequency of three to four times each May over the period from 1890 to 1990. In some years, frost came as many as seven or eight times during the month. In both 1907 and 1929, many climate observers reported frost on 12 days during the month, causing crop damage to early-planted fields.

Occasionally, frost was reported as late as Memorial Day (May 31), as was the case in Chippewa County in both 1897 and 1947.

For generations of citizens, this was an important climate feature that prevented farmers from wanting to plant corn too early in May and bearing the risk of frost damage.

Today, farmers worry more about getting crops planted on time, handling early-season weed control, and timing split applications of fertilizer than they do about frost in late spring. This is certainly justified for both climatic reasons and new seed technologies.  Most corn hybrids have been bred for cold tolerance as well as early-growth vigor.

Even when a rare late-May frost occurs well after corn emergence, it is often the case that the growing point inside the stock is still below the soil surface and remains undamaged. In most cases, the crop is set back for a day or two and perhaps shows some damaged leaf tissue, but it soon recovers and resumes vigorous growth.

With that point aside, however, the climate frequency of May frosts in western and southern Minnesota counties has declined remarkably since 1992. In fact, over the past 25 years, the frequency of frosts in May across the state is about half of the historical frequency before 1990. This changed feature of the Minnesota climate is logical, in that average May minimum temperatures have increased by several degrees over recent decades in conjunction with increased measured water vapor (dew points).

Though we still occasionally record frosts during the second half of May, as was the case in 2002 and 2014, the climatic trend shows that this is becoming an even more unusual occurrence than it has been in the past.

We’ll see what 2018 brings. Don’t be surprised not to see signs of frost after the first week of the month.

Seeley is professor emeritus of climatology at the University of Minnesota.