The risk of frost is quickly going away until fall

Photo: Max Pixel

CEDAR RAPIDS, Iowa (KCRG) - Monday morning saw temperatures dip into the 30s in many places, but the forecast has many overnight lows in the 50s. With this and what’s happened in the past in mind, the potential for frost is virtually over for the season.

We can get patchy frost when temperatures fall to around 36 degrees (since the temperature at the ground may be several degrees colder than where the thermometer is). The average last 36-degree low in Cedar Rapids is May 8; the earliest was April 9, 1896 and latest was June 4, 1945. The average date of the last freeze, 32 degrees, is April 25. The earliest was March 29, 1999 and latest was May 26, 1925. Finally, a hard freeze of 28 degrees has an average last date of April 13. The earliest was March 21, 1895 and latest was May 25, 1925.

There are a few rules-of-thumb that all essentially recommend waiting until about the second week of May to plant, and that includes the Czech legend of the Three Frozen Kings. The legend goes something like this: three saints, or kings, were frozen while fishing at sea – Pankrac on May 12, Servac on May 13, and Bonifac on May 14. Then, on May 15, Saint Zofie thaws them from their icy situation.
The moral of the legend is that it’s not a bad idea to wait until after May 15 to put in your sensitive plants, just in case a late frost comes along. It turns out there’s a meteorological basis for that!

Here are other frost and freeze dates for eastern Iowa:

Dubuque
36 degrees: Average last: May 6; earliest: April 10, 1985; latest: June 11, 1972
32 degrees: Average last: April 24; earliest: March 28, 1878; latest: May 29, 1965
28 degrees: Average last: April 11; earliest: February 27, 1878; latest: May 10, 1966

Iowa City (dates may be one day later than the actual date because of when the observation is taken)
36 degrees: Average last: May 6; earliest: April 9, 1955; latest: May 31, 1897
32 degrees: Average last: April 23; earliest: March 24, 1998; latest: May 25, 1925
28 degrees: Average last: April 14; earliest: March 23, 2017; latest: May 11, 1966

Waterloo
36 degrees: Average last: May 11; earliest: April 15, 1985; latest: June 4, 1945
32 degrees: Average last: May 3; earliest: April 7, 1955; latest: May 31, 1897
28 degrees: Average last: April 19; earliest: March 24, 1998; latest: May 14, 1895