Eager gardeners will soon be approaching the traditional May 15 “frost rule,” a local guideline that marks a safer window for planting tender crops outdoors in southern Wisconsin. The rule reflects typical last frost dates for the region and helps reduce the risk of cold damage to gardens.

While it’s not an official regulation, the May 15 date has become something of a benchmark in Green County and much of southern Wisconsin. According to data from the National Weather Service and the University of Wisconsin Extension, it’s rooted in historical weather patterns that show that the likelihood of frost drops significantly by mid-May.  And the concept is fairly straightforward.