The growing season is often defined as the number of days between the last time the temperature was 32 degrees in spring and the first time the temperature falls to 32 in fall.
The latest frost in spring is important to gardeners as they seek to plant after a long winter, and for most plants it is wise to do so when the temperatures are likely to remain warmer than 32 degrees.
For our region, based on temperature observations between 1981 and 2010, the median date of the last frost is between May 1 and May 10. That is why many gardeners consider Mother’s Day a good day to plant.
One reason for the variation in the last frost date is how the first frost date is defined.
Giving the median date of last frost means that there is still a 50 percent chance that a frost will occur after this date. Some definitions require that there is only a 10 percent chance of a 32 or lower temperature.
Some sites use a different 30-year period, for example, between 1951 to 1980. As it turns out, our nighttime minimum temperatures have been getting warmer, so the last frost date has been moving earlier in the spring.
The change is not much for Dane County, but in northwestern Wisconsin, the last frost date now occurs about two weeks earlier than it did in 1950.
According to the Wisconsin State Climatology Office, for Madison, based again on the 1981-2010 time period, the median date for a temperature of 32 is May 2, and it isn’t until May 20 that the probability of an occurrence of 32 has dropped to 10 percent. On April 18, there’s a 90 percent chance of a later occurrence of 32 degrees.
So, if you have not yet done your planting, now would be a good time.