Warm Season Plants and Frost-Free Date

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As we approach the end of April and the temperatures warm into the 70's, we get the "itch" to plant our gardens. But when is it safe to plant our warm-season vegetables and to move our tropical house plants outside?

Our warm season vegetables like cucumbers and squashes need a soil temperature of 70 deg. F.  Tomatoes and green beans need soil temperatures in the mid-50's and minimum temperatures that are above 45 deg. F.  For us here in this part of Illinois, we typically can not expect those kind of temperatures until after our last expected frost.

According to the Illinois State Water Survey here in central Illinois, a frost can happen up until May 7 through May 12 depending on your location. For ease of remembering, I tell people to plant warm season crops on or after Mother's Day.

If you are willing to protect cold-sensitive plants, then you might be more interested in the median (average) frost date, which varies from April 14 to April 22. This year we had temperatures in the mid-20's on April 16 and 17, so just remember a median or average frost date means that 5o percent of the time the temperature will be above or below 32 deg F on that date.

Another important point to remember is the number of frost-free growing days we have here in central Illinois. On average, we can expect to have about 175 to 185 days. I normally figure on 180 days. This means that if I want to grow artichokes or ginger, I am going to have to either start them indoors or provide them some sort of a season extension structure to grow in, such as a hoop house or greenhouse.