Joel Rosen lives at Park Lake Farm in Mahtowa MN, in Carlton County. When we talked to him (on April 26th) freezing drizzle had put down a quarter inch of ice at the farm. "Whether weather like this is a setback to planting depends on your soil," Joel observed. "Light soil dries faster than heavy soil."
In the 20 years Joel Rosen operated Park Lake Farm he kept records about the growing season. Those records are the basis for a spreadsheet he originally developed for the Duluth Community Garden newsletter. "The spreadsheet should work for the entire KAXE listening area," he said, "if you apply two main concepts--the number of frost-free days in a season and growing degree days." Frost dates determine how many frost free days there are on average before frost will occur. Growing degree days measure warmth. Warmth is important in planning and planting vegetables. "Sweet corn can be problematic. It will take a very light frost but it will rot if the soil is wet and under 60 degrees, unless it's treated." One time the weather was so cool and aggravating that Joel tried planting sweet corn on July 1. The ground was still too cold. The seed rotted. According to his spreadsheet June 15 is the last date to plant sweet corn.
"The MN DNR has a climatology website with a color map. There are between 1200 and 2000+ growing degree days across Minnesota," Joel said. Temperatures get warmer as you travel westward from Lake Superior across the northern part of the state. Where it's warmer it's easier to grow more warm weather plants. Plants don't grow much when temperatures fall below 50, so growing degrees are daily high temperatures minus 50 degrees.
Joel advises that the best things to seed directly in the ground this time of year (around May 1--weather permitting) are spinach, arugula and lettuce. Joel says these seeds are relatively safe because they don't rot in cold, wet soil (although if it's really cool it'll be a while before they actually grow). He recommends avoiding peas in spite of common folklore advising early planting. "Unless they're treated they will rot if the soil is cold and wet."
Joel retired from farming 5 years ago. His garden is downsized since he was a market gardener, and he doesn't conduct many trials anymore. However, he says that climate change is forcing him to re-think how he grows garlic. "There won't be any garlic this year. I have never had to mulch it in the past but plan to mulch it next year for the first time."
Listen to Joel's entire interview and his advice below. For a PDF version, click HERE.