The biggest question each spring for Michigan State University Extension horticulture educators and Master Gardeners answering hotline questions is, “I am planning my vegetable garden now: When is the last day my garden will get a frost?”
Frost free dates are based on historical information. The frost free date is going to be when there is no danger of frost. The problem is these are based on what has happened, not what is going to happen.
There are 78 weather reporting stations in Michigan that keep track of dates. It is important to choose the one that is closest to where your garden is located. MSU Extension’s Gardening in Michigan website has the frost free chart on its website. (Go to the Vegetables page and it’s under the “Garden planning calendar” section.) The chart gives several dates for each location. It lists the first opportunity for no frost, a 75, 50 and 25 percent chance of no frost, and the last chance for no frost.
Occasionally, new gardeners will think these dates are predictions of what is going to happen, but more experienced gardeners know that each spring in Michigan is a Magical Mystery Tour of unusual weather.
Seeds that are planted and have not come up will not be affected by a frost, but they may not germinate and grow if the soil is too cold. The soil temperature is taken in the top 1 to 2 inches of soil early in the morning before the sun has warmed it. For example, cool season vegetables like spinach, parsnips and lettuce can be planted when the soil temperature is 35 degrees Fahrenheit. This doesn’t mean that they will grow; the seeds just won’t die. Seeds put into cold soil could rot, but these vegetables would germinate and grow when the soil temperatures were between 50 and 75 F. The closer to the higher temperature, the more rapidly the plants begin to grow, so there is little motivation to put seeds into cold soil.
For warm season vegetables like beans, the seeds will germinate at 60 to 85 F soil temperature, but on the cold end, seedlings sometimes look deformed and twisted. Again, there is little reason to put seeds in too early.
Consider what Grandma used to say about transplants and warm season crops like tomatoes, peppers, eggplant beans and corn. Her recommendation was to go to the Memorial Day parade in town, come home and put these in the garden. But since we are living in Michigan, it’s always good to pay attention to the weather forecast and cover transplants if the night temperatures are dipping into the mid-30s. Smart gardeners recognize that Mother Nature always has a wild card to play each spring.
This article was written by Gretchen Voyle for Michigan State University Extension. For more information, visit http://www.msue.msu.edu. To contact an expert in your area, visit http://expert.msue.msu.edu, or call 888-MSUE4MI (888-678-3464).