- The last freeze date, also known as the last frost date, is the last time overnight temperatures reach 0 degrees Celsius or 32 degrees Fahrenheit. According to the Weather Channel, the average last freeze date for southern Michigan is May 1; for northern Michigan it is May 15. These dates tell you the earliest you can plant your vegetables, but since they are averages the last frost each year could be earlier or later.
- The first freeze date, also known as the first frost date, is when overnight temperatures reach 0 degrees Celsius or 32 degrees Fahrenheit after the last freeze date. For the south third of Michigan (south of Grand Rapids or Flint), this is normally around Oct. 15. For northern areas of Michigan, this will fall around Sept. 15 or Oct. 1. The date occurs earlier the farther you are from the lake shore. Subtract the time it takes a plant or seed to reach maturity from these dates to ensure that your vegetables have a long enough growing season.
- The soil must reach a certain temperature for seeds to germinate. According to Weekend Gardner, you can determine the soil temperature by placing a soil thermometer into the ground 4 inches. Then, take the temperature of the soil at the same time for several days (midday is best) for several days in a row. Then, average the temperatures to determine your soil temperature. The soil temperature will vary depending on how much sun an area gets, so, for the best results, take the temperature in several locations.
- Cool season vegetables grow best in cooler weather and can handle some freezing temperatures. Cool season vegetables that can grow when the soil temperature is down to 40 degrees Fahrenheit include lettuce, kale, parsnips, peas, radishes and spinach. Cool season vegetables that can grow when the soil temperature is down to 50 degrees Fahrenheit include Chinese cabbage, onions, potatoes, Swiss chard and turnips. Cool season vegetables that can grow when the soil temperature is down to 50 degrees Fahrenheit include asparagus, beets, broccoli, Brussels sprouts, cabbage, carrots and cauliflower.
- Warm season vegetables cannot handle any freezing temperatures and must be planted after any danger of frost. Most warm season vegetables require a soil temperature of 70 degrees Fahrenheit, including cucumbers, eggplant, muskmelon, watermelon, pumpkins, sweet potatoes, squash and tomatoes. Beans can be planted when the soil temperature is at 60 degrees Fahrenheit, provided all danger of frost has passed.
