Gardening in Michigan: zones, frost dates and when to plant

Two peninsulas, two calendars

MSU Extension no longer uses one statewide planting schedule. It publishes separate gardening calendars for the Lower Peninsula and the Upper Peninsula, because differences in latitude, elevation and Great Lakes influence can move planting dates by several weeks.

The Lower Peninsula calendar reflects earlier soil warming and a longer season across most of that peninsula. The Upper Peninsula calendar is built for a short season with later spring frosts and earlier fall frosts. MSU says UP gardeners often cannot plant warm-season crops until early to mid-June. In the UP, most fall vegetables go in from July through early August rather than into September. Use the frost dates for your city above to fine-tune either calendar.

Within the Lower Peninsula, the difference shows up in spring too. MSU’s Smart Gardening sheet on planting vegetables says hardy crops like onions, peas and spinach usually go in from late March to early April in southern Michigan, and one to three weeks later farther north.

The Great Lakes effect

MSU’s Department of Geography explains that the lakes mostly affect weather close to shore, especially on the lee side (the eastern and southeastern shorelines).

• Spring and early summer: from April through August, and especially in spring, the lakes are colder than the air and pull heat out of it. Lake breezes keep summer afternoons several degrees cooler near the shore, usually only a few miles inland. Shoreline areas also get more fog in spring and early summer. The Lake Superior shore is the foggiest.
• Winter: areas about 20 miles downwind of Lake Michigan have somewhat milder minimum temperatures than places farther inland.
• Snow: NOAA notes that the south shore of Lake Superior in the UP and the eastern shore of Lake Michigan get far more snow than the rest of the state. Parts of the UP average more than 180 inches.

What limits gardeners here

Beyond the short UP season, the big risk is a false spring. NOAA reports that Michigan has warmed almost 3°F since the start of the 20th century, mostly in winter and spring. Warmer springs lengthen the season but can raise the risk of a damaging freeze. In 2012, record March warmth pushed fruit trees into early bloom, and an April freeze then destroyed much of the crop. NOAA also reports more frequent heavy rainstorms in recent years.

What MSU Extension recommends

Both MSU calendars put soil temperature ahead of air temperature. Cool-season crops such as lettuce, spinach, kale, carrots and onions germinate best once soil reaches 55°F or warmer. Warm-season crops such as tomatoes, peppers, squash, cucumbers and corn need 75°F or warmer soil to establish well. Cold soil can damage roots and stunt warm-season crops. MSU Enviroweather posts current soil temperatures.

Other calendar advice:

• Start spring transplants indoors about four weeks before they go outside, and harden them off for 7 to 10 days.
• Cool-season crops tolerate light frost and often taste better after a chill, which makes them good fall crops.
• For fall planting, count back from your first frost using the days to maturity on the seed packet.

The Smart Gardening sheet adds germination thresholds. Some extra-sweet corn varieties germinate poorly unless soil at seed depth is above 60°F. Beets, carrots, radishes and turnips will germinate at 40°F. Early sweet corn varieties can usually go in a couple of weeks before other warm-season vegetables. Seeds of warm-season crops sown too early can rot in cold, wet soil.

Soil testing

MSU’s calendars include soil testing among the seasonal tasks. The Upper Peninsula calendar lists it in spring as the ground thaws and again in fall if the ground isn’t frozen. MSU Extension’s Home Lawn and Garden Soil Test Mailer is sold through county MSU Extension offices and the MSU Extension Bookstore.

For a garden sample, collect 10 random slices about 7 inches deep, mix them, and air-dry 1 to 2 cups without artificial heat. The report gives soil type, pH, organic matter, phosphorus, potassium, calcium and magnesium, plus nitrogen and lime recommendations. MSU notes that most garden plants do best at a pH around 6.5. Blueberries, rhododendrons and azaleas are among the exceptions.