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

North, south and the lakeshores

NOAA’s state climate summary explains why Wisconsin’s weather swings so widely. With no mountains to the north or south, the state gets bitter Arctic air in winter and warm, humid Gulf air in summer. The south has cold winters and mild to hot summers. The north has frigid winters and generally cool summers, with occasional short hot spells. Lake Superior and Lake Michigan moderate temperatures for places near their shores. Annual average temperatures run from about 39°F in the north to 50°F in the south.

UW-Madison Extension’s planting advice follows the same pattern, in four broad areas:

• Southern Wisconsin: the baseline for Extension planting dates.
• Central Wisconsin: plant about a week later than the south.
• The lower Lake Michigan shore: also about a week later than the south.
• Northern Wisconsin: later again, and the season is short.

What limits Wisconsin gardens

A short season up north. UW-Madison Extension’s The Vegetable Garden (A1989) warns that the northern season is sometimes too short to fit in succession plantings.

Cold, wet springs. The NOAA summary expects more winter and spring precipitation, which means a continuing risk of delayed spring planting. Heavy silt loam and clay soils make this worse, because they hold water and warm slowly. Sandy soils dry and warm faster, but they hold little water or nutrients.

Late freezes after warm spells. NOAA notes that a “killer frost” in 2012 followed an unusually warm March and badly damaged fruit crops. An early warm spell is no sign that the danger of frost is over. Check the frost dates for your city above before setting out tender plants.

Drought. The same summary calls severe drought a natural part of Wisconsin’s climate, and Extension’s site selection article recommends a garden with an easy water source for dry spells.

What UW-Madison Extension recommends

The Vegetable Garden is the state’s core home garden guide. It covers garden layout, soil, fertilizer, planting and care, and includes a planting guide with dates for southern Wisconsin. Its two sections on timing word the northern adjustment slightly differently. One says to plant about two weeks later in northern Wisconsin, and the other says 10 to 14 days. Either way, the gap is about two weeks. Central Wisconsin is about one week behind the south.

Other points from the guide:

• Where to plant first: sandy or well-drained soils, especially on south-facing slopes, dry and warm sooner and can be planted earlier.
• Heavy soils: use raised beds and organic matter to improve drainage, and work clay only when it is a bit dry, or it packs into clods.
• Feeding: on sandy soils, side-dress heavy feeders like sweet corn, tomatoes and peppers later in the season.
• Clear plastic mulch: it warms soil more than any other mulch, which makes it especially useful in northern Wisconsin. Weeds grow under it, so it works best for the first six weeks or so.

Go by soil temperature

UW-Madison Extension in Dane County argues that soil temperature is a better guide than a calendar date, especially in springs that turn warm early or stay cold and wet. Take readings on three mornings in a row, at 2 inches deep for seeds and 4 to 6 inches for transplants. Minimum soil temperatures for planting include:

• About 45°F: lettuce, spinach, carrots, beets and radishes.
• About 50°F: peas.
• About 65°F: cucumbers and peppers.
• About 70°F: tomatoes and squash.
• About 72°F: beans.

Choosing the site

The Extension article on site selection calls for at least six hours of direct sun. It also says to avoid low spots where frost settles in late spring and early fall. Keep gardens out of the root zone of black walnut trees, whose juglone harms many vegetables. Before planting, work 2 to 3 inches of compost into the soil.

Soil pH and testing

UW-Madison Extension sources give slightly different pH targets. The Vegetable Garden puts the best range for most vegetables at 6.0 to 6.5 and says not to add lime unless a soil test calls for it. The site selection article calls about 6.5 optimal but says most vegetables grow very well anywhere from 6.0 to 7.2.

For a test, contact the UW Soil and Forage Lab in Madison and request a kit. It comes with sampling instructions, the submission form and mailing bags. The lab does not test for lead. UW-Madison Extension points gardeners to the Wisconsin Occupational Health Laboratory for lead testing.