Gardening in Wisconsin

Hardiness zones in Wisconsin

Wisconsin covers USDA hardiness zones 4a through 6a. That range spans five zones in all: 4a, 4b, 5a, 5b, and 6a. A hardiness zone marks the average coldest winter temperature a spot in the state sees in a typical year. A lower number, like 4a, means colder winter lows than a higher number, like 6a. The zone number says nothing about summer heat, rainfall, or soil, so two gardens in the same zone can still need different care. If you garden in Wisconsin, check which of these five zones covers your spot before you pick plants rated for a minimum winter temperature. Plants rated for zone 5 or colder will handle most of the state, while a plant rated only for zone 6 may struggle in the northern two-thirds.

• Zone 4a
• Zone 4b
• Zone 5a
• Zone 5b
• Zone 6a

Frost dates

Across Wisconsin's weather stations, the last spring frost has come as early as April 19 and as late as June 3, depending on where you are. The first fall frost has arrived as early as September 18 in some spots and as late as October 29 in others. Those four dates mark the typical 50% chance of frost at a given station, not a guarantee. Use your local last frost date to judge when it's safe to move tender seedlings outside. Add a week or two of buffer if your spot sits low or near open water, since cold air settles there. Count back from your local first fall frost date to see how many weeks of warm-season growing remain. The state's frost dates span more than five weeks in spring and six weeks in fall. A planting calendar written for one part of Wisconsin will not fit another. Find the station nearest your garden for the most useful numbers.

Climate

Wisconsin's climate data comes from 201 weather stations across the state. That means the numbers below reflect real variation from the far north to the state line with Illinois. Winter hardiness zones run from 4a in the coldest spots to 6a in the mildest, a three-zone spread within one state. That spread shows up in frost timing too. The earliest last spring frost on record for a station is April 19, while the latest is June 3. That's a gap of more than six weeks. Fall works the same way. The earliest first frost is September 18 and the latest is October 29, a gap of about six weeks as well. Put together, a station's frost-free growing season runs from as short as 100 days to as long as 192 days. Almost a hundred extra growing days separate the shortest season from the longest, even though both are still within Wisconsin's borders. If you garden in the north, near zone 4a, plan on a shorter season and colder winter lows. A gardener near zone 6a in the south sees the opposite. None of these figures say anything about summer rainfall, humidity, or heat, since the data here covers only hardiness zones, frost dates, and season length. For anything beyond that, look at the station record closest to your own garden rather than a single statewide average.

Season length

A growing season in Wisconsin can run as short as 100 days or as long as 192, depending on the station. That's nearly twice as many growing days in the mildest spots as in the coldest ones. A 100-day season leaves room mainly for fast, cold-hardy crops started close to the last frost date. A 192-day season gives you enough time for long-season vegetables and a second planting after an early crop finishes. Check which end of that range your own area falls closer to before you choose what to grow and when to start seeds. Local frost dates, not the calendar month, are the better guide for timing in a state with this much spread.