Growing vegetables in Wisconsin requires mastering the calendar due to the state’s short, intense growing season. Precise timing is necessary for a bountiful harvest. Planting calculations are based on average weather patterns, soil conditions, and the specific temperature needs of each crop. This guide outlines the Wisconsin planting season, from the earliest spring sowings to preparing transplants indoors.
Defining Wisconsin’s Growing Season
The gardening year is defined by the risk of freezing temperatures. Wisconsin requires planning around two critical dates: the last expected spring frost and the first expected fall frost. The period between these events is the true growing season for most temperature-sensitive vegetables.
The state spans USDA Hardiness Zones 3b (far north) to 6a (Lake Michigan shoreline). The last expected frost date varies significantly, ranging from mid-April in southern counties to mid-June in northern regions. Gardeners must use the average last frost date for their specific location as a starting point for outdoor planting calculations.
This “average” date carries a 50% chance of a subsequent frost, acting as a guideline and requiring local weather monitoring. The growing season concludes with the first expected fall frost, which occurs from early September in the north to mid-October near Lake Michigan.
Timing for Cool-Season Vegetables
Cool-season vegetables thrive in spring and fall, tolerating light frost and preferring soil temperatures below 70°F. They should be direct-sown as soon as the soil is workable, typically four to six weeks before the last expected spring frost date. This early start allows them to mature before summer heat causes them to “bolt” (prematurely produce seed stalks).
Crops ready for this early planting include peas, spinach, radishes, lettuce, and carrots. Peas can often be planted in late March or early April in the southern half of the state, as they germinate well in cooler soil. Spinach and kale benefit from early planting, as flavor improves in cooler conditions.
For a continuous supply, utilize succession planting by sowing small batches of seeds every two to three weeks. A second harvest can be planned by planting these crops again in mid-to-late summer (mid-August to early September). This timing ensures vegetables mature during the cooling temperatures of early fall, often resulting in sweeter produce after a light frost.
Timing for Warm-Season Vegetables
Warm-season vegetables require consistently warm conditions and cannot tolerate any frost. These crops, including tomatoes, peppers, squash, beans, and corn, must only be transplanted or direct-sown after the last expected spring frost date has passed. Planting too early exposes these frost-sensitive plants to a high risk of permanent damage.
Soil temperature is a crucial factor for warm-season success, even beyond air temperature. Most seeds and transplants require the soil to reach a minimum of 60°F for optimal germination and root development. Air temperatures remaining below 55°F, even without frost, can severely stunt the growth of young plants like tomatoes and peppers.
For direct-sown crops like bush beans and corn, waiting until the soil is reliably warm in late May or early June is necessary for quick, strong germination. Transplants need a gradual transition to the outdoors before permanent placement. This process, known as hardening off, involves slowly exposing the plants to sun, wind, and cooler night temperatures over seven to ten days.
When to Start Seeds Indoors
Starting seeds indoors provides a head start to ensure warm-season vegetables reach maturity during Wisconsin’s short summer. This involves calculating the required indoor growth time and counting backward from the target outdoor planting date. The goal is to produce a sturdy, compact seedling ready for immediate outdoor planting after the last frost risk has passed.
Peppers should be started earliest (eight to ten weeks before the last expected frost) due to their longer development period. Tomatoes require an indoor start six to eight weeks before the outdoor transplant date. Starting crops too early can result in tall, spindly plants that are less resilient when moved to the garden.
Broccoli and cabbage are often started indoors four to six weeks before the last frost to ensure they are robust enough for early-spring planting. Sufficient light and a warm, stable temperature during this indoor period are necessary to grow healthy transplants.