The successful cultivation of onions in Wisconsin requires navigating the state’s short growing season and unpredictable spring weather. Onions need a long, cool period for leaf growth before they begin forming a bulb, making correct planting timing paramount for a successful harvest. Maximizing the vegetative growth phase before the summer solstice triggers bulbing helps avoid pitfalls like premature bolting or underdeveloped bulbs.
Selecting the Right Onion Variety
Onion bulb formation is controlled by the number of daylight hours, a phenomenon called photoperiodism. Onions are classified as short-day, intermediate-day, or long-day types based on the light duration needed to initiate bulbing.
Given Wisconsin’s northern latitude, which experiences 14 to 16 hours of daylight during the summer, only “long-day” varieties are suitable for producing large, mature bulbs. Planting a short-day variety here will result in small bulbs forming too early.
Long-day varieties need a minimum of 14 hours of daylight to transition from leaf production to bulb enlargement. Examples include ‘Walla Walla’, ‘Redwing’, or ‘White Sweet Spanish’. Choosing a long-day variety ensures the plant develops sufficient leaves, which directly correlates to the final size of the mature bulb.
Determining Optimal Planting Times
The optimal planting time is determined by the material used (sets, transplants, or seeds) and the condition of the soil in early spring. Transplants and sets should be planted as soon as the soil is workable, often late March or early April in southern Wisconsin.
This timing is typically four to six weeks before the last expected spring frost, allowing plants to establish roots in the cool soil. Onions are a cool-season crop and tolerate light frost, so planting should not be delayed until all danger of freezing temperatures has passed.
Starting onions from seed indoors offers the greatest variety selection and is the preferred method for the largest bulbs, but it requires a significant head start. Seeds should be sown indoors approximately 10 to 12 weeks before the anticipated outdoor transplant date, usually in late January or February.
The goal is to have seedlings about a quarter-inch thick when moved outside. Direct sowing seeds into the garden is less common in Wisconsin because the short growing season often prevents plants from reaching full size.
If direct seeding is attempted, it should be done as soon as the soil can be prepared, provided the soil temperature is above 40 degrees Fahrenheit for germination. The primary window for moving transplants or sets outdoors is mid-April, when the ground is thawed and ready. This early start maximizes the vegetative growth period before bulbing is triggered.
Planting Techniques and Initial Care
Onions are heavy feeders with shallow root systems, so proper soil preparation is fundamental. The planting site requires full sun and loose, well-drained soil, ideally a sandy-loam with a pH between 6.0 and 6.8.
Incorporating organic matter, such as compost, before planting improves soil texture and drainage, which is important in heavier Wisconsin soils. A balanced fertilizer, such as a 10-20-10 mixture, should be worked into the soil at planting time to provide immediate nutrients.
When planting sets or transplants, the depth must be shallow to encourage proper bulb development. Transplants should be placed so only about one inch of the lower portion is buried. Planting too deep inhibits swelling.
Sets should be planted just deep enough to cover the bulb, typically one to two inches. Spacing is a factor in the final size of the onion, with 4 to 6 inches between plants recommended for full-sized bulbs.
The most demanding aspect of initial care is weed control, as onions are poor competitors against emerging weeds. Their thin leaves offer little shade, making it necessary to weed consistently throughout the early growth stage.
Consistent moisture is necessary during the early stages because of their shallow roots. Watering should be reduced as the tops begin to yellow and fall over, signaling that the bulbs are nearing harvest.