Onions require strategic timing due to Minnesota’s short growing season and northern latitude. They need a long period of vegetative growth before forming a bulb. Understanding the crop’s specific needs, particularly its sensitivity to day length, is the first step in ensuring a healthy and abundant harvest. Onions are cool-weather tolerant, allowing them to be planted early, provided the soil is prepared and the correct variety is selected.
Selecting the Right Onion Type for Minnesota
Onion development depends on photoperiodism, categorized by the duration of daylight required to initiate bulbing. Minnesota’s far northern latitude experiences very long summer days, making long-day onions the necessary choice for forming large, mature bulbs. Long-day varieties need approximately 14 to 16 hours of sunlight to transition from leaf growth to bulb formation. Planting a short-day variety will cause the plant to bulb too early, resulting in small bulbs because the foliage has not developed sufficiently. Since each leaf corresponds to one ring in the mature onion, maximizing leaf production is crucial for size. Reliable long-day varieties include ‘Walla Walla,’ ‘Redwing,’ and ‘White Sweet Spanish.’
Optimal Planting Windows
Onions should be planted in early spring, as soon as the soil is dry and workable. They are cold-tolerant and can withstand light frosts, allowing them to be planted before many other crops. The planting window generally opens when the soil temperature consistently reaches 50°F, though some plant as cold as 45°F. This timeline typically falls four to six weeks before the average last spring frost date, meaning planting occurs from late April through early May. This early start allows plants to establish a robust root system during the cool spring weather. Planting too late delays the vegetative growth phase, causing the plant to begin bulbing before achieving sufficient size. Fall planting is occasionally attempted for overwintering varieties like scallions, but Minnesota’s severe winters present a significant risk for the survival of most bulbing onions.
Planting Methods: Seeds, Sets, and Transplants
The three primary ways to start onions—seeds, sets, and transplants—each have a distinct timeline.
Seeds
Growing from seed offers the widest selection of long-day varieties but requires a substantial lead time. Seeds must be started indoors approximately 10 to 12 weeks before the target outdoor planting date, usually in late January or early February. This indoor head start ensures the plants are mature enough to utilize the full summer growing season outdoors.
Sets
Onion sets are small, immature bulbs grown the previous year and forced into dormancy, offering the easiest and fastest path to a harvest. However, commercially available sets are often short-day varieties and are more prone to bolting if planted too early. Sets should be planted with the pointy tip facing upward, just deep enough that the top of the bulb is barely covered by soil, which helps prevent rot and encourages proper bulbing.
Transplants
Transplants are seedlings started by a grower and sold in small bunches. They offer the best balance of maturity and variety selection for the average gardener. These seedlings should be planted about two inches deep and spaced three to four inches apart. Transplants tolerate light frost and are planted concurrently with sets in the early spring window, giving them a significant advantage over direct-sown seeds.
Post-Planting Care and Harvest
Once onions are in the ground, they require consistent care because they are shallow-rooted and easily outcompeted. The plants need uniform moisture, typically one inch of water per week, especially as the bulbs swell. Weeding must be done frequently and shallowly, as deep cultivation can damage the surface-level root system. Onions are heavy feeders and benefit from side-dressings of nitrogen fertilizer early in the season to support leaf growth. Applying too much nitrogen later, however, results in thick necks that are difficult to cure and soft bulbs with poor storage quality. Fertilization should cease when the onion bulb begins to push the soil away and the plant starts to mature. Onions are ready for harvest in mid-to-late summer when about half of the foliage yellows and flops over naturally. After pulling the bulbs, they must be cured for long-term storage. Curing involves drying the onions in a warm, dry, and well-ventilated location, ideally between 75°F and 90°F, for two to four weeks. The process is complete when the outer skin is papery and the neck connecting the top to the bulb is completely dry and tight.