Growing onions successfully in Michigan requires understanding the state’s climate and specific horticultural practices. The northern latitude presents unique challenges related to day length and the short growing season. By selecting the correct variety and managing planting and maintenance, home gardeners can harvest large, flavorful bulbs. Success depends on an early start and consistent care throughout the summer.
Selecting the Right Onion Variety for Michigan
Successful onion cultivation in Michigan depends entirely on choosing Long-Day varieties. Onions are photoperiodic, initiating bulb formation only after daylight reaches a specific duration. In Michigan, the summer solstice provides the 14 to 16 hours of sunlight required to trigger bulbing in long-day types.
Planting a Short-Day variety, which only needs 10 to 12 hours of light, would result in very small bulbs because they would begin forming before the plant developed sufficient foliage. Appropriate cultivars for this region include ‘Walla Walla,’ a popular sweet onion, and the pungent ‘Yellow Sweet Spanish.’ For better storage longevity, varieties like ‘Red Zeppelin’ or ‘Copra’ are excellent choices.
Planting Methods and Timing
Gardeners have three options for starting their onion crop: seeds, sets, or transplants, each dictating a different timeline. Starting from seed offers the widest variety selection and the potential for the largest bulbs but requires a significant head start indoors. Seeds should be sown in late January or early February, approximately 12 to 16 weeks before the last expected spring frost.
Onion sets (small, dormant bulbs) and transplants (young seedlings) are the easiest options for beginners. Plant these outdoors as soon as the soil is workable in early spring, typically four to six weeks before the last frost date. This early timing allows the plant to establish a large root and leaf system before bulbing is triggered. When planting sets or transplants, position them about one inch deep, ensuring the roots are covered and the growing tip is above the soil line.
Essential Care and Maintenance
Onions are shallow-rooted and require loose, well-draining soil with a slightly acidic pH between 6.0 and 6.8. Because the roots do not penetrate deeply, the crop depends on consistent nutrient and water availability in the topsoil. Amending the bed with organic matter before planting improves soil structure and retains moisture.
The plants need a steady supply of nitrogen early in the season to fuel the growth of the green tops, which directly determines the final bulb size. Apply a nitrogen-rich fertilizer, like ammonium sulfate, every three to four weeks after planting until the bulbs begin to swell. It is important to stop all nitrogen applications around mid-July; applying nitrogen too late will delay maturity, resulting in thick necks and soft bulbs that do not store well.
Consistent moisture, about one inch of water per week, is crucial, especially during rapid bulb expansion. Aggressive weed control is necessary because onions cannot effectively compete with weeds for water and nutrients. Carefully hand-pulling or shallowly cultivating the soil around the plants avoids damaging the developing bulbs.
Harvesting, Curing, and Storage
The onion crop signals its readiness for harvest when the green tops turn yellow and begin to fall over naturally. This process usually happens in late summer, typically 90 to 120 days after planting, depending on the variety. Once about two-thirds of the tops have fallen, the bulbs are ready to be lifted from the soil using a garden fork to avoid damage.
After harvesting, the bulbs must undergo curing, which is necessary for long-term storage. Curing involves drying the neck and outer skin layers in a warm, dry, and well-ventilated location, such as a covered porch or garage, away from direct sunlight. The process lasts two to three weeks until the necks are completely dry and the outer skins are papery.
Once cured, the dried tops can be trimmed to about one inch above the bulb, and the roots can be clipped. Properly stored onions require a cool, dark environment with temperatures between 32 and 40 degrees Fahrenheit and moderate humidity. Storing the bulbs in mesh bags or hanging them in braids ensures continuous air circulation, which prevents moisture buildup and spoilage.