Growing onions successfully in Michigan depends on precise timing due to the state’s short growing season. Onions are photoperiodic, meaning the length of daylight hours triggers the bulb formation process. Michigan requires long-day varieties that need 14 to 16 hours of daylight to swell their bulbs, a condition met during the summer months of June and July. The goal is to establish a robust, leaf-producing plant early in the spring so it is ready to bulb once the summer solstice approaches.
Optimal Timing for Seeds, Sets, and Transplants
The method chosen for starting onions dictates the precise planting window. Onion sets, which are small, dormant bulbs, are the easiest and should be planted first. Sets can go into the ground as soon as the soil is workable, typically from late March to early April in most of Michigan. Planting sets this early allows the plant to develop a strong root system before the crucial long-day period begins.
Transplants, small seedlings started indoors, offer the next best option for reliable bulb size and are generally planted out in April or May. Gardeners should start these seeds indoors approximately 10 to 12 weeks before the anticipated outdoor planting date, usually in February or early March. The seedlings are then hardened off and moved to the garden after the danger of a hard freeze has passed.
Direct sowing seeds into the garden is possible, though it is the least common method for achieving large bulbs in a single season. If choosing to direct sow, the seeds should be planted four to six weeks before the last expected frost, generally placing the window in early to mid-April. This ensures the plants are well-established and have produced a substantial amount of green foliage by the time bulbing is triggered.
Essential Soil Preparation and Site Selection
A successful onion crop requires selecting a site that receives a minimum of six hours of direct sunlight daily. Onions possess a shallow root system, making them highly susceptible to competition and poor drainage, so a full-sun location is necessary. The soil must be loose and crumbly, ideally a sandy loam, to allow the developing bulb to expand without restriction.
Heavy clay soils are not suitable because they retain too much moisture and compact easily, which deforms the growing bulbs. Amending the planting area with aged compost or well-rotted manure before planting is necessary to improve both the soil structure and drainage. Onions thrive in slightly acidic to neutral soil, with a preferred pH range of 6.0 to 7.0.
A soil test can confirm the pH level, and if the soil is too acidic, applying ground limestone can help raise it. Before planting, a balanced fertilizer should be incorporated into the bed to ensure immediate nutrient availability. A nitrogen-rich feeding is beneficial early on to support vigorous vegetative growth, as bulb size is directly related to the number of green leaves the plant produces.
Post-Planting Maintenance and Harvesting
Onions require a constant supply of moisture because their shallow roots cannot reach deeper water reserves during dry spells. Providing the plants with approximately one to one and a half inches of water per week, either through rainfall or irrigation, is sufficient.
Rigorous weeding is necessary throughout the growing season because onions cannot compete effectively with weeds for water and nutrients. Any weeds near the plants should be removed carefully by hand to avoid disturbing the shallow onion roots. Continue watering and feeding until the onion tops begin to signal that the bulbs are reaching maturity.
The harvest is ready when the green tops start to yellow and fall over, usually occurring in late August or early September. Once approximately half of the tops have bent over, stop watering completely to begin the drying process. After the tops are mostly dry, the bulbs can be gently lifted from the soil and moved to a warm, dry location for curing. Curing for two to four weeks in a warm area (ideally between 75°F and 90°F) allows the outer skin to dry and the neck to constrict, which is necessary for long-term storage.