The successful cultivation of bulb onions relies entirely on precise timing relative to the local climate, a principle that holds particularly true for gardeners in USDA Hardiness Zone 5b. This northern zone presents a specific growing challenge that demands attention to both the calendar and the unique daylight requirements of the Allium cepa plant. Understanding the spring and fall planting windows, along with the necessary variety selection, ensures a plentiful harvest of mature bulbs. Following a practical planting schedule is the foundation for a productive onion crop.
Zone 5b Climate Requirements and Onion Selection
Zone 5b is characterized by significant photoperiod change, making the selection of the correct onion variety crucial for bulb formation. The average last spring frost typically falls between mid-May and late-May, marking the end of the risk of damaging young plants. However, the critical factor for onion bulbing is day-length, not temperature. Onions are photoperiodic, meaning they require a specific number of daylight hours to trigger the switch from producing green tops to swelling into a bulb.
Gardeners in Zone 5b must plant either Long-Day or Intermediate-Day varieties to ensure proper bulb development. Long-Day onions require 14 to 16 hours of daylight to begin bulbing, a duration achieved during the summer solstice in northern latitudes. Planting Short-Day varieties, which only need 10 to 12 hours of light, will result in premature bulbing and very small, unusable onions. Suitable Long-Day varieties include ‘Yellow Sweet Spanish’ and ‘Walla Walla Sweet,’ while Intermediate-Day varieties like ‘Candy Onion’ are also well-suited for Zone 5b.
Spring Planting Timing and Initial Care
The primary planting window begins well before the last frost date, capitalizing on the onion’s natural hardiness. Starting seeds indoors early is the most reliable method for obtaining large bulbs, giving the plants a head start on vegetative growth. Seeds should be sown indoors approximately 8 to 10 weeks before the average last frost date, typically placing the sowing window in late January or early February. These early starts require supplemental light, often from grow lights, to ensure strong initial growth and prevent them from becoming leggy.
Onion seedlings or sets (small dormant bulbs) can be transplanted outdoors as soon as the soil is workable and has thawed, usually about 4 to 6 weeks before the last expected frost. This early spring planting generally occurs from late March through mid-April, as onions are tolerant of light frosts and cold soil temperatures. Planting sets or seedlings too late will result in smaller bulbs, as the plants will not have enough time to grow sufficient green tops before the summer day-length triggers bulbing.
When transplanting, set the small plants shallowly, with the base of the bulb or the white portion of the seedling about one inch below the soil surface. Proper spacing is necessary for bulb size, requiring 4 to 6 inches between plants to produce large, mature bulbs. If using sets, select those under three-quarters of an inch in diameter, as larger sets are more prone to bolting later in the season. The soil must be consistently moist, and initial watering helps settle the soil around the root system.
Fall Planting for Early Harvest
An alternative planting schedule involves overwintering specific hardy onion varieties for an earlier spring harvest. This technique is best suited for shallots, multiplier onions, or specialized winter-hardy bulb onions, which are planted in late summer or early fall. The goal is to allow the young plants to establish a robust root system before the ground freezes for the winter.
Timing this planting is delicate; it should occur 6 to 8 weeks before the average first fall frost, typically in early to mid-September. If planted too early, the onions may grow too large and bolt prematurely when spring arrives. Planting too late prevents adequate root establishment. Once the ground cools and the plants enter dormancy, apply a heavy layer of protective mulch, such as straw, to insulate the bulbs and prevent damage from frost heave. This strategy yields small, fresh onions in late spring, often before the main spring-planted crop reaches full size.
Ongoing Cultivation and Harvesting
Once established, the onion crop requires consistent maintenance to direct the plant’s energy toward bulb development. Onions are shallow-rooted and compete poorly with weeds, so diligent weeding is necessary throughout the growing season. They are also heavy feeders, requiring consistent soil moisture and regular side-dressings of nitrogen fertilizer every few weeks until the bulbs begin to swell. Watering should be reduced as the tops begin to mature and fall over, signaling the approach of harvest time.
The primary cue for harvesting is when the leafy tops naturally begin to yellow and fall over, a process that typically starts around mid-summer. This natural lodging indicates that the onion has ceased producing foliage and has begun the final stage of bulbing and maturation. Onions should be gently lifted from the soil, ideally after a period of dry weather, and then moved to a location for curing.
Curing is a necessary post-harvest process that prepares the bulbs for long-term storage by drying the outer skins and neck. For Long-Day storage varieties, the bulbs should be placed in a warm, dry, and well-ventilated area, protected from direct sunlight, for one to three weeks. The onions are fully cured when the necks are completely dry and the outer skin is papery and brown. After curing, the dried tops can be trimmed, and the onions are ready for storage in a cool, dark, and dry location.