Onions are a unique garden crop, classified botanically as biennials, though they are cultivated commercially as annuals for a single season’s harvest. The question of how long they can remain in the ground has a two-fold answer, relating both to the initial growth period and the limited time they can stay in the soil after reaching full maturity. Understanding the onion’s biological clock and its response to environmental signals is necessary for a successful harvest and long-term storage.
The Maturation Timeline
The time it takes for an onion to form a large, mature bulb is directly linked to its genetic response to day length, a process called photoperiodism. Onion varieties are categorized into three groups: short-day, intermediate-day, and long-day types. Short-day onions initiate bulbing when daylight reaches 10 to 12 hours, making them suitable for southern latitudes. Long-day varieties require 14 to 16 hours of daylight, which is why they are grown in northern regions. Maturation typically spans 85 to 120 days from planting, though planting the incorrect variety may result in the onion bulbing too early, remaining small, or never bulbing at all.
Identifying Harvest Readiness
The most reliable sign that an onion has finished its subterranean growth is the natural softening and collapse of its above-ground foliage. This event, called “lodging” or “neck fall,” signals that the plant is redirecting energy toward sealing the bulb for dormancy. The neck, the point just above the bulb, softens and loses its rigidity, causing the tops to topple over. Growers should prepare for harvest when 50 to 80 percent of the tops have fallen naturally. This cue indicates the bulb has reached maximum size and begun forming protective outer skin layers, ensuring full flavor and maximum dry matter content for better storability.
Post-Maturity Ground Storage Limits
Once the onion tops have fallen over, the bulb is fully mature, and a countdown begins for its removal from the soil. The maximum safe period for a mature onion to remain in the ground is generally no more than one to two weeks. Exceeding this brief window significantly compromises the bulb’s quality and its ability to store long-term. Leaving the bulb in the moist soil past the two-week limit increases its susceptibility to fungal diseases, such as neck rot, because the collapsed neck provides an easy entry point for soil-borne pathogens.
Onions are biennials, meaning they attempt to complete their life cycle by producing a flower stalk in the second year. After the initial dormancy phase, the mature bulb may begin to re-sprout or initiate new root growth if the weather remains mild and moist. This re-growth draws stored energy from the bulb, making it soft, reducing its pungency, and severely shortening its shelf life. For storage varieties, a tight, dry neck is necessary for proper curing, and prolonged contact with damp soil prevents this essential sealing process.
Environmental Factors That Reduce Storage Time
The one to two-week post-maturity window is an ideal maximum and can be drastically shortened by unfavorable environmental conditions. Wet soil from heavy rain or poor drainage is the primary factor that accelerates rot and disease development. Extended periods of wetness around the bulb provide the high-moisture environment that fungal and bacterial pathogens require to thrive and infect the mature onion. High ambient temperatures combined with high humidity also encourage bacterial decay and increase the overall incidence of bulb rot. If a period of heavy rain or high heat is forecasted immediately after the tops fall, the onions should be pulled from the ground immediately, even if the full two weeks have not passed. Cultural practices, such as excessive nitrogen fertilization late in the season, can also delay maturity and result in wider, thicker necks that are more vulnerable to disease while still in the ground.