It is possible to leave onions in the ground over winter, a process called overwintering, but the success of this practice is highly dependent on both the specific variety of onion and the local climate. The primary reason gardeners choose to overwinter onions is to achieve an earlier harvest the following spring or early summer, sometimes weeks ahead of those planted in the spring. Overwintering involves planting small bulbs, known as sets, or young seedlings in the late summer or fall, allowing them to establish roots before the ground freezes. This method exploits the plant’s natural hardiness to gain a head start on the next growing season, rather than leaving a mature bulb from the current season in the ground, which would typically result in rot.
Differentiating Onion Types for Overwintering
The type of onion dictates whether overwintering is a viable option for a spring bulb harvest. True bulbing onions, which form the large, globe-shaped bulbs used for cooking and storage, are sensitive to day length and temperature fluctuations, making them prone to bolting if planted improperly for winter. Successful overwintering requires specific varieties, often intermediate- or long-day types, that are bred to tolerate cold exposure without triggering the flowering response. For most bulbing onions, the goal is to plant small sets or starts that reach a size of about four to five leaves—roughly the thickness of a pencil—before the deepest winter cold arrives. If the plants are too large going into winter, they are more susceptible to bolting prematurely in the spring.
Hardy bunching onions, such as scallions, Welsh onions, or Egyptian walking onions, are far more resilient and naturally suited for overwintering. These varieties are grown primarily for their green tops and do not form a large, single storage bulb, making them much less likely to suffer from the issues that plague bulbing onions. Egyptian walking onions, for example, can endure temperatures as low as -30 degrees Fahrenheit, making them a choice for extremely cold climates. While overwintering is most consistently successful in milder regions, such as USDA Hardiness Zones 6 through 8, specialized, bolt-resistant bulbing varieties can be successfully grown in colder zones with adequate protection.
Leaving a fully mature, summer-harvested onion bulb in the ground is distinct from overwintering sets or starts. A mature bulb is a biennial plant that has completed its first season of growth and will naturally attempt to flower and produce seed if left to experience a second cold period. This process leads to the core of the bulb becoming tough and woody, which is generally not the intended outcome for a food crop.
Essential Soil and Protection Preparations
Preparing the planting site is essential for the survival of young onion plants throughout the winter months. Onions, regardless of type, are highly susceptible to rot, making excellent soil drainage a necessity for overwintering. Planting in a raised bed is often recommended, especially in regions with heavy clay soil or excessive winter precipitation, as this helps prevent waterlogging that can lead to the sets rotting in the ground. Adding well-rotted compost before planting improves soil structure and provides a slow release of nutrients to support root establishment.
The timing of planting is a delicate balance, as the sets must establish roots but not grow too large before the hard freeze arrives. Generally, fall planting is recommended about two to three weeks before the first hard frost, or between late August and early October in many northern latitudes. Once the plants are established, the application of mulch serves as the primary method of protection against the harsh winter elements. A layer of organic material like straw, pine needles, or dry leaves, applied several inches deep, insulates the soil and helps maintain a more consistent temperature.
This insulation is important for preventing the damaging cycles of freezing and thawing, which can physically push the shallow-rooted onion sets out of the ground, a phenomenon known as frost heave. In zones with consistently severe cold, below Zone 6, additional protection, such as a low tunnel or a horticultural fleece cover, may be required to raise the microclimate temperature by several degrees. Even hardy varieties benefit from this protection if the climate is both extremely cold and wet.
Understanding the Spring Growth Cycle
Once temperatures begin to warm in the spring, the successfully overwintered onions will resume active growth, often providing a harvest several weeks earlier than those planted in the spring. However, the exposure to the winter cold, followed by the warmth of spring, can trigger bolting in many bulbing varieties. Bolting is the plant’s attempt to complete its reproductive cycle by sending up a thick flower stalk, or scape, to produce seeds.
When an onion bolts, the plant diverts energy away from bulb enlargement into the flower stalk, which causes the center of the bulb to become woody and inedible for long-term storage. Gardeners must harvest these bolted onions immediately, as soon as the scape is visible. This early harvest should be used quickly, as the bolted onions will not cure properly or store for an extended period. Bunching onions, in contrast, can often be harvested continuously throughout the spring. Once the threat of hard frost is past, the protective mulch should be removed to allow the soil to warm and the bulbs to begin their final phase of rapid spring growth.