Growing onions in winter depends on the variety planted, the local climate, and the grower’s goal. Onions are biennial plants, typically cultivated as annuals for their bulb in the first year. Successful winter growing requires the plant to survive a dormant cold period without prematurely initiating its second-year reproductive cycle. By choosing cold-tolerant types and adjusting planting times, gardeners can harvest fresh greens through the colder months or secure an earlier bulb harvest in the spring.
Onion Varieties Suited for Cold Tolerance
An onion’s response to winter is determined by its photoperiod, the amount of daylight required to trigger bulb formation. Short-day onions (10 to 12 hours of daylight) are typically planted in the fall in warmer climates (Zone 7 and above) to grow through the mild winter and bulb as days lengthen. Long-day onions (14 to 16 hours of daylight) are generally planted in early spring in northern regions (Zone 6 and colder), where long summer days initiate bulbing. Intermediate-day onions fall between these two, suiting mid-latitude regions.
For intentional overwintering, specialized “overwintering” varieties, often intermediate-day types, have been developed for cold-hardiness and resistance to premature flowering, or bolting. Certain types, such as the Welsh or bunching onion (Allium fistulosum) and multiplier onions, are inherently more cold-tolerant and perennial. These hardy types prioritize leaf growth for continuous harvest of greens, making them reliable for winter survival rather than large bulb formation.
Overwintering Bulb Onions for Spring Harvest
The objective of overwintering bulb onions is to establish a robust root system in the fall so the plant survives dormancy and resumes growth early in the spring. Planting small onion sets or transplants in late summer or early fall must be timed so plants reach four to five leaves before the hard freeze, without forming a bulb. This established root system allows the plant to withstand winter chill and begin growing as soon as temperatures rise.
A major risk is bolting, the premature formation of a flower stalk and seeds. Since onions are biennials, a period of cold temperatures (below 45°F) followed by warmer weather can trick the plant into entering its second, reproductive year early. Once an onion bolts, the bulb quality is diminished, and it will not store well, requiring immediate consumption.
To mitigate bolting and ensure survival, consistent protection is necessary, especially in colder climates. Applying a heavy layer of mulch, such as straw or leaves, helps stabilize soil temperatures and insulate the bulbs from extreme cold. Using low tunnels or row covers can further improve the microclimate, offering consistent temperature and protection from harsh weather. Providing nitrogen-rich fertilizer in late winter can also encourage vegetative growth and suppress the plant’s instinct to flower.
Cultivating Green Onions During Winter
For a continuous harvest of fresh greens throughout the winter, the focus shifts from bulb dormancy toward leaf production. Perennial varieties like bunching onions are excellent for this purpose; they are hardy and produce edible leaves that can be harvested repeatedly. These plants grow well outdoors, often surviving harsh conditions and resuming growth quickly in the spring.
Another effective method involves forcing green onions indoors for active growth during the coldest months. This technique uses the vegetative base of store-bought scallions or small onion sets planted in soil or placed in water near a sunny window. The plant utilizes stored energy in the base to rapidly produce new green shoots, which can be harvested in seven to ten days.
This indoor forcing method allows for a steady supply of fresh leaves, as the plant will regrow if a few inches of the white base are left intact when harvesting. While outdoor perennial types offer resilience, the indoor method provides a reliable way to sustain a winter harvest, prioritizing greens over a dormant bulb.