The onion, Allium cepa, is a cool-season crop with a respectable tolerance for cold weather compared to many garden vegetables. “Frost hardy” refers to a plant’s ability to survive temperatures near or slightly below freezing (32°F or 0°C) without permanent damage. Onions are generally classified as hardy, meaning they can withstand heavy spring frosts. However, their survival and eventual quality depend heavily on the severity and duration of the cold event, allowing them to be planted early or overwintered in milder climates.
Cold Tolerance Based on Growth Stage
Cold tolerance changes significantly based on how the plant was started. Onion sets, which are small, immature bulbs, possess the greatest resilience to cold. They are often planted in the fall for overwintering because the stored energy allows them to survive winter temperatures and resume growth in spring.
Directly sown seeds develop a moderate tolerance once they have germinated and formed a root system. Seedlings or transplants started indoors are the most sensitive to freezing temperatures. These young plants lack the stored energy of a set and have not been “hardened off,” making them susceptible to damage from an unexpected late frost.
The plant’s development stage is also a factor related to bolting. Onions are less vulnerable to cold-induced problems before they have produced six true leaves. Once the plant has established six or more leaves, cold exposure can trigger a physiological response that negatively impacts the harvest.
Specific Temperature Limits and Damage
Onions can tolerate light, short-duration frosts, down to the range of 28°F to 32°F (-2°C to 0°C), especially if they have been slowly acclimated. The danger zone begins when temperatures drop below 25°F (-4°C) for several hours. A temperature consistently below 20°F (-6°C) constitutes a hard freeze and is often lethal to younger plants.
Cold exposure causes two main types of damage: physical and physiological. The first is physical damage, which appears immediately after thawing, presenting as translucent or water-soaked outer scales. If the damage is severe, the tissue inside the plant becomes mushy a few days after the freeze, indicating cell death and an unrecoverable plant.
Physiological damage is known as bolting. Bolting is the premature development of a seed stalk, triggered when young onions are exposed to sustained temperatures below 50°F (10°C) for two weeks or more. This cold stress tricks the biennial plant into diverting energy into reproduction instead of bulb formation. Once an onion has bolted, the resulting bulb will be smaller, woody, and will not store well.
Protecting Onions from Extreme Cold
Gardeners can take several proactive steps to minimize the risk of cold damage when a freeze is predicted. Applying a thick layer of organic mulch, such as straw or leaves, around the base of the plants provides insulation. This mulch helps retain residual heat in the soil, protecting the shallow onion bulbs and roots from freezing.
A temporary physical barrier is also effective for short, severe cold snaps. Covering the plants with a row cover or a tarp can greatly reduce freeze damage, particularly when temperatures are expected to drop below 20°F (-6°C). Watering the plants well the day before a hard freeze is beneficial, as moist soil holds warmth better than dry soil, adding another layer of insulation.
For seedlings started indoors, hardening off increases natural resilience. This involves gradually exposing the transplants to outdoor conditions, including cooler temperatures and wind, for increasing periods before final planting. This slow acclimation strengthens the plant structure and improves its ability to withstand the shock of a late spring frost.