Onions are a popular crop, but gardeners often worry about their resilience when temperatures drop. While these plants possess a notable degree of cold hardiness, their ability to survive cold weather is not limitless and depends on the severity and duration of the freeze. Understanding the specific temperature thresholds and the factors influencing their tolerance helps determine when to intervene to protect the developing bulbs. This nuance allows them to be successfully grown in many climates, often planted in the fall for an early spring harvest.
Specific Temperature Tolerances
Onions exhibit a respectable tolerance for cold, generally surviving light frosts without significant damage. Most established onion plants can withstand temperatures down to approximately 20°F to 23°F (-6°C to -5°C) for short periods of time. Temperatures in this range are often referred to as a moderate freeze, and while the foliage may suffer, the developing bulb and root system can usually recover.
The duration of the cold is often more damaging than the absolute lowest temperature reached. A brief dip to 20°F is survivable, but a sustained period of several hours below 25°F (-4°C) constitutes a hard freeze and can be lethal to the plant tissue. When the bulb freezes and then thaws, the cells can rupture, leading to a mushy texture and subsequent rotting. Damage is often visible as water-soaked or translucent outer scales on the bulb soon after a freeze event.
Hardiness Based on Planting Material and Variety
The cold hardiness of an onion plant is greatly influenced by its stage of growth and the type of material used for planting. Onion sets, which are small, dormant bulbs grown from seed the previous year, and transplants, are generally more resilient to cold than plants grown directly from seed. Young seedlings that have just sprouted are the most vulnerable and require protection from heavy freezes.
Varietal differences also play a role in how a plant handles cold exposure. Long-day onion varieties are typically adapted to northern climates and can handle more significant temperature drops because they are naturally more cold-tolerant. Short-day varieties, which are better suited for mild winters in the southern regions, are less robust against deep cold, although some can survive temperatures as low as 17°F (-8°C) if they are well-established. These day-length classifications reflect the plant’s genetic programming for cold resilience in its native growing region.
Protecting Onions from Extreme Cold
When temperatures are predicted to fall below the plant’s natural threshold, specific actions can be taken to mitigate cold damage. Applying a thick layer of organic mulch, such as straw, hay, or shredded leaves, creates an insulating barrier over the soil and around the plants. This mulch should be at least two inches deep and helps hold heat in the soil around the bulb and roots.
Another effective technique is to use physical barriers like row covers or cold frames to shield the plants from biting wind and direct cold. Row covers made of spunbonded fabric or perforated plastic can raise the temperature around the plants and provide a degree of frost protection. Watering the plants thoroughly before a predicted hard freeze is also beneficial, as moist soil holds heat better than dry soil, providing additional insulation for the roots.
Growth Consequences of Cold Exposure
Even when an onion plant survives a cold snap, the exposure can trigger undesirable physiological changes that impact the final harvest. The most common negative outcome is bolting, the premature formation of a seed stalk. Bolting occurs when the plant is subjected to sustained cool temperatures, typically below 45°F to 50°F (7°C to 10°C), especially after the plant has developed a certain number of leaves.
When an onion bolts, the plant redirects its energy into producing a flower stalk and seeds, which effectively halts the development of the edible bulb. The resulting bulb is often smaller, tougher, and less palatable. This cold-induced stress can also lead to issues like bulb splitting or a reduction in the overall storage life of the harvested onions.