What temperature range is fatal to onion plants?
Written by
Tina Carter
Reviewed by
Prof. Charles Hartman, Ph.D.Onions have a balancing act with extremes in temperature. While mature plants can survive frost as low as 20°F (-7°C), anything less than 15°F (-9°C) will kill the leaves and damage the bulbs. I know this firsthand: a sudden and unexpected cold snap in Wyoming cost me my entire crop of onions. Now I watch forecasts intensely and always have row covers at the ready.
Critical Thresholds
- 28°F (-2°C): Seedlings sustain leaf damage
- 20°F (-7°C): Mature plants survive 4-hour exposure
- 15°F (-9°C): Bulb cell walls rupture irreparably
Protection Strategies
- Apply 2-inch (5 cm) mulch before frost
- Use 1.5 oz/yd² (50 g/m²) row covers
- Water soil before freeze to buffer roots
Recovery Actions
- Trim blackened leaves above bulb neck
- Apply 5-10-10 fertilizer post-thaw
- Check bulbs for mushiness after 48 hours
During periods of extreme cold, soil insulation is extremely important. In my garden in Montana, I use black plastic mulch to keep 5F (3C) more ground heat. For containerized plants, wrap the pots in bubble wrap and keep them on a wall. I have kept onions on my patio this way during April snow storms.
The factors addressed during post-frost will determine the salvageability of the crops. Inspect plants 48 hours after air temperatures have returned above freezing. Use fingers and gently apply pressure to bulbs, if the bulb feels firm it is likely to survive. If there are soft spots, cut the bulbs out immediately. I was able to salvage approximately 60% of my crop last winter through rapid inspection and selective pruning.
Read the full article: When to Plant Onions: Expert Guide for Perfect Bulbs
