Onions Soil Temperature Guide
A cool-season crop that germinates in cold soil and tolerates light frost.
| Minimum germination temp | 45°F (7.2°C) |
|---|---|
| Optimal germination temp | 75°F (23.9°C) |
| Maximum germination temp | 90°F (32.2°C) |
| Danger zone | Below 45°F — seeds rot or fail to sprout |
Is your soil warm enough for Onions?
Minimum needed: 45°F
Why soil temperature matters for onions
Soil temperature directly controls enzyme activity inside onions seeds. Below 45°F (7.2°C), germination slows dramatically or fails entirely—seeds may rot before they sprout.
When to plant onions in your region
Sow onions outdoors when your soil consistently reads 45°F (7.2°C) at 4-inch depth. For the fastest germination, wait until it reaches 75°F (23.9°C).
Signs your soil is ready
The easiest method is to track your local soil temperature forecast until it's predicted to stay above 45°F for the next 7 days—sign up for notifications when it's time to plant onions. For more precise readings in raised beds, pots, or different spots in the garden, push a soil thermometer 4 inches deep in the early morning.