What Temperature Is Too Hot for Seedlings?
Above 85°F, seedlings stop growing. Above 95°F, most are damaged. Above 100°F, they die within hours.
Heat stress looks different from cold stress. Leaves wilt, curl, and crisp.
Temperature Guide
| Temp | Effect |
|---|---|
| 65-80°F | Ideal |
| 80-85°F | Slightly stressed |
| 85-90°F | Growth slows |
| 90-95°F | Wilting, leaf curl |
| 95-100°F | Damage, sunscald |
| Above 100°F | Death within hours |
Signs of Heat Stress
- Wilting despite moist soil
- Leaf curl (upward)
- Brown, crispy leaf edges
- Flowers dropping (no fruit)
- Bolt (going to seed early)
What to Do
- Move to shade. Especially afternoon shade.
- Water more often. Morning watering.
- Mulch. Keeps soil cool.
- Increase airflow. Fan or open window.
- Move indoors if possible.
Which Plants Are Sensitive
| Plant | Heat Tolerance |
|---|---|
| Lettuce | Low (bolts above 80°F) |
| Spinach | Low |
| Cilantro | Low |
| Broccoli | Low |
| Tomatoes | High (up to 95°F) |
| Peppers | High |
| Basil | High |
| Zucchini | High |
Common Questions
Will heat-stressed seedlings recover? Yes, if the damage isn’t severe.
Should I water more in heat? Yes, but only in the morning.
Can I mist leaves in heat? Only in early morning.
What’s “bolting”? When cool-season plants go to seed early because of heat.
Should I move plants inside during heatwave? If you have AC, yes.
How do I know if a plant is dead from heat? Crispy brown leaves + mushy stem = dead. Firm stem + wilted leaves = recoverable.