Why do tomato leaves curl upward at night?
Written by
Tina Carter
Reviewed by
Prof. Charles Hartman, Ph.D.Gardeners often wonder why their tomato leaves appear to be curling up in the evening. This is most typically an example of a natural phenomenon known as nyctinasty. I've observed this in several cultivars, with leaves relaxing by mid-morning if the plant is healthy. However, persistent curling during the day usually indicates a plant experiencing heat stress or more likely water stress, and, as such, appropriate action would be required.
Healthy Nyctinasty Signs
- Upward curl starts at dusk
- Leaves flatten by 10 AM next day
- No discoloration or stunted growth
Stress Indicators
- Daytime curling with leaf cupping
- Edges turning brown or yellow
- Soil cracks or surface algae
Track leaf positions by using time-stamped images. A client recently confirmed their 'issue' was simply natural rhythm after reviewing 7 AM and 2 PM leaf positions. Use a thermal gun to check leaf surface temperatures, healthy leaves stay below 80°F (27°C) of shade.
Combat heat stress with intelligent irrigation. I recommend watering at 6 in the evening in periods of heat, moisture will be absorbed into the soil before nighttime. Supplement with a light-colored mulch to aid in reflecting sunlight. A test done in a vineyard on a tomato planting reduced apple curling by 60% by using crushed marble mulch.
Read the full article: Tomato Leaves Curling: Causes and Solutions
