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
