Tomato leaves will sometimes curl as a result of imbalanced growth above and below the soil. Gardeners may see vigorous top growth during mild spring weather. Below the soil the roots may not be keeping up. When the weather turns hotter during the summer the roots are not established enough to support the size of the upper growth. Curling leaves, in this instance, is a physiological condition that enables the plant to reduce its surface area and conserve water. Leaves may also become tougher and leathery.
Physiological leaf curl is most often seen as the seasons change from spring to summer but can also be caused by heavy cultivation that has damaged the roots, improper fertilization and poor watering practices. Though the plants typically self-correct with time and as conditions improve, prolonged periods of time with improper care can affect yield. To avoid leaf curl, it is important to properly harden off tomato seedlings. Use a layer of mulch around tomato plants to regulate soil moisture and temperature. Apply fertilizer as needed based on soil testing. You may also choose determinate (bush-type) varieties of tomatoes which are less susceptible to leaf curl than the indeterminate (vining) varieties.
Cynthia Domenghini, Extension Agent
