A tomato plant is a tender, warm-season perennial that gardeners grow as an annual in summer and fall gardens. Tomato plants tend to bear fruit about 60 days after planting. Tomato plants can be planted in the spring after the last expected frost. They will survive until the first fall frost. If grown in pots, they may survive after early fall frosts if brought inside while temperatures are below freezing. Under certain conditions, the normally flat leaves of a tomato plant may curl up.
Tomatoes grow best in full sun and in places where the air won't become too cold at night, or too hot during the day. They need rich soil that drains well. In some cases, tomatoes may have problems with pests. They also may suffer from damage from herbicides sprayed nearby. Problems with conditions, pests and herbicides are common causes of leaf curl.
If you notice curling, shoestringing or cupping of leaves on your tomatoes, find out if someone in your household or a neighbor has recently applied herbicides in the vicinity of the tomato patch. Leaf curl is one of the indications of potential herbicide contamination. If herbicides have been applied in the general vicinity, stop applying the herbicides. In many cases, the tomato plants will recover, unless they were directly sprayed.
Leaf curl, or leaf roll, is a natural response of the tomato plant to a sudden period of unusually warm weather. The leaf curl is a natural physiological response during hot weather. The plant senses that top growth has exceeded root growth, resulting in rolled leaves, which reduces growth until the roots catch up with the above ground growth. The leaves on some varieties of tomatoes also thicken and become more leathery when the leaves curl.
When unusually heavy rains saturate the soil, a tomato plant's leaves may curl. Plant roots need air, just like the above ground parts. When heavy rains saturate the soil, the roots can't get enough air. This can result in root damage, which causes the tomato to react in a similar way to sudden hot weather. The plant senses that there isn't enough root growth for the overall plant size, and the plant furls its leaves to reduce above ground vegetative growth.
In some cases, leaf curl can be an indication of a pest. If herbicides aren't the culprit, the weather has been stable and the ground around the tomato plant hasn't been saturated, the curl could be caused by cucumber mosaic virus or tomato mosaic virus. Transmitted by aphids, the best way to prevent this disease is to keep aphids off your tomato plants. If you have an infestation, indicated by leaves that curl, then become stringy and light green, remove the infected plant and sterilize all garden tools used in that part of your garden.