1. Tough Growing Conditions
Nature can throw a variety of challenges at tomato plants as they grow. Less than ideal growing conditions are the most common cause of tomato leaf curl, according to experts, and also the easiest to remedy. “Leaf curl is primarily driven by hot weather, lack of moisture, and heat stress, " says Dr. Ajay Nair, Extension Vegetable Specialist at Iowa State University. In overly hot and dry conditions, the leaf margins roll upwards. Nair says that you’ll usually see curling more on the lower leaves. The science behind this stress-induced leaf curl reveals that it’s a way that plants actively protect themselves. When hot, dry conditions persist, tomato plants are not able to take up as much water as they lose through evaporation. This internal water deficit causes leaflets to curl up. A curled leaf absorbs less of the sun’s energy and loses less water. Leaf curl is actually a self-defense mechanism. What to do: Reducing environmental stress that causes leaf curl can help the problem and prevent it too. Start with good watering practices. Aim to provide tomato plants with 1 inch of water a week. If plants don’t receive that amount from rainfall, supplement by delivering water right to the root zone with a hose or drip line. Avoid overhead watering to prevent the spread of disease. Add a 2-inch layer of mulch around tomato plants to limit soil moisture evaporation. Leaf curl brought on by tough growing conditions usually resolves when the conditions improve. Don’t expect long-term effects. A short battle with leaf curl “does not significantly reduce plant growth or yield,” says Nair.
2. Too Much Pruning
While pruning tomato plants can help promote fruit development, it can cause problems when too much foliage is removed at once. The plant senses the sudden loss of energy-generating leaves and then curls its remaining leaves as a stress response. What to do: The best remedy for excessive pruning is to water the plant well and give it time to recover. Let new leaves remain. The plant should return to good health in a couple of weeks.
3. Transplant Shock
Moving seedlings or young starts into your garden can stress tomato plants. Temperature fluctuations and root disturbance associated with transplanting cause some tomato varieties to curl up their leaves in self-defense. What to do: Plants often recover on their own from leaf curl caused by transplant shock within a couple of weeks. Provide plenty of water as your tomatoes settle in. To minimize transplant shock in the future, make sure to slowly acclimate seedlings to garden conditions before planting, and gently handle the root ball. Do your transplanting on a cooler, overcast day or give your newly transplanted tomatoes some temporary shade with a tarp or other material that will block direct sunlight.
4. Weed Killers
A weed-free lawn could be why your tomato leaves are curling. “Off target drift of herbicides such as 2,4-D or dicamba is a frequent offender,” says Nair. If wind blows weed killer onto your tomato plants, the chemicals will affect foliage growth and may even kill your tomatoes. Plants with herbicide injury have leaves that bend downward and the individual leaflets bend upward in a cup-like shape. This looks different than the tightly curled or rolled leaves caused by hot, dry growing conditions. Another source of herbicide-related leaf curl is contaminated compost or mulch. Nair says the contamination comes from long-lived pasture herbicides such as picloram, clopyralid, or aminopyralid within the commercially available compost materials. When the compost is spread over the garden, it can impact tomatoes. What to do: There is no cure of herbicide-induced leaf curl. Take a “wait and see” approach. Some plants will overcome the effects and produce a crop. Other plants will die without fruiting. In the future, avoid using weed killers near tomato plants. Know the source of your compost and mulch, making sure to purchase from a reputable dealer.
5. Tomato Diseases
Though not very common, some viral tomato diseases could be why you’re seeing curling leaves. If a virus is to blame, you’ll see twisting and twining new growth, as opposed to curled older leaves common for plants stressed by tough growing conditions. Individual new leaflets often curl. What to do: There is no cure for tomato viruses. Remove the entire plant from the garden to help prevent spread. In the future, choose newer varieties that offer disease resistance to viruses.
