Seeing curled leaves on a tomato plant can give gardeners quite a scare. Curled leaves are not always a symptom of a serious problem, but it is worth investigating just to make sure.
So, why do your tomato plants have curled leaves? A common cause of curled leaves on tomato plants is physiological leaf roll, due to dry air, wind, lack of water, or cool and damp weather. Other possible causes include wind damage, herbicides, viruses, and pests.
Luckily, there are ways to prevent some of these problems and protect your tomato plants. Let’s look at each of the causes of curled leaves in more detail, along with options for prevention and treatment.
Curled Leaves Due to Physiological Leaf Roll
Physiological leaf roll occurs when a plant’s leaves curl or twist in response to environmental conditions such as dry air, wind, or lack of water.
Dry Air (Low Humidity)
When humidity levels are low, plants lose more water through their leaves at a faster rate. To prevent this, tomato plants will curl up their leaves on dry days.
Since dry days tend to coincide with sunny days, this strategy also reduces the surface area of the plant exposed to the sun. This, in turn, reduces heat stress on the plant.
To prevent this from happening, grow your tomato plants in a greenhouse to increase humidity levels on dry days. If your plants are young, you can use a cloche to cover individual plants (or a larger one for an entire row) to increase local humidity.
Otherwise, there is not much you can do to change humidity levels. Just be sure to keep your plants watered.
Combined with dry air, a strong wind can make leaf roll even worse. When the wind is blowing, plants lose water through evaporation more quickly.
A tomato plant will curl its leaves to prevent this from happening. However, if you want to extra protection against leaf curl due to wind, you will need to take action (more on this below).
Lack of Water
If a tomato plant’s roots cannot supply all of the leaves with enough water, then some of the leaves will curl up. This can happen even if the soil is moist, since the leaves of a plant will sometimes demand more water than the roots can absorb and transport.
This is more likely in potted and hanging tomato plants, which have a natural limit on root growth, but no limit on the growth of foliage. Be sure to keep your plants watered, but remember not to overwater them or keep the soil damp, since this can cause root rot.
Curled Leaves Due to Wind Damage
When strong winds arrive, they can cause damage to plants in a number of ways. The wind itself can knock over tall tomato plants that are not staked and tied properly.
The wind can also blow dust around, which can damage the leaves of plants and cause them to curl. Strong winds also cause leaves to lose water rapidly due to accelerated evaporation. Again, a plant’s leaves will curl up in an attempt to slow down the pace of water loss due to evaporation.
As mentioned above, you can use cloches to cover your young tomato plants and protect them from strong wind. You can also use row covers, although these might not provide as much protection as cloches.
A more effective solution is to use a windbreak, such as a hedgerow, fence, or wall, to protect your garden. For more information on these methods, check out my article on how to protect your plants from wind.
Curled Leaves Due to Herbicides
The leaves on your tomato plants may also curl up due to herbicides and other artificial chemicals used in farming and gardening. This problem is a little more difficult to solve, since you don’t always have control over what others use on their properties. Also, it can be difficult to trace the source of herbicides, which can arrive at your yard on the wind or in compost, manure, or mulch.
Herbicides Carried to Your Garden by Wind
According to Texas A & M University, a wind speed of only 5 miles per hour can carry herbicides up to a mile away from their origin*. Stronger winds may carry herbicides even further.
So, if you live within a few miles of a farm or pasture that uses herbicides, there is a chance that these chemicals will end up in your garden, on your tomatoes and other plants. Unfortunately, there is not much you can do except ask these farms or pastures to avoid using herbicides, which may not be feasible for them.
Even if you are not near a farm or pasture, your nearby neighbors may still be using herbicides in their lawns and gardens. This is more difficult to trace, since the herbicides could come from any neighbors within a mile or more of your garden. Again, one of your only options is to spread the word and ask your neighbors to avoid using herbicides.
You can also try to grow your tomatoes in a greenhouse to avoid herbicides being blown in from elsewhere. If your greenhouse is short, you may want to make sure that you plant determinate tomato varieties, since they will stop growing at a certain height.
*For more information, check out this article from Texas A & M about curled leaves on tomato plants.
Herbicides in Compost, Manure, or Mulch
Herbicides have another way to get into your garden, and it may not require any wind at all. Remember that if you use compost, manure, or mulch in your garden, you may be adding herbicides without realizing it!
When you make compost, you often use grass clippings as one of the ingredients. Unfortunately, grass clippings may contain traces of herbicides, either blown in on the wind or used by lawn care companies to prevent weeds.
This is much more likely if you take grass clippings from neighbors to use in your compost pile. Always ask your neighbors if herbicides were used on their lawns before taking grass clippings. Also, avoid the use of herbicides on your own lawn.
Manure is the waste and bedding from animals such as horses, goats, and chickens. Unfortunately, these animals eat plants such as grass and weeds, which are often treated with herbicides.
As a result, their manure may also contain traces of these herbicides, which can make their way into your garden soil when you add manure. Your best bet is to ask farmers, horse boarders, and neighbors if the animal food has been treated with herbicides before taking any manure for your garden.
Perhaps you have mulch delivered from a landscape or garden center, as many people do. If so, remember that the mulch pile may have been exposed to herbicides from the wind if the garden center uses them. Always ask before you order mulch from one of these places.
Curled Leaves Due to Viruses
There are a number of viruses that can affect tomato plants, and some of them will cause curled or twisted leaves. Here are a couple of the common ones.
Tomato Yellow Leaf Curl Virus
According to Wikipedia, tomato yellow leaf curl virus is found in tropical and subtropical regions. It causes leaves to twist and curl, and is transmitted between plants by the whitefly.
One way to prevent the spread of this virus is through the use of insecticides. If you find that you have infected plants in your garden, you will need to pull them out and destroy them.
Do not compost the infected plants, since the virus may survive in your compost pile and infect your garden in subsequent years.
For more information, check out this article from Wikipedia on tomato yellow leaf curl virus.
Tomato Curly Top Virus
According to Wikipedia, curly top virus is often found in the Western U.S., and it causes curled leaves and stunted growth in plants.
This virus is transmitted by the sugar beet leafhopper, and it affects beets, tomatoes, spinach, peppers, beans, potatoes, and cucumbers.
For more information, check out this article from Wikipedia on curly top virus.
Curled Leaves Due to Pests
In addition to transmitting diseases, such as the viruses mentioned above, pests can also directly cause curled leaves due to their feeding.
For instance, the broad mite is microscopic, and it causes curling leaves and eventually kills new growth on a plant. They like areas with high humidity and low temperature, so you may find them in shady areas inside your greenhouse.
If you find infected plants, pull them out and destroy them. Do not compost the plants, for the same reasons mentioned above: the broad mite may survive a winter in the compost pile to come back and infect your garden again next year.
For more information, check out this article from Wikipedia on broad mites.
By now, you should have a better idea of what could be causing curled leaves on your tomato plants. If you take steps to treat and prevent the problem, you will be one step closer to a bountiful tomato harvest.
I hope this article was helpful. If you have any questions or advice of your own about curled leaves on tomato plants, please leave a comment below.