Seeing curled leaves on a tomato plant can scare any gardener, whether beginner or experienced. 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 hot & dry air, wind, root damage, excessive fertilizer, lack of water, or cool and damp weather. Other possible causes include wind damage, herbicides, diseases, and pests.
Of course, it helps to know which of these things is causing the leaves on your tomato plant to curl so that you can treat the problem.
In this article, we’ll look at each of these causes of curled leaves on tomato plants. We’ll also talk about ways to prevent or solve the problem.
Let’s get started.
Curled Leaves Due to Physiological Leaf Roll
- heat and dry air
- root damage
- excessive fertilizer (nitrogen)
- lack of water
Curling occurs on the older leaves first (the ones that are lowest on the plant). They tend to roll upwards in these cases of environmental stress.
Curled leaves due to physiological leaf roll tends to appear at the transition from spring to summer. Let’s take a closer look at some of the specific causes, starting with heat & dry air.
Heat & Dry Air (High Temperature & Low Humidity)
Normally, plants “exhale” water through their leaves into the air (called transpiration). When the air is dry, humidity levels are low.
Low humidity means that the air can absorb more water. Higher temperatures also allow the air to absorb more water.
As a result, plants lose water from their leaves at a faster rate on hot, dry days. To prevent this, tomato plants will curl up their leaves on dry days.
Dry days are often sunny days, so this strategy also reduces the surface area of the plant exposed to the sun. This reduces heat stress on the plant.
To prevent curled leaves due to physiological leaf roll, there are some steps you can take:
- Use a cloche for young tomato plants – this will increase local humidity near each plant.
- Grow tomato plants in a cold frame or greenhouse – this increases humidity for all tomato plants inside the structure on dry days.
- Use a shade cloth – this will protect tomato plants from intense sunlight, and can reduce their temperature by up to 10 degrees Fahrenheit.
- Keep plants watered – feel the soil with your fingers, and water your tomato plants if the soil feels dry. Check often in hot, dry weather. Consider a drip irrigation system on a timer.
Otherwise, there is not much you can do to change temperature or humidity levels.
Combined with dry air, a strong wind can make physiological leaf roll worse. When the wind is blowing, plants lose water through transpiration even faster.
A tomato plant will curl its leaves to prevent this from happening. However, if you want extra protection against leaf curl due to wind, you will need to take action (more detail on this later!)
Root damage during transplant (a form of transplant shock) is another common cause of stress and leaf roll in tomato plants. There are a few ways root damage can occur:
- pulling apart seedlings – if planted too close together, seedling roots get tangled. Separating them will sometimes damage the roots. To prevent this, grow each seedling in its own separate cell (as in a seed tray).
- rough handling of transplants – if you aren’t careful, you can damage the roots of tomato transplants when moving them from indoors to the garden. To avoid this, leave the soil on the root ball when you transplant.
- sun exposure of roots – it is common to take tomato transplants out of their tray and leave them in the sun while digging holes for them. On a hot, dry day, this will soon cause root damage. Instead, leave each transplant in the tray until it is time to transplant it.
To avoid curled leaves from transplant shock, handle tomato plants carefully when transplanting. To ensure a successful transplant, use hardening off to help plants to get used to outdoor conditions gradually.
Excessive Fertilizer (Nitrogen)
Although tomato plants do need plenty of nutrients, it is possible to over fertilize them. This is especially true if too much nitrogen is applied.
Over fertilizing can burn the roots of plants. This will eventually cause the same symptoms as root damage, including curled leaves on tomato plants.
To avoid over fertilizing, use a blend that has lower NPK numbers on the package. Follow the instructions on the package to avoid adding too much fertilizer at once, and water it in when you apply it.
Lack Of Water
If a tomato plant’s roots cannot supply all of the leaves with enough water, some of them will curl up. This can happen even when the soil is moist.
In that case, the leaves of the plant demand more water than the roots can absorb and transport.
This problem is more likely in plants that received frequent and shallow watering. This type of watering encourages shallow roots to develop.
Instead, water plants deeply and infrequently. This will encourage strong, deep root systems that will see plants through a drought.
Lack of water is also more likely in potted and hanging tomato plants. THis is true for 2 reasons:
- 1.) Tomato plants in a container have a limit on their root growth (the size of the container).
- 2.) The soil in containers tends to drain faster, since they are elevated.
One way to prevent this is to use plastic pots, which tend to retain water in soil a little better than clay pots. Also, keep an eye on your plants and keep them watered.
Just remember not to over water 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 and dirt around at high speeds. This fast-moving dust and dirt can damage the leaves of plants and cause them to curl in response.
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 water loss due to evaporation.
As mentioned above, you can use cloches to cover your young tomato plants and protect them from strong winds. 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 to protect your garden. Some examples include:
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. These are often artificial chemicals that are used in farming and gardening.
This problem is a little more difficult to solve, since you don’t control what others use on their property. Also, it is difficult to trace the source of herbicides, which can arrive in your yard in many ways, including:
- on the wind
- in compost (for example, grass clippings from a lawn treated for weeds)
- in manure
- in 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. Unfortunately, there is not much you can do, besides asking 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, the same problem can occur. 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 plant determinate tomato varieties.
Determinate tomato varieties will stop growing at a certain height. Indeterminate tomato varieties can grow up to 8 feet tall or more, which may exceed the limits of your greenhouse.
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, 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.
Herbicides might be blown in on the wind. They may also be 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 Diseases
There are a number of diseases that can affect tomato plants, and some of them will cause curled or twisted leaves, including:
- Bacterial Wilt of Tomato
- Tomato Blight (Early & Late)
- Tomato Yellow Leaf Curl Virus
- Tomato Curly Top Virus
Bacterial Wilt Of Tomato
According to Wikipedia, Bacterial Wilt is a number of diseases that affect many plants, including:
In tomatoes, Bacterial Wilt is caused by Ralstonia Solanacearum. This bacterium infects plants by damaged roots or by carriers (such as root-knot nematodes).
The disease is most common in moist soil at temperatures over 85 degrees Fahrenheit (30 degrees Celsius).
The first signs are a few leaves on top of the plant wilting and curling. Later, the entire plant wilts.
Finally, the stem turns brown. The plant’s growth may also be stunted throughout this period.
Bacterial Wilt clogs the vascular system of a tomato plant. This prevents it from moving water and nutrients through its tissue.
Eventually, the plant will succumb to this disease. There is no cure, so it is best to remove infected plants right away to prevent the disease from spreading.
Choose resistant tomato varieties (such as Neptune or Tropic Boy) to avoid the disease.
Tomato Blight (Early & Late)
There are two types of tomato blight:
- Early Blight – caused by Alternaria tomatophila or Alternaria solani. Early blight spreads more rapidly in damp conditions, such as after rainy weather, watering your garden, or heavy morning dew.
- Late Blight – caused by the fungus Phytophthora infestans. It spores spread on the wind, and it does well in cool, wet conditions, with an ideal temperature range of 60 to 70 degrees Fahrenheit (16 to 21 degrees Celsius).
Of the two diseases, late blight is easily the more fatal for tomato plants.
Luckily, there are some blight resistant tomato varieties, which you can read about in my article here.
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.
Tomato Curly Top Virus
This virus is transmitted by the sugar beet leafhopper, and it affects beets, tomatoes, spinach, peppers, beans, potatoes, and cucumbers.
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 a microscopic plant pest. It causes curling leaves and stunts the growth of leaves.
Eventually, broad mites destroy the new growth on a plant. They like areas with high humidity and low temperature.
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.
Now you know what might be causing curled leaves on your tomato plants. You also know how to treat and prevent the problem, which takes you one step closer to a bountiful tomato harvest.
I hope you found this article helpful – if so, please share it with someone who can use the information.