Why are my tomatoes leaves curling? Here are some common reasons why and drift from chemicals is not the main reason for curling leaves.

Leaves curled on my patio tomatoes image from Becky's Greenhouse

     ​4th of July is now over, where did spring and part of summer go?  I am glad that the temperature is a little cooler and less humidity.  Old Farmers’ Almanac said that July would have below normal temperatures and above normal precipitation.  It is time for that to start….It was really hot at 90 degrees on the blacktop and working with the plants.  I watered, and then watered again in a day.  Plants are looking good yet.  I am  open Wed, Thursday and Friday from 9-6.  I just looked at the calendar and Monday July 10th, we will be working at the Franklin Co Fair.  We are taking baskets and pots to help decorate. I will be there with my flower wagon, and a display of lots of other plants at the site by the popcorn stand.  Franklin Co Fair will start on Wed. July 12th.  I am now stressing out with all I want to plant and take.  I will get it done, I will, I will.
I had my Gardeners of North Iowa here 2 weeks ago, and one question was on curling leaves on her tomatoes.  She wondered if it was drift.  Here is what I found out as other causes of that problem with herbicides only being on.  I would have to say I think it was caused by the hot temperatures we had and the lack of rain.  Hope this helps. 
Tomato Plant Leaves Curling – 9 Causes and How To Fix It
Growing tomatoes can be a challenge. Tomato plant leaves curling up is one of the most common issues to arise. Thankfully, the root cause of the problem can usually be solved with some simple adjustments to your gardening routine.
In this article, I will go over several possible reasons for your tomato plant leaf curl. I’ll work through from the most likely cause to the least likely, providing the best solutions for each.
1. High Temperatures
The most common cause of tomato leaves curling up is high temperatures. During the hot summer months, tomato leaves may begin to curl as a defense mechanism.
If you have had a heatwave and your tomato leaves have begun to roll upwards, this is almost definitely the issue. The leaves will tend to roll up in the shape of a cylinder under high-heat conditions to prevent water loss.
This type of leaf curling in plants is known as physiological leaf roll, and it is a permanent change in the shape of the affected leaves. In other words, once the tomato leaves have curled, they will not un-curl.
Solutions:  Provide afternoon shade (with garden fabric or similar).Keep evenly watered during a heatwave. Keep large plants trimmed. Harden off plants gradually. Under hot conditions, your best bet is to help the plant stay cool during the hottest part of the day. This is usually during the mid-afternoon hours. Without protection, tomato leaves may also begin to wilt, and can even turn white due to sun scald.
Provide some shade from the sun if possible, or if your plants are in pots, move them to a shady location for a few hours until the heat passes. Also, ensure the plants have plenty of water, as they will drink significantly more in hot and dry weather.
Thankfully, this type of tomato leaf roll is not a major concern. As long as the entire plants are not showing rolled leaves, they will survive. After all, the plant is simply having a natural response to its environment.
2. Root Bound Plants
If you are like me, you tend to get a bit antsy during the winter months. However, you should never plant your tomato seeds too early!
If your plants are ready to go outside before the weather is warm enough, the plants may end up stuck in a pot that is too small. This almost always leads to a root bound tomato plant, which can cause leaf curling.
A root bound plant essentially has an entangled root system due to inadequate soil space. If the plants in question are in small containers, waiting to be transplanted, then this may be your cause.
Solution: Transplant the plant into a larger pot or into the plant’s final location as soon as possible! Next time, make sure you have a planting timeline established to avoid constraining the plants during early, rapid growth.
 3. Pest Damage
Broad mites, aphids and other tiny insects can be a huge headache for tomato growers. These hard-to-spot pests can show up unannounced and cause massive damage if left untreated.
I always recommend checking for pests on a regular basis by inspecting your leaves, top and bottom. Many of the most problematic pests blend in with the foliage, making them tricky to find.
If an infestation happens, you will eventually see signs of their presence. One of the common signs is tomato leaves curling. This type of curling will usually affect smaller, younger foliage, as this is what many pests are attracted to.
Solutions: Plant flowers and other companion plants to attract predatory insects. Spray the plant with a hose to knock off aphids. Bottom prune leaves to keep foliage off the ground.Spray tops and bottoms of affected leaves with insecticidal soap. Neem oil is an all natural insecticide that works by smothering insects, so be sure to spray the solution thoroughly on affected leaves. This can help reduce populations of aphids and other insects, while doing no harm to beneficial insects like ladybugs.
Note: Any pesticide should be a last resort, as they often kill beneficial insects too.
Diatomaceous earth is another natural remedy that can help prevent wingless insects from reaching the base of your plant. Sprinkle it on the surface of the soil in an unbroken circle around the base of each plant.
4. Viruses and Disease
Finally, we reach the most dreaded cause of tomato leaves curling, disease. While there are a wide array of diseases that affect tomato plants, don’t immediately assume your leaves are curling due to disease.
There are many causes for curling and yellowing tomato leaves aside from disease, so be sure to rule out other possible causes first.
Many viruses cause tomato leaves to wrinkle, curl or roll. Some common ones include mosaic virus, tomato yellow leaf curl, streak virus, and crumple virus. The names of these speak for themselves, essentially describing the visual effects of the condition.
Solutions:  Remove and burn the infected plants. Be sanitary in the garden. Bottom prune tomato leaves to avoid soil splashing onto them. Unfortunately, the only recommendation I can give for a diseased plant is to remove it from the garden. If you can, burn the affected plant far from your garden. Never compost a diseased plant!
The reason for this drastic measure is that most diseases can spread from one plant to another. Viruses can also be seed-borne, so avoid saving seeds from affected fruits.
To help avoid disease in the future, use good hygiene while gardening. Always wash hands before and after entering the garden, and never smoke in the garden.
Most viruses lay dormant in the soil until a suitable host and conditions are found. Prevent them from reaching your tomato leaves by bottom pruning away low leaf branches throughout the season.
Also, apply a thick layer of mulch around the base of your tomato plants. This will help dampen rainfall, reducing splashing.
5. Overwatering
Too much water is never a good thing for tomato plants. Ideally, your plants should be evenly watered, meaning not too wet, but not too dry either.
If you are overwatering, your plant’s roots will essentially begin to drown. One of the earliest warning signs of overwatering is tomato leaves curling.
Solutions: Use well-draining potting mix and allow potted plants to drain. Amend ground soil with perlite, vermiculite, or sand to improve drainage. Water only when top 2-5 inches of soil is dried out.If you have an automatic irrigation system, consider adjusting the schedule to reduce the amount or frequency of watering. Note that hot weather will lead to more frequent watering.
Note: Perhaps just as important as drainage is moisture retention. By mulching around the base of tomato plants, you can prevent evaporation of the moisture in the root system.
6. Drifting Herbicides
Do you live near a commercial, inorganic farm? How about a railroad? Maybe a neighbor with an exceptionally lush, green lawn?
Chemical herbicides, like dicamba, are used in many applications to destroy weeds and unwanted plant life. It is usually applied to foliage in a liquid or dust form.
If an herbicide like dicamba is used in large quantities at a nearby location, excess chemical may drift to your home garden via the wind. Even in tiny quantities, chemical herbicides can be devastating to sensitive tomatoes.
If your plants have been exposed to certain herbicides, the leaves will curl upwards at all of the edges. The tip of the leaves will also tend to curl up, eventually folding over. The edges of the leaves may also begin to turn yellow.
Solution: Gently spray all affected tomato plants with water after sunset.Avoid using chemical lawn fertilizer anywhere near the garden Don’t compost affected plants It is also possible that the lawn fertilizer you are using contains chemical herbicides. These may work well for suppressing unsightly weeds in the yard, but they can be horrible for your vegetable plants. Consider changing to an organic solution!
7. Excess Nitrogen  Tomatoes are fast-growing plants that require lots of nitrogen. However, any plant has its limits. If you provide too much plant food, your tomato leaves may begin to curl.
Nutrient burn will usually present itself with brown spots at the edges of leaves, in addition to curling. These spots will usually become dry and crispy if left untreated.  Solutions:Reduce fertilizer amount. Use a more gentle fertilizer (organic fertilizers are typically less concentrated) The solution is to reduce the amount and/or frequency of fertilizer. In extreme cases, you may wish to flush out the soil by watering and draining potted tomato plants several times. This effectively reduces the concentration of nutrients available in the soil.
8. Cold Weather
As the growing season draws to a close, tomatoes and other warm weather crops will begin to stress. Cool nights can cause leaf curling in tomatoes as the plants attempt to deal with the cold.  While freezing temperatures are inevitable in many locations, there are some things you can do to prolong the growing season.  Solutions:
Use cheese cloth or garden fabric to insulate plants from cold.Use a mulch around the base of plants to keep the roots warmer.Move potted plant into a garage or similar when expecting a freeze.Harvest any near-ripe tomatoes before a cold night.Keep in mind, it is always a few degrees warmer next to a house or building. If your plants are in containers, simply moving them near the side of your house may keep them alive a few weeks longer during late fall!
9. Wind Damage
In addition to hot weather, windy and dry weather can cause tomato leaves to curl in a similar fashion. Excessively windy conditions can stress both the leaves and stems of your tomato plants.  If you are planting in an open field, this could likely be your issue. Strong winds can steal moisture from the plants, usually causing the leaves to twist and curl.  Solution:  Provide some protection from the wind. This can be done by planting sturdier plants like bushes or trees to take the brunt of the breeze. Use a weathervane or similar to learn the typical direction of wind in your location and plant accordingly.  You can also protect young plants from wind damage by building a low wall around your garden beds. This is best for permanent, in-ground gardens as it can be costly.
What Can I Do About Curling Tomato Leaves?
Hopefully, you now have a better idea of the root cause of your curled tomato leaves. Each section above describes the best way to deal with each issue. But what can you do about the already curled leaves? Should you cut them off?
In short, curled tomato leaves will not un-curl, but new foliage can be improved by adjusting plant care. The curled leaves don’t need to be removed, as they can still contribute photosynthetic energy.
Taken from https://tomatogeek.com/tomato-plant-leaves-curling/
Till next time this is Becky Litterer, Becky’s Greenhouse, Dougherty Iowa beckmall@netins.net  641-794-3337  cell 641-903-9365