In my experience, when new gardener see their tomato leaves curling, it can often cause panic, and their instinct is to water the plant more, use a fertiliser, or spray it, and that can actually make the problem more severe for your tomatoes.
I’ve personally been growing tomatoes for decades and have made my share of reactionary mistakes! After years of experience, he is the diagnostic framework that I now use to accurately assess the problem and why the direction, location and even the vein colour of the curling leaf is your biggest diagnostic clue.
Read the Direction First: It Narrows the Cause by Half
This is the most important and useful bit of intel I have for diagnosing your curling tomato leaves…Before we check the soil or look for pests, we need to look at which direction the leaves are actually pointing!
This is an important distinction, as upward and inward curling is an indication that the problem is environmental or cultural.
Your tomato plant is protecting itself from too much evaporation from the leaves, which can be caused by heat, wind, drought or more likely a combination of the 3.
The leaves of the plant are still green, and the texture is characteristically leathery, but all in all, healthy. The plant is acting in its own self-preservation.
Downward and cupping leaves that curl notably when the leaf twists to the side or is distorted in some way are an indication of chemical or biological problems, for example, herbicide drift, aphids or viral infections can all cause downward or irregular curling leaves rather than a clean inward roll that is a characteristic hallmark of heat stress.
We need to get this distinction right before we do any more investigating, as it saves time, money and the tomato plant itself.
The Taco Leaf: When Curling Is Actually Clever
As I recall, it was 2 summers ago we hit a particularly hot heatwave in July where the thermometer didn’t drop below 95°F for a full week, and I walked out onto the patio to find my Cherokee purples (a particular favourite of mine) looking stressed with leaves rolled up so tight they looked like little green tacos.
My first thought was to get the hose and give them a generous soak, but experience taught me to check the soil first. I checked an inch into the soil, and it was still moist, so the plant wasn’t thirsty.
The leaves were doing what botanists call physiological leaf roll, where the curl reduces the leaf surface area, which reduces the amount of water loss through transpiration and reduces the area that the sun can shine on it.
It was defending itself from the environmental conditions rather than any sinister sign of disease.
So, I waited until evening fell and the temperature dropped, and the leaves breathed a sigh of relief and gently uncurled themselves.
You see, if I had panicked overnight and given it a good hosing, I could have had waterlogged roots on a hot and humid night, which could result in root rot.
How do I confirm it’s heat stress?… I would do a test over 24 hours, which is to move a container plant into bright light rather than full sun (or use a wind break for my tomatoes planted in the ground) and give the base of the tomato plant a deep, slow soak in the evening. Check back the next morning, and if your leaves look uncurled and healthy, then you were just dealing with heat stress, and there is nothing to worry about; however, if they are still curled and the morning air is cool, that’s a more sinister problem afoot.
The Chopstick Test: Stop Guessing About Moisture
Here is a diagnostic test that I use instead of relying on a water meter (which in my experience are not as accurate as you need them to be).
Take a chopstick or a BBQ skewer and push it 4 to 5 inches deep into the soil near the base, leaving it for 10 seconds and then pull out.
Think of this like a cake test…a dark, wet coating on your skewer means the roots are in saturated conditions, and the curling is because there is not enough oxygen around the roots rather than drought, so adding more water compounds the problem.
If the soil is only slightly damp with a few soil particles on it, then the moisture levels are okay, and the reason it’s curling is due to something else.
So it takes 30 seconds and removes doubt on one of the most common misdiagnoses in tomato plant growing.
If your roots are soaking, then you need to skip watering for a while to let them dry out, and it should recover; however, if it’s left, then your tomato plant can die of root rot.
Whenever I see this problem, it’s usually because the tomatoes are planted in clay soil, which drains too slowly (in which case, plant your tomatoes in pots instead) or because the pots do not have drainge holes in the base. Drainge is very important to avoid curling leaves, so always choose pots with drainage holes in the base of the pot.
When the Curl Goes Downward: Herbicide Drift
Last year, I had a row of San Marzanos (the best tomatoes for making sauce), and the leaves started curling in a different way in a sort of downward, twisting formation. Also, the new growth was stringy and weirdly almost fern-like, so I knew it wasn’t a watering issue.
I peeked over my back fence into my neighbour’s garden to see his lawn was all of a sudden looking immaculate without a dandelion in it. What happened was that he had sprayed a broadleaf weedkiller containing 2,4-D on a windy day (why on earth?!) which landed in my garden.
Tomatoes have a big reputation for herbicide drift, where even a tiny amount of 2,4-D landing on the leaves causes something botanists call epinasty, which is a hormone disruptor that produces the twisting, downward, fern like distortion in the leaves that cannot be addressed with water or fertilizer.
If you are seeing curling leaves on your tomato with unusual-looking new growth, then you may have the same problem! I discarded the plants to be on the safe side, and fortunately, I had some San Marzano plants in a greenhouse that were unaffected.
Where on the Plant: Old Leaves vs New Growth
We need to note that the age of the affected leaves is another important clue for diagnosing our plants.
Curling starting on old, lower leaves after a bout of heavy rain or boggy soil is usually physiological, and the plant is reacting to too much moisture around the roots, which stops the roots from respiring and the lower canopy curls and turns yellow.
Curling on brand-new, emerging leaves at the top of the plant is a different clue indeed. The newer growth that emerges is a different matter entirely, as new growth that is tightly curled up or crumpled with even a purple tinge is not environmental but instead…
- Pests such as broad mites and thrips that feed on the new, more vulnerable tissue. So, check the undersides of affected leaves with a magnifying glass if you have one and even without seeing the mites themselves, you can often see fine silver webbing on the leaf.
- Viral infection: Tomato leaf curl virus is spread by whiteflies and results in the new leaves curling upward, turning pale annd stunt and once the plant is infected, there isn’t a cure, so remove it and throw it in the bin to prevent whitefly spread.
The rule is that environmental causes affect older tissue first, as the plant prioritises protecting the newer growth, whereas pests and viruses target new growth first because it’s the easiest to attack.
The Vein Colour Check
Turn the tomato over to inspect the veins on the underside, as this is a vital clue…
Do the veins turn a distinct purplish-blue on the curling leaves? If so, then this suggests that either the plant has a phosphorus deficiency, which, surprisingly enough, can be caused by cold soil, which is a common problem early in the season as tomato plants struggle to uptake nutrients when the soil temperature is below 55°F.
Veins staying perfectly green while the areas between the tissue turn yellow is likely magnesium deficiency, which is a mobile nutrient issue that occurs on older leaves initially, and I find it responds well to an Epsom salt treatment.
Veins and leaf tissue both uniformly affected without any pattern between the veins suggests an environmental problem or moisture stress as opposed to a nutrient problem.
The Quick Diagnostic
| Curl Direction | Location | What To Look For | Likely Cause |
|---|---|---|---|
| Upward / inward | Older leaves | Damp soil, hot weather | Physiological heat or drought stress |
| Downward / clawing | New and old growth | Distorted, stringy new leaves | Herbicide drift |
| Upward / crumpled | New growth only | Silvery scarring, stunted leaves | Pests (mites, thrips) or virus |
| Upward | Lower leaves | Saturated soil, heavy rain | Overwatering/root oxygen deprivation |
| Any direction | Older leaves | Purple veins, cold weather | Phosphorus deficiency or cold soil |
The One Thing Worth Remembering
Most of the tomato leaf curling I find is just the plant sensibly reacting to hot weather.
I only get concerned when the leaf curls still happen on cool mornings, affecting new growth or downward and distorted shape or have discolouration and pest damage too.
Dealing with curling tomato leaves and not sure which category fits? Describe the direction, which leaves are affected, and your recent weather in the comments, and I’ll help you work it out.