The longer we nurture our plants, the more we understand them. When you see their leaves curling inward, you know your plant craves your attention to correct something.
The terrific news is that inward leaf curl is entirely reversible.
So, relax, take a breath, and continue reading to explore the causes of this seemingly irrational behavior.
Once you understand what’s happening, you will can promptly fix the leaf curling issue on your plant and have it back to its splendid beauty rapidly.
Leaves curl inward to conserve water either from underwatering or in response to abiotic stress. Potential causes include temperature fluctuations, insufficient nutrients, improper watering, pest infestations, or restricted root growth from either being pot-bound or wrongly fertilized.
Did you know that up to 95% of a plant can be entirely water?
Suffice to say, without enough, the plant cannot function.
They need water primarily for photosynthesis. It is how plants make food for survival. Sun is the main compound responsible for photosynthesis, but it can only happen with sufficient water content in the plants ecosystem.
Water is evaporated from the leaves, but in extreme temperatures, it will evaporate from the potting mix before the roots get a chance to soak in the nutrients to transport it through the stems to the leaves.
When that happens, the result is leaves curling, inward usually, but when plants get extremely thirsty, the leaves can curl to the extent that it is effectively rolling over on itself to create a cylindrical shape.
Within each leaf, there is (or should be) a constant amount of water.
This is referred to as “turgor” and it is the constant pressure exerted on the cell walls of plant tissue.
When too much moisture is in the leaves, they swell. Without enough water, turgor reduces, leaves curl, then wilt.
In that respect, congratulations are in order for noticing the leaves curling inward, because if you fail to fix the leaf curling then within a few days, the leaves are likely to be wilting when there’s even less moisture available.
Check the soil and if the top inch is dry to the touch, then it will need to have water added.
Take a note of when you water because if the plant is going through water faster than usual, something will be causing the soil to dry out faster than it should be.
Regardless of the plant species, overwatering is bad. It will always lead to a negative outcome.
The problem with too much water is that it causes roots to rot.
Root rot kills plants. The reason is that the roots need oxygen and they cannot get that from soil that has become compacted from having too much moisture.
A plant with roots buried in compacted soil is totally different from an aquaponics setup or hydroponics.
If you know of a plant species that a friend uses in their aquarium with the roots fully submerged in the water yet the plant grows just fine, it is because of the air pump.
Container plants with a potting mix or vegetables planted in a raised garden bed have to rely on air pockets throughout the potting medium. Without those, there is no oxygen.
The longer the roots are drowning, the faster the plant’s demise. Root rot does not happen instantaneously. It is a slow (ish) progression.
Starting off, the leaves lose their color, turning yellow first. Then they go brown or black when they die. The roots turn to mush, then the stems go soft.
When the roots are rotted, no water can be transported to the plant leaves. Because of that, the leaves will look as though they need watered.
They do, but not because it is not available, but rather because there is far too much available that the air pockets that were in the soil are flooded.
As such, the roots wither, rot, then die.
In all soil and potting mixes, there are air pockets. It is not all dirt below the topsoil.
When soil becomes compacted, the oxygen supply is reduced. That is when the real concern of overwatering is raised up a notch.
When the soil becomes compacted from water saturation.
If the potting mix is saturated to the point that water pools on the surface, the plant needs to be removed from the potting mix then given time to dry, and then repotted in a fresh potting medium.
Whether you can save a plant from root rot depends on the severity.
If all the roots are rotted, little can be done to prevent the plant from dying. If a partial amount of roots are rotted, those can be cut off the plant, and the healthy roots only repotted in a fresh soil/potting mix.
3. Extremely high temperatures
High temperatures go hand-in-hand with underwatering because water evaporates faster in heat. Different plants respond differently too.
Some plants roll their leaves into cylinder shapes to shrink the amount of leaf surface that is exposed to direct sunlight.
Others tilt to be parallel with sunlight so that the entirety of the leaf surface is not exposed.
Broadleaf plants with two or more color tones will turn their leaves so that the lighter-colored leaves (or part of them) are facing the sunlight because light colors reflect sunlight. Dark colors absorb it.
The more dramatic leaf changing characteristics happens when temperatures approach extreme levels such as triple digits in the midst of summer at the hottest part of the day – noon to 3 pm.
When high temperatures are behind leaf curling, adding more water to replace the moisture lost through evaporation is not the solution.
The plant will continue its heat dissipation cycle, which has three stages. Water evaporation, convection and conduction.
The solution to reduce the temperature of the leaf surface is to increase air flow.
The higher the temperature, the less dense the air is resulting in a reduction to air flow.
Rather than attempt to replace water lost through transpiration, turn fans on to improve air flow, which is what the plant needs to help it regulate the leaf temperature.
4. Insufficient nutrients
Plants are smart. Just like they curl their leaves to reduce the leaf surface as an assist with transpiration and respiration, they will also sacrifice the old leaves to assist the young new growth when nitrogen is in short supply.
Nitrogen is one of the primary chemicals plants need to survive. It relies on it for chlorophyll production (the green pigment in the leaves) which is what plants needs to have for photosynthesis – the process it uses from the energy obtained from sunlight to make sugar from the water you feed it. It’s own food.
The early signs of nitrogen deficiency is yellowing leaves, but not all over the plant. The oldest leaves at the base of the plant will yellow first, then brown, then drop. That part is natural. Further up the plant new growth is happening, which should not be yellowing.
Old leaves yellowing is natural because the plant will reserve and redirect as much nitrogen as possible into new growth. As a result, the older leaves near the base of plants will lose their greenery, display slight leaf curling, then brown and drop from the plant, eventually.
You can speed the process along by pruning off yellowing leaves at the base of the plant. If the yellowing is happening further up the plant on new foliage, the issue will be something else.
5. Pest infestations
Plant pests are a frequent cause of leaves curling in on themselves due to their feeding habits. They pierce the leaves of plants to drink the sap in the leaves resulting the plant becoming dehydrated.
The three most common insects responsible for eating plant leaves of container plants are aphids, whiteflies and thrips. Those are also the few tiniest ones making them difficult to spot.
Larger leaf-piercing plant bugs include leafhoppers, stinkbugs, scales, and squash bugs which are more common in garden plants.
All of these pests are fast breeders and will infest a plant in a short space of time. When these are present, they congregate on the underside of leaves.
Looking at the leaves from any other angle than from the bottom, you will notice yellowing, inward curling from the leaf edges, and possible spotting on the leaf surface.
Another indicator of a plant having a pest presence is the appearance of ants. Ants do not feed on plants, but they do feed on the honeydew which is the excrement of insects. It is the sugary substance of honeydew that ants feed on.
When sap-sucking insects are present, ants can be nearby protecting the colony of insects. Getting rid of the ants will not be possible without eliminating their food source,which will be the pests feeding on the plant.
Treating plants for a pest infestation can be done with a strong blast of water across the undersside of all the leaves, or using rubbing alcohol, or an appropriate insecticide.
One of the safest treatments for banishing bugs is neem oil. It is a natural product that acts as a contact poison on insects.
6. Root/Pot Bound
Once a plant outgrows its pot, the roots can no longer contain themselves within it. Instead, they grow out of the drainage holes, in turn blocking the holes where excess water should be draining out of.
The result is the same as you would have had if you overwatered the plant by watering before existing moisture could drain freely like it should.
The roots are not necessarily always going to grow through the drainage holes in plant pots. Sometimes, they can just as easily grow too close together inside the pot, forming a dense mat of roots with little soil/potting mix between them.
With too many roots inside a pot that is too small, there may not be enough nutrients in the mix to support all of the roots. This would result in a lack of nitrogen, regardless how much fertilzer was used.
To know if your plant is root bound, you need to take the plant out of its current container to inspect the root system.
Just by attempting to remove the plant from the pot can give you a good indication of whether it needs to be in a bigger pot.
If the pot size is correct, the plant will slide out with little force required. A slightly root bound plant will take a little wiggling to prise it out of its container.
A heavily root bound plant is likely to require a knife or similar object inserted around the edges of the pot to prise it away from the container.
In extreme cases of plants being constrained to a pot that is too small, there may be little alternative to breaking the container by cutting through plastic nursery pots.
Once the plant is out of its current container, the roots can be inspected, trimmed by up a to a third to repot in its current container with a fresh potting mix, or depending on the plant species, you may be able to divide and propagate the plant into multiple pots.
If you have never repotted a plant before, be sure to learn how to repot a plant because it involves more than taking it out of one container to pop into another.
If you want to keep the plant at its current size, repot in a container slightly bigger than the one it is in. Typically, one to two inches larger is the guideline, but that does depend on the size of the root system.
What to keep in mind is that the root size and leaf size is interdependent. The larger the roots beneath the surface, the more nutrients can be supplied to the leaves up top.
The less nutrients each root can use, the less nutrients can reach the leaves. That is when you begin to see symptoms like leaves curling upwards, inward, downward, crinkling, and sometimes just downright dropping from the plant.
To keep the leaves in tip-top shape and great health, the roots absolutely need to be cared for properly.
The size of pot has a direct impact on nutrient absorption, water retention, and oxygenation within the soil or potting mix. Too small or too large of a pot will have negative consequences on the plants health.
7. Herbicide drift or contaminated compost or fertilizer
Just because you have not used chemicals near your plants does not mean they are immune to herbicide damage.
Some chemicals are short lived and effected garden waste can be disposed of sparingly in the garden compost bin. Other, stronger grade herbicides can take years to break down.
Herbicides containing clopyralid (which can be applied to hay and straw) can take a few years to break down. Gardeners may inadvertently buy straw or hay to use as mulch in garden beds, then see their plants leaves curl, wilt, and degrade.
It can happen if hay and straw is contaminated with clopyralid as that leaches into the soil dosing the root system with a herbicide, unknowingly to the gardener.
In 2020, the Norwegian Institute of Bioeconomy Research (NIBIO) conducted an analysis of organic fertilizers that were suspected to be the cause of inward leaf curling on tomato and cucumber plants grown by hobby growers in Denmark and Sweden.
At low levels of 1 μg (microgram) /kg of clopyralid or aminopyralid, it is harmless. What the analysis found was 4μg/kg to 1200 μg/kg of clopyralid mostly in liquid fertilizers. Dangerously high levels!
The adverse effects of leaf curling differed based on the plants sensitivity and the level of exposure.
Strawberry plants can withstand higher concentrations (up to 1,000µg/kg) than tomato plants, cucumber, lettuce, beans, and lentils that can only tolerate up to 1 µg/kg.
If you have recently applied a fertilizer that is different from your usual, try going back to your original, or use an alternative fertilizer to see if it makes a difference.
Compost is another aspect to be careful with when using selective herbicides such as those used for lawncare. Herbicide drift is the concern for gardeners when applying these products.
Taking care to spray weeds only on clear days with little wind to avoid the weed killer blowing onto other plants.
Where the problem of contamination can arise is placing grass clippings or fallen leaves into the compost bin soon after the herbicide has been applied.
This can lead to the compost becoming contaminated, which can then leach chemicals into the soil impeding root growth of plants, which then shows on top growth with inward leaf curl.
When your plant behaves differently from its usual, always consider what growing conditions have been changed as that will be the likely cause.
Daniel has been a plant enthusiast for over 20 years. He owns hundreds of houseplants and prepares for the chili growing seasons yearly with great anticipation. His favorite plants are plant species in the Araceae family, such as Monstera, Philodendron, and Anthurium. He also loves gardening and is growing hot peppers, tomatoes, and many more vegetables.