The Reason Why Plant Leaves Curl Is Simple. It’s Responding To Local Environmental Conditions To Control Water Loss, Temperature Etc. That is What This Article Is About.

What History (& Science) Tells Us About Plant Movements & Leaf Curling

Why Plant Leaves Curl. Image attribution: Jon Sullivan, Public domain, via Wikimedia Commons

That plants exhibit movement isn’t new. Charles Darwin wrote about it, in 1880, in his groundbreaking book The Power of Movement in Plants. Darwin observed that many plant parts, especially leaves, respond to a range of extrinsic (environmental) and intrinsic (physiological) factors. Among the most significant extrinsic factors are light intensity (phototropic), light direction (heliotropic), water content (hydrotropic), and temperature (thermotropic). Notably, thermotropic movements are most commonly observed in plants growing in hot, dry environments, where leaves curl or move upward and become vertical to avoid excessive light absorption.

Leaf Curling, In Simple Language …

Plants have limited ways in which they can respond if it gets too hot, wet, dry etc. But they have some, and one of the most obvious is the way it uses its leaves.

When it rains we can move out of the rain, plants can’t. When the sun is too hot we can move into shade, plants can’t. We have many ways to respond to the weather and local conditions simply because we can move and make other responses. Plants however are much more limited.

In the following sections of this article I will show you how plants respond to environmental change. I’m not talking about those that have evolved to cope with specific conditions such as cacti that can store water and have reduced their leaves to spines so they lose little water when it is scorching. Or plants that shed their leaves in winter .. or even those that purposely shed leaves in summer (there are several that do in v various parts of the world!). I’m talking about vegetable plants such as tomatoes and other salads.

Why Do Plants Curl Leaves?

The question above is the short version of the many I’ve seen online. Some of the variations I’ve seen on Facebook recently include those at the end of this post.

The tomatoes equivalent of moving into the shade it to create its own shade. By curling its leaves it reduces the amount of leaf in full sun and partially shades itself.

Curling the leaves upon also has another effect. It reduces the area where it can rapidly transpire. If a leaf goes from being flat to form more of a cylinder shape then airflow is reduced and that means reduced transpiration. So leaf curling reduces the amount of water being loss by creating shade and less surface where the sun and breeze can remove moisture from the leaf.

Of course before this happens the plant will have closed its stomata, those minutes pores mainly found on the underside of leaves that allow gaseous exchange. Curling is resorted to when closing tomato alone isn’t sufficient. In extreme cases the leaves will shrivel up and die. In some cases that means death for the plant. But in some plants the loss of leaves like this is just part of the process and the plant will use its reserves to grow more leaves when conditions improve, PROVIDED it has enough reserves to do so. Not all plants do!

The above all leads me to the next reason why tomatoes and other plants curl their leaves. They do so to control moisture loss when they are under watered.

Overwatering can also be an issue. While less frequent, overwatering can also cause curling. In this case, the leaves might curl downwards and feel mushy.

Humidity levels can also cause leaf curling, or worse. Where the humidity levels are low then transpiration is speeded up and the plant loses moisture. The plant responds in much the same way as it does when it gets too hot. The difference here being that leaf curl occurs even when its not that hot.

Other Temperature Response Mechanisms In Plants

Have you ever noticed how rhododendrons respond to snow and frost. They hang their leaves downwards. This thermotropic movement helps them conserve energy by becoming pendent rather than vertical.

Light Level Issues That Lead To Leaf Curling

Too much light

If plants receive excessive direct sunlight, the leaves off some plant species might curl inwards or cup upwards to protect themselves from the intense light. This applies especially to those plants that like lower light levels such as some woodland plants. In the home it might happen to plants such as Ficus benjamina, which copes with relatively low light levels but struggles more with direct sunshine

Insufficient light

In low-light conditions, leaves may move to capture more available light. I’m told that they also curl so they can absorb more light, though I find this more difficult to comprehend and I’m yet to be convinced that the explanation I’m given, that it enables them to capture reflected light, is a satisfactory answer. ALWAYS question everything until it makes sense!

Pest or Disease Causing Leaf Curling

Many insects can cause plant leaf curling.

Aphids can suck the sap from leaves, causing them to curl and distort. Spider mites also feed on plant sap and can cause leaves to curl and turn yellow. Thrips are fast-moving insects can cause leaves to curl and develop silvery patches.

Herbicide Damage Causing Leaf Curling

A number of herbicides, even invert low doses, can cause leaf curling in susceptible plants. Ion my cases the culprit is a herbicide that contains a growth regulatory plant hormone. These are derived from, or based on, natural plant hormones that control growth in plants. But when used in herbicides the quantity is such as to induce abnormal growth leading to leaf curl and / or severe contortion of the plant stems and leaves which ultimately leads to death.

It isn’t always necessary to spray these herbicides on your plants. They can drift on the wind if sprayed nearby, or can be transported to susceptible plants on clothing.

The one that is most usually in the news in recent years are the aminopyralid and related compound herbicides that are used to control weeds in grass fields where they do no harm to the grass. Aminopyralids are however persistent and survive in the grass. If this is then fed to cattle or horses it is excreted in the dung and any manure made from it can cause problems if used to grow susceptible plants.

Not all plants are susceptible. However beans and tomatoes are very susceptible and can soon die when in contact with the aminopryralid.

I’ve written about aminopyralids in other articles and explain how to test for them.

More Leaf Curling Questions Asked Online!

Why are the leaves curling on my tomato plants?

Why are my tomato plant leaves curled?

Why are my tomato plant leaves curling?

Why are the leaves curled on my tomato plants?

Why are the leaves on my plant curling?

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Image attribution: Jon Sullivan, Public domain, via Wikimedia Commons
Tag: Why Plant Leaves Curl

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