How Curling Leaves On Rhododendrons Act Like A Thermometer

Close up of pink rhododendron flowers on a shrub
(Image credit: Yaroslav Litun)

Do you ever look at your plant leaves to tell the temperature outside? You could, if you have rhododendrons, and people have been doing it for over a century. When the weather drops, you’ll see your rhododendron leaves curling - and yes, it’s a real phenomenon called “thermotropic” leaf movements. Read on for all the details.

Leaves Curling on Rhododendron

Whether you have rhodies in your garden or not, you likely know what the leaves look like. Normally, rhododendron leaves are flat and extend horizontally from the plant. But when temperatures dip significantly in winter, you may see those leaves coiled and hanging like dark green beans, as one gardener expressed it.

That this happens is indisputable. As early as the 1800s, scientists noticed this phenomenon and tried to understand it. Why do the leaves droop and curl in the cold, as if they were hugging themselves? Even today, many gardeners wonder the same thing. And many theories have been forwarded.

Theories About Leaf Curl on Rhododendrons

Many potential reasons have been proposed for cold weather causing curling rhododendron leaves. But almost all of them have been discounted or disproved.

One theory was that this leaf reorientation was nature’s way of reducing snow load on this broadleaf evergreen. The proponents suggested that if the rhodie leaves remained outstretched, the snow buildup could break branches. However, it turns out that a substantial amount of snow can accumulate on the branches of these plants regardless of leaf position and without doing the plant any harm.

Another theory was that rhododendrons curled leaves in order to reduce water loss. When the leaves droop and curl, they could be upping the humidity of the air around their leaves and lowering their risk of desiccation. However, rhododendron do not open their stomata during the colder months, so there is no net transfer of water into or out of the leaves in winter.

Explanation for Leaf Curl in Rhododendrons

The current theory for why rhododendrons curl their leaves in winter involves separate reasons for the two different movements: leaf curling and leaf droop. Leaf curling, where the edges curl and cup inward, happen when temperatures dip below freezing. The latest thinking is that this helps to protect the vulnerable underside of the leaves by blocking them from the cold air with the waxy cuticle of the upper side of the leaves.

Leaf droop involves a change in the angle at which the leaf is positioned. This doesn’t occur at freezing, but at a lower temperature, around 25 degrees F. This is when the foliage is most vulnerable to irradiation. Sunscald is more dangerous to rhododendron species in winter than in summer, since during their growing season, they are protected from the sun and temperature extremes by a deciduous canopy.

However, this protection is gone in winter. The leaf cell membranes that handle photosynthesis are particularly vulnerable and can be permanently damaged. When the plant lowers its leaves to a vertical position, it reduces both excess cold and light.

This all means that rhododendron leaves curl and droop in the cold. Next time temperatures drop, take a look at your rhododendron and see for yourself.

Teo Spengler

Teo Spengler has been gardening for 30 years. She is a docent at the San Francisco Botanical Garden. Her passion is trees, 250 of which she has planted on her land in France.