Rhododendron Leaf Curl
Checking the winter landscape recently, I noticed the rhododendrons looking most distressful. Their leaves were curled under into green bean looking tubes droopily hanging from the branches. It was during the very cold weather experienced here in Connecticut last week. I have seen this other years in the same plant so I know they will recover. The evergreen leaves will uncurl and perk up once the temperatures rise to about 35 degrees F.
I did a little research to find rhodies are ‘thermotropic’; sensitive to temperature changes and respond with leaf movement. Charles Darwin wrote a book in 1880 titled ‘The Power of Movement in Plants’ in which temperatures causing movement is covered. As air temperatures drop below 35 degrees, the curling and drooping begins. The lower the temps drop, the tighter the curl and more vertical the hanging leaf. Some people have come to recognize the actual temperature by how far their rhododendron leaves have curled and drooped. Different species of rhodies curl at varying temperatures, so you will have to watch your particular plant and the thermometer to develop this talent!
Rhody leaf curl is widely thought to be a protective measure taken by the plant to ward off the drying winter air and winds causing moisture loss. The rhododendron’s leaves have tiny valve openings on the undersides called stomata. This is where the plant releases moisture. When the leaf curls, the stomata are concealed. The vertical drooping catches less wind than a horizontal leaf, resulting in less drying.
Another theory proposed goes into more detail saying the curling is to protect the leaf from the sun. Rhododendrons naturally grow in part shade but in winter the deciduous trees are lacking leaves exposing the evergreen rhodies to more light than in summer. This causes the leaf temperature rise, thaw out and make food in the leaves. When night comes, the temperature drops, freezing the leaves and water in the leaf. Water expands as it freezes, forming ice crystals in the leaf cells, cutting the cell walls. Leaf curling reduces the amount of leaf tissue exposed to the sun therefore reducing the amount of photosynthesis taking place. It is the daily thawing and freezing causing the damage.
Time (and research) will tell which theory proves correct. Or, it may end up being a combination of the two. Either way, they both say that Rhododendron’s leaves curl in below freezing weather to protect the leaves from being damaged. I will watch them for a little sign of spring as I wait for them to uncurl and stay there!