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!
-Carol
Rhododendron leaf curl - Carol Quish
Uconnladybug's Blog 
February 5, 2009 at 2:27 pm
This article was great because it is so timely. My friend and I were just discussing how the plants are our personal temperature gauge during this bitter cold winter. One glance out the window and you pretty much know how cold it is out there. I had no idea there was another theory as to why the leaves curl! Keep up the good work of informing us fellow plant lovers.
February 11, 2009 at 5:24 pm
I enjoyed your article, Carol. I always watch the rhododendrons outside my kitchen door. During one of our Connecticut ice storms in January, the plants became thickly coated with ice while the leaves were still open. A day or two later the leaves had curled and the “ice leaves” were still intact above the “green beans.” I snapped a picture, but my camera was not up the capturing the detail.
March 13, 2009 at 9:14 am
Thanks for this article. I used it construct few important arguments and ideas on my blog.
April 1, 2009 at 10:40 am
I felt better when I read your article. Do the temperatures have to consistently be above 35? We have had some nice days in the low 50’s, but the leaves on my rhododendrons are still curled. Is it because the nights are still below freezing? This is the first winter with them and I hope they’ve survived.
April 1, 2009 at 1:14 pm
Once it is above 35 degrees F the leaves should uncurl. I suspect you have another reason for the leaf curl. Check the roots and lower bark for rodent chewing damage. Look for holes in the ground around the roots. Check the bark of the shrub for any boring insect holes. Or it could be winter dessication from drying winds and the cold weather on tender, young plants that caused the damage. Wait and see if they perk up and show signs of life once spring is finally under way. If not, they may need to be replaced.
May 4, 2009 at 6:54 pm
Thank you for this article. I moved into a house this past fall with a rhododendron bush. It is currently in poor shape. Not only have the leaves curled, but it is also brown and it honestly looks dead. The top portion looks this way, but the bottom looks healthy. Can I save the bush by trimming/pruning? If so, when and how should I do this? Cut close to the tree base or closer to the leaves? Also, should I cut the healthy branches? Or is it beyond help? Thanks for your help.
May 4, 2009 at 7:06 pm
Hello Rosanne,
Prune any dead branches back to live tissue. Look for any borer holes in the branch that has died. If hole is found, but below it to make sure insect is cut out with the branch. Put branch in garbage and remove from the property. Do not compost it as insect can continue to develop in the branch. Healthy branches do not need to be cut.
-Carol
May 20, 2009 at 6:38 pm
I have 5 rhododenrones on the northeast side of my house between the house and sidewalk in Omaha, NE There are cedar trees about 5 feet from them. One of the rhododenrones has extreme leaf curl and looks dead. I dug around the roots and noticed soil erosion on two sides. We had a downspout near the plants come off once during a hard rain but not other plants seemed affected. I filled the erosion parts with soil for now. Is the bush dead–it’s looked like this for two weeks.
May 26, 2009 at 4:54 pm
Hello Sheila,
If the rhododendron leaves are tightly curled during this month of May, the plant is probably dead. Snap some twigs to see if they are dry and dead. If there is green inside, there is still life and hope of resurrection. If the plant does not perk up in the next two weeks, it time to remove it.
July 23, 2009 at 6:13 pm
Thanks for your article. I too experience curling of the older leaves on my rhododendron every year,but,simultaneously, all newly formed leaves are perfect. On inspection in earlier years no cause could be seen. However this July I examined the curled leaves and inside each one was a green spider with web and eggs. Despite being sprayed, these hatched into tiny green elongated fast moving bugs. Any idea what they are? I do not live in a cold area.
July 23, 2009 at 8:02 pm
Hello Paul,
Spiders hatch out of eggs into tiny spiders. Therefore I think you either have two different insects or the original green green spider is not a spider but something else. I suggest you take a curled leaf containing the insects and eggs to your local extension service. They are aligned with your local university.
– Carol
August 22, 2012 at 6:08 pm
It is now mid August with a daytime temp of 75-80F. One of our rhodes which is next to another healty one has curled like in the dead of winter. It’s been curled for a few days. They are both planted atop our well. Why is one perfect and the other curling? Is it dying. Rick from Southern Maine. Please RSVP.
August 22, 2012 at 7:17 pm
Hello Rick,
Your rhododendron has something interfering with water uptake delivered to the leaves and stems. It could be a disease for an insect. Check the base and stems of the plant for small holes of the rhododendron borer. See fact sheet link here. http://www.clemson.edu/extension/hgic/pests/plant_pests/shrubs/hgic2051.html
Phytophthora root rot will travel in the soil and infect the roots. I would expect the other rhody would show signs of the same disease if it is in close proximity. See fact sheet link here. http://extension.psu.edu/plant-disease-factsheets/all-fact-sheets/azalea_rhododendron-diseases
Regards,
Carol Quish