How would you like to be able to tell how cold it is outside before you head out of the door? Look at your rhododendron. Drooping leaves occur when the temperature drops.
The leaves begin to droop when temperatures reach zero. Below zero, rhododendron leaves curl. That’s a defense against sun damage because surprisingly, shrubs can be exposed to more light exposure in the winter, despite shorter days.
It happens when they’re shaded by a canopy in the summer when everything in the garden is in bloom. By fall, leaves shed leaving the canopy wide open, exposed to direct sunlight.
Rhodies having drooping leaves in cold weather is normal. When it happens persistently, a little detective work would be in order to make sure your evergreen shrub stays healthy.
Possible Causes of Rhododendron Leaves Drooping
Bugs Draining Fluids from The Leaves
The main cause of leaf droop on any plant is a watering problem. Those are not always directly. When the plant has insufficient water or too much, leaves behave strangely. When they droop, it indicates low moisture in the soil.
Assuming you are using well-draining soil, the next issue is ensuring that moisture is not being stolen by pesky bugs. A number of tiny insects suck the sap from the leaves, leaving it looking limp. Aphids, scales and mealybugs are the usual culprits for draining fluid from plant leaves.
The simplest natural way to get rid of bugs on outdoor plants is just blasting water from a garden hose over the leaves to knock the bugs to the ground.
If no bugs are draining the leaves of fluids…
Move on to Root Care
The root ball is the critical factor to keeping rhododendrons healthy. Unlike some of the larger garden shrubs, these perennials do not have thick taproots. The roots are extremely fine and expand outward from the root ball in search of moisture.
This is why rhododendrons should be planted in a hole roughly twice the size of the root ball.
On that same topic, you can find that if you have planted the root ball too deep, the leaves will droop because of low moisture content, but worse, planting too deep is often the cause of a rhododendron not flowering. Kind of defeats the point of planting it.
Newly Planted Rhododendrons Need a Drip Watering System
Rhododendron roots take time to mature. When you first plant them, the finer feeder roots are not present. No matter how much moisture is in the surrounding soil, without the feeder roots, the root ball will be limited to direct moisture.
That’s the moisture content directly in contact with the shrubs’ main root system.
Until it develops the finer feeder roots to obtain moisture from the surrounding soil, a drip feed irrigation system can be used to slowly keep the roots watered.
Generally, you can expect new rhododendrons to require direct root watering for the first year to two years. After that, the feeder roots will be established enough to obtain moisture from the surrounding soil.
Water young rhodies more, and then reduce the watering frequency based on the presence of feeder roots.
Feeder roots generally extend up to 10cm from the root ball and remain near the soil line. These are the roots to keep hydrated on mature rhododendrons.
Care for the feeder roots and those will keep the root ball moist. Until your shrub develops those feeders though, you need to water the root ball directly.
Root Weevil Larvae
Vine weevils can do real damage to the rhododendron roots.
If you have your rhododendron potted, then these are more likely to be a persistent pest. Not so much the weevils, but the larvae they leave behind.
The black vine weevil is the adult. This does no damage to the roots. Instead, these cause cosmetic damage by nibbling on the leaves at night during the summer months. That is when these are most active. The most damage happens over the winter months beneath the soil.
All black vine weevils are female and reproduce parthenogenetically meaning they do not need a partner. And, all of their offspring will be female too.
The larvae are laid in the soil or the mulch. By the winter, larvae mature into white grubs that feed on the roots. That is when the damage can get to extreme levels.
The video below shows the life cycle of a black vine weevil. The black adult beetle and the leaf damage it causes is shown, as is the closeup of the weevil larvae.
Look for these in your soil and around the roots of your rhododendron because as these grow larger, they begin to feed exclusively on plant roots.
Phytophthora Dieback and Root Rot
Phytophthora is a serious threat to the Rhododendron. It is rare and it is a soil problem that effects the shrubs roots directly.
Drooping and wilting limp leaves on your rhodie are symptoms. These can persist for up to a year on mature plants. Younger Rhododendrons won’t have the strength to fight this infection.
As Phytophthora is a fungal infection that thrives in wet soil, you can try aerating the soil, giving the roots time to dry out and treating it with a fungicide.
The good news is that the vast majority of rhododendrons are bred for resistance to this fungus. If your unlucky to have chosen one that isn’t, it will be difficult to treat.
Try following the steps laid out here for how to save a dying rhododendron, because assuredly, if the soil is contaminated with the Phytophthora fungi, the plant will die. It’s only a case of when.
Mature plants can hang on for up to a year, however the damage progresses as the disease progresses so expect your plant to get worse until the fungus is eliminated from the soil.
Photooxidation Caused by Hot Weather or Too Much Sunlight
The green leaved plants most of us are familiar with are the ones that use photosynthesis. C3 plants to be exact.
The Rhododendron is one of those. It opens its stomata during the day then they close at night because without sunlight, they can’t photosynthesize, which is how they create sugars for food to keep them alive and healthy.
But What Happens if The Sun Comes up and the Stomata Remains Shut?
This is called photorespiration and it happens when the temperatures are too hot or the light too intense for the plant. It will keep the leaf pores closed to prevent excess water loss.
This is the reverse of photosynthesis and is a protective mechanism. If it goes on for too long though, photooxidation can kick in.
Photooxidation is a chemical reaction within the plant that occurs when the levels of carbon dioxide remain at high levels for too long. Photooxidative stress for evergreen perennials is more prevalent during the winter, usually because rhododendrons do not have larger trees, shrubs or blooms above the canopy to diffuse the sunlight.
Any type of stress will cause leaves on rhododendrons to droop. Once the light balance is restored, the stomata will open, then photosynthesis can happen. Until the light intensity and temperatures drop, leaf movements on the rhododendron will continue.
Take a look around your rhododendron to see what’s usually around it. If there’s usually some shade from a taller plant or tree diffusing the sunlight that’s shed its leaves in the fall, the shrub will be exposed to a higher light intensity.
The vast majority of rhododendron species prefer dappled sunlight year-round. Not full sun. If the leaves are drooping and curling in the winter only, consider the level of sunlight reaching the plant. It may be too strong requiring uprooting and repositioning somewhere more shaded.