11 Common Problems With Rhododendrons You Shouldn’t Ignore
Are you Rhododendrons looking a bit worse for wear this season? There are many common issues that can impact the way your rhododendrons look. In this article, gardening expert Jill Drago shares the most common problems that may plague your Rhododendrons, and what to do about each issue.
Rhododendrons are beautiful, large flowering shrubs. These shrubs are acid-loving, and versatile in your gardens due to the many varieties available, and the wide array of colors the flowers bloom in.
Once these shrubs are established in your garden, they are relatively low maintenance. Requiring partial shade and little water, you will not need to do much other than admire the beautiful flowers that these shrubs produce.
Despite their ease of care, all garden plants are susceptible to issues. Luckily most issues are not fatal to your rhododendrons. Below I have listed the most common problems you may encounter while growing rhododendrons in your garden.
Finding yellow leaves throughout your rhododendron is a very common occurrence. This yellowing is called chlorosis – a yellowing of the leaf’s tissue due to a lack of chlorophyll. Chlorosis can be caused by multiple different causes, including nutrient lockup due to alkaline soil pH, damaged or compacted roots, nutrient deficiencies, or poor drainage.
Before treating for chlorosis, consider your environment. Do you have claylike soil that holds a lot of excess moisture? Is your soil pH high? Have you fertilized your plant recently? Is it in a container where it might have become rootbound? Usually, one of these factors will be the culprit. For fertilizing or soil pH concerns, perform a soil test to identify the causative factor. For claylike soils or rootbound plants, a more physical solution such as repotting it in a larger pot or amending the soil to promote better drainage may be all it needs to recover.
Another reason that your rhododendron leaves may be yellowing is simply senescence or the aging and dropping of leaves. Leaves that are going through this process will be entirely yellow. Senescence may occur during a heat wave, but we most often see senescence in action in the fall as trees drop their leaves and go dormant for the winter.
Here, though, autumn senescence should not be occurring; most rhododendrons are evergreen shrubs. A few older leaves will still turn yellow and drop on occasion as the plant renews its foliage, so you shouldn’t see a dramatic shift in the entire plant. If you do, you’re dealing with chlorosis, not senescence.
Brown Spots on Leaves
Brown spots can be caused by a variety of issues, but the most common is one of many forms of leaf spot. These are fungal or bacterial diseases that are manageable but, if left untreated, can severely damage or kill the rhododendron.
In the case of many fungal leaf spot causes like alternaria or septoria leaf spot, often an application of either a sulfur-based or copper-based fungicide will help prevent further spread. However, you will need to prune off already-damaged leaves prior to treatment to ensure that any spores cannot shift to healthier leaves during spraying.
Bacterial leaf spots are harder to treat. These are best avoided by watering at the base of plants rather than on the foliage and ensuring your plant remains healthy with no damaged tissue where a bacteria might enter. With bacterially-infected plants, removal and disposal of the infected plant will reduce the risk of that bacteria spreading to other plants in the garden.
In both cases, stressed plants are more at risk. Ensure your plants have the right amount of water, sunlight, and nutrients they need, and you won’t see significant leaf spot issues.
Die Back of Branches
If you notice that one or more of your branches have died completely, you may have fungal dieback. However, if most of your rhododendron seems healthy, you can diagnose fungal dieback by scratching the bark of the dead branches.
If you reveal green plant tissue beneath the bark, your rhododendron is healthy. If you see red underneath the bark, you have some work on your hands treating and dealing with fungal dieback.
The first thing to do here is to prune away the affected branches. Remove these branches from the area, and do not add them to your compost pile, as fungal diseases can spread easily via fungal spores on the infected material. Next, you will want to begin treating your rhododendron with a copper fungicide.
Leaves are Curling
In the wintertime, it is a common sight to see your evergreen rhododendron leaves curling up on themselves. I can understand why this would be a concerning sight to see. The good news is this is what rhododendron leaves do when the temperatures drop. This will help the rhododendrons from losing too much water.
Another reason your rhododendron leaves could be curling up on you is drought. In the heat of the summer months, when rain is sparse, these large shrubs can suffer from drought stress.
Similar to the reaction rhododendron leaves have in the winter, the curling of the leaves in the summer is a defense against the heat and will help keep the leaves from losing too much water.
Buds are Not Opening
Occasionally your rhododendron will produce beautiful flower buds, but come spring, these buds may not open. This is usually due to a late spring frost.
If you notice that your previously green flower buds have suddenly turned brown in the spring, this is likely what has happened to your rhododendrons.
Unfortunately, if your flower buds have become frost damaged, there is no resurrecting them for this growing season. Applying an anti-desiccant may help in the future. This will provide a waxy coating that will help to protect the plant tissues from cold temperatures as well as chilly winds. If you’d prefer not to use an anti-desiccant, consider providing some form of plant protection such as burlap or frost blankets during cold weather.
No Flower Buds
Rhododendron flower buds are produced in June, not long after the bloom period has ended. These buds will persist through the rest of the summer and even over the winter. Right now, my rhododendrons and their perky flower buds are sitting happily with a light dusting of snow on them.
If spring comes and you see that there are flowers, take a harder look. Are there any flower buds? Flower buds can vary in size but are ovate in shape and will form at the ends of the branches.
If you have found no buds, there is one very likely reason: pruning. The pruning of rhododendrons should be done immediately after blooming has ended. If you have waited too long, you likely nipped the buds right off of the plant unknowingly.
The good news here is that the flowers will return in the next growing season. The bad news is nothing can be done about it except to wait and plan your pruning better the next time.
Lichen on Branches
Lichen is a common sight on shade trees and shrubs. So what does it mean if you have lichen on your rhododendron? Luckily lichen itself is not an issue for the rhododendron. However, the sign of lichen is a reason for concern. Lichen will form on plants that are growing in less than optimal conditions.
Generally speaking, lichen forms on plants that are either growing too slowly or that don’t move, and it requires damp or humid conditions to truly thrive. This means that rhododendrons that are growing too densely or too close together may be at risk. The lack of airflow through the plants can cause conditions that make lichen more likely to grow. Consider giving your plant a rejuvenating prune to help air circulation and allow sunlight to reach all parts of the plant.
Rhododendrons are susceptible to root rot. This is a fungal disease that can readily if your plant sits in soil that does not drain well.
Chlorosis (mentioned earlier) can be caused by root rot, particularly if the yellowing is accompanied by drooping or wilting of softer branches. If you expose the roots of the plant and discover that they are mushy or discolored, it indicates your plant’s experiencing rot.
At this point, you will need to make a decision about the overall health of your rhododendron. If only a few roots are mushy but most of the root system appears intact and healthy, rinse off the current growing medium, trim off damaged roots, and replant in soil that drains more readily.
The best way to prevent root rot is by paying attention to how well your soil drains. If your plants are directly planted in your native soil, you can dig a hole, fill it with water, and watch to see how long it takes to drain; if it’s more than 30-60 minutes, your soil needs to be amended. Those growing in containers may find it easier to simply repot in a better- mix with good drainage holes in the base of the container. Finally, try to ensure your plants get about one inch of water per week.
If you notice brown or rust-colored leaves on your rhododendron when spring comes around, it is likely that you have some winter burn on your shrub. This is not a disease or the result of an insect infestation. Instead, it is an unfortunate outcome of the plant’s location in your yard or the weather conditions you have experienced during the winter months.
The reason this can happen to your rhododendron leaves is mainly due to a lack of moisture. The leaves on rhododendrons are broadleaf and have a large surface area, making it easy for moisture to get wicked right out of them in the winter winds.
It is more likely for your rhododendrons to get some winter burn if they are planted in direct sunlight in the winter months. This can happen if they are living under deciduous trees and are suddenly in full sun rather than partial shade.
If you know that your rhododendron is planted in a situation where it will receive too much sun or wind in the winter, you can apply an anti-desiccant, creating a waxy coating on the leaves. This will help hold the water in the leaves throughout the winter. Apply the anti-desiccant in the late fall for the best results. When applying, the temperatures should be 40-50 degrees Fahrenheit with no projected rain in the near forecast. This ensures the anti-desiccant has time to dry fully and solidify on the leaf surfaces.
Protection via a frost blanket or wind barrier can also reduce the risk of winter leaf scorch.
Yet another fungus that rhododendrons may struggle with is petal blight, and sadly this fungus attacks its beautiful flowers. Once the flowers have bloomed, you may notice small brown spots on the flower petals, and from there, it will spread rapidly.
The good news here is that this will likely only affect your rhododendron for one growing season. If you’re quick to act when you see the first symptoms, you may be able to stop the spread before it moves to another flower by removing the fungally-infected ones.
Infected petals will fall to the ground once the disease has taken hold. Once the petals are down, cleaning them up and removing them from your garden is the best way to prevent this disease from returning year after year, along with using mulch to prevent soil from splashing up onto the flowers during watering. Do not add these petals to your compost pile! The spores on the petals can remain viable even after composting.
For the most part, the insects that are found on rhododendrons attack the lush foliage. Black vine weevil, lace bugs, and spider mites all have their own methods, but the end result is pretty much the same: leaf damage.
Black Vine Weevil
Black vine weevil will start feasting on your rhododendrons roots and make their way up to the leaves. These insects spend their entire lives feasting on different parts of the rhododendron. You will not even notice these insects until the leaf damage has begun. Apply beneficial nematodes to the soil at the first sign of plant droop and identification of weevil damage on the foliage. The nematodes will attack and kill the larvae in the soil.
Lace bugs attack rhododendrons that are growing in full sun and may be under a bit of drought stress. These insects will leave yellow spots on the leaves. This is a tricky bug to control since it moves around frequently, but a broad spectrum pesticide is your best move.
Spider mites are very tiny insects that feed on many parts of a plant. They are so small in fact, that you will likely only notice their webbing and not the insects themselves. Spider mites like to feed on the leaves of plants during drought. Spray your rhododendrons with a hose to remove these pesky little bugs. A horticultural oil application can also be effective.
Woodland and shady gardens are not complete without the glory of the rhododendron. The larger varieties are great for privacy screens, while the smaller or dwarf varieties are great in borders or containers. If you encounter any of these issues above, try not to worry. They are manageable, and I hope these tips will make caring for your shrubs easier.