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Rhododendrons have to be one of my favourite shrubs, both the evergreen and deciduous varieties, including Rhododendron luteum, a more recent addition to my garden. I’m lucky to have worked as a professional gardener for over 20 years. My family also runs a small nursery where we sell a large selection of rhododendrons.
Over the years, I’ve seen and experienced my fair share of problems that affect rhododendrons in my garden, my customers’ gardens, and the nursery.
These include common issues I see repeatedly, such as drooping leaves that eventually drop. I’ve actually written a guide on dropping leaves here. This problem is the most common one that gets asked about at the nursery every year. There are actually a few causes, which I will go into more detail further down in my guide.
Do you have drooping/wilting leaves
However, I will briefly cover it now. If it’s winter and there has been a period of cold weather and frost, rhododendrons often droop to a point where they look like they’re wilting. This is usually not a problem, and it’s definitely more common with rhododendrons grown in pots which is why we see it every year in the nursery. Once the weather warms up, they always perk right back up.
Just double-check that the soil isn’t dry or that there has been a period of drought, as they could be wilting from lack of water. This is particularly important in autumn and winter, as this is when the buds form. If rhododendrons don’t get enough water while the buds form, they might not form, or worse, they will drop off before opening in spring.
What out for vine weevils, which eat the edges of leaves
Now, a couple of more serious problems also cause the leaves to wilt. One is vine weevils, which are little black beetles. However, it’s their larvae that really cause some damage, as they eat the roots and can cause serious damage. This is more of a problem for rhododendrons grown in pots, as the roots are so confined. If you have a rhododendron with wilting leaves, I’ve written a more in-depth guide here. One more cause of drooping leaves is root rot, usually caused by waterlogged soil, which I will discuss in more detail below.
Rhododendrons need acidic soil to get the nutrients they need, with yellowing leaves being the first signs
The second most common problem I see is yellowing leaves, usually caused by nutrient deficiencies in the soil. This is usually caused by the soil being alkaline, in which case you can try to make it more acidic, but this is no easy task. I typically recommend you grow them in large pots in ericaceous compost instead. Rhododendrons need acidic soil, which you can quickly test with a soil pH tester.
You can usually prevent most problems by planting rhododendrons in ideal growing conditions. This means acidic soil or ericaceous compost in a partially shaded position. Rhododendrons like a bit of shade. They naturally grow in woodlands under the canopy of trees. They prefer moist but free-draining soil. By providing ideal growing conditions, you can prevent most issues in the first place.
Watch out for caterpillar damage
One issue I get myself, which is difficult to prevent, is the leaves getting eaten by pests, usually caterpillars. If the holes are inside the leaves and not around the edge, these are usually caterpillars; if they have edges eaten, this is a sign of vine weevil damage. If your rhododendron is grown in a pot, check the roots for tiny white larvae with brown heads. If you find them, remove them by hand and treat the roots with a vine weevil killer that targets the large. Provardo vine weevil killer usually does the trick.
Powdery mildew and bud blast are the two main diseases to watch out for on rhododendrons
When it comes to diseases, rhododendrons are prone to a few different diseases, but the two most common diseases I’ve come across in around 90% of disease cases are powdery mildew, a common disease on many plants but not too serious although unsightly and bud blast. With powdery mildew, it can be treated with a fungicide spray, of which there are many, and by removing badly affected leaves.
However, bud blast is more serious and harder to control but is easy to identify as the buds turn brown but don’t fall off. They often turn a silvery greyish colour. I always recommend removing effects buds as soon as you notice them to help prevent the spread to other buds. It’s thought leafhoppers may spread it, so controlling this can reduce the chances of getting a bud blast. However, controlling these is easier said than done, so all you can do is look out for them.
That covers the most common issues with rhododendrons, but there are a few more pests, such as scale insects, I want to mention. They are also susceptible to diseases such as Powdery Mildew and even root rot and dieback disease, which I cover in more detail below.
Pests that attack Rhododendrons
Vine weevils and their larvae
The biggest pest you are likely to face is vine weevil, and it’s easy to identify them from the damage they do to the leaves, as pictured above. The adult beetles eat the leaves from the outside, which is how the damage caused by vine weevil is different from general caterpillar damage, which leaves holes on the inside of the leaves. Vine weevil beetles themselves don’t really cause any significant serious damage, though it’s very unsightly.
It’s actually the larvae that live in the soil that eat the roots of your Rhododendron and cause serious damage that can actually kill the plants. However, this is only a real problem for rhododendrons grown in pots as the roots are so confined.
The adult weevils are most active in the spring and at the end of summer, while their larvae start to eat the roots between summer and the subsequent spring. This is a widespread insect problem across the UK. Realistically, all you can do is inspect your plant regularly and pick off the weevils you see them, trap the adults with sticky barriers around your garden, or introduce natural enemies like birds and frogs, which basically means encouraging them into your garden if you don’t already do so.
If you have Rhododendrons grown in pots, then you can treat them with Provado Vine Weevil Killer, a systemic pesticide used to drench the roots, killing the vine weevil larvae and this is what we actually do on the nursery as we get problems with vine weevil now and again.
Scale Insects
Scale insects are another problem and they will suck the sap out of your plants and are fairly common, although not really an issue until you get a big infestation. The real problem is that they excrete a sticky substance on your foliage, and that eventually causes black sooty mould, which, although not serious, can make plants look very unhealthy with black sticky leaves.
There are many different species of scale insects, and they are active all year round. Some are small as 1mm, whilst some are larger than 10mm, and if you see scales or tiny bumps on the stems or underside of your leaves, it is indicative of scale insects. You can spray them with organic sprays or pesticides to remove them, but they are very persistent and might reappear. There are also pesticides you can use, and you need to follow the instructions accordingly. Pesticides are best treated when young because pesticides seem to be less effective as they get older and form a waxy shell that repels the sprays.
Other diseases that affect Rhododendrons
Powdery Mildew
Powdery mildew is another prevalent disease, but it’s often easier to live with it as it’s not usually fatal, but it’s very common. Usually, you will actually see white powdery growth on the leaves. I usually find that it can get much more prolific if your Rhododendron is grown in a damp location or somewhere very shaded. And in some circumstances, this happens purely because of the location. I recommend you plant Rhododendrons in partial shade with some protection against strong winds.
It’s very important to remove any dead leaves you see, even those that are on the ground around the base of the plant, and to improve the airflow so plenty of space around them. If you notice powdery mildew has already spread on your Rhododendron, you can attempt to control it by spraying with a fungicide.
It’s worth noting that on evergreen Rhododendrons, it can be difficult to spot powdery mildew, whereas it’s easy to spot on deciduous varieties. On evergreen varieties, it also causes patches of purple, brown, light green or yellow on the surface of the leaves. Even though the manifestation of the disease is different, it is still a fungal disease, and most Rhododendrons thrive despite this disease. If you see any of these symptoms on the leaves from the previous season you can apply a fungicide to prevent them from spreading further, however, you need to be thorough about the application of the fungicide to prevent any airborne spread effectively. I would not recommend removing the leaves because it would cause severe defoliation. Unless very severe, these types of infections ideally would be tolerated.
Root rot
Rhododendrons thrive in moist conditions, but they need to be well-drained as they don’t like wet feet. However, on the opposite end of the scale, you need to be careful about root rot, which is also a common disease and often caused by poor draining soil. Root rot is a soil fungus that can infect plants in different locations and eventually will cause the leaves to yellow, wilt and die.
If you don’t do anything about root rot, the plant itself will die as it can’t take up nutrients or water from the soil as the roots die. You can rectify this problem by replanting a Rhododendron to a location with better drainage, or improving the drainage before planting which is the ideal scenario. With root rot one of the problems is that the preliminary symptoms appear underground and those that appear above ground are very similar to many other diseases and common problems.
Dieback
Dieback is when you see the leaves on your Rhododendron wilting and the entire branch dying back, typically as the result of a fungal infection. If you have a very dry season and your Rhododendron gets stressed, you need to be very careful about pruning because any wound, crack in the bark, or scars will allow the infection to enter and spread.
Cut back any affected branches right back to a good healthy section of the stem and remove all affected sections of the plant.
Other problems with rhododendrons
Weather damage
Aside from diseases and pests as covered above, Rhododendrons will experience damage if they are exposed to windy or cold weather, usually in exposed sites. Excess frost or winds can damage the leaves and the flowers, especially if we get a late frost.
Buds dropping or buds not forming
Rhododendrons prefer moist but well-drained soil, and when the soil is not moist (at any time of the year), it can cause the buds to form incorrectly or not form at all. Often they will dry up and just drop off in spring. The only solution to this is to prevent it in the first place. This means watering during times of very little rainfall and adding a good layer of acidic mulch around the base of plants to help retain moisture.
Soil deficiency
Rhododendrons are susceptible to different problems, the first of which is called chlorosis. This is basically when the leaves turn yellow, and this can happen at any time of the year. In almost all cases, it’s completely harmless to the growth cycle of your plant, and it’s simply indicative of a nutritional deficiency.
If you look at the plants and you see the veins, in particular, are starting to yellow and further down the line, you also notice things like wilting or leaf drop, which is often an indication there is a problem with iron or magnesium deficiency. It is for that reason you need to be particularly careful about planting your Rhododendron in the right soil. Rhododendrons need acidic soil, so you should test the quality of the soil ahead of time, and if you happen to have alkaline soil, it’s better for you to grow them in pots, in ericaceous compost.
