Rhododendrons are among the most popular garden plants in Germany. Caring for them is easy if the location meets the needs of the rhododendron. The rhododendron needs loose, well-drained, acidic soil. The shrubs do not tolerate lime at all. If the soil is too calcareous or the pH value of the soil is too high, the plant will show signs of deficiency in the form of yellow leaves. In case of pest infestation and too much sunlight, the leaves get brown spots. Quick help is needed to allow the plant to recover.
Diseases
If the leaves of the rhododendron turn pale yellow, then it is suffering from chlorosis, also known as calcareous chlorosis. The disease is a typical deficiency symptom when the soil is too calcareous. Because of the lime in the soil, the plant cannot absorb enough magnesium, iron and nitrogen.
Tip: Lime can also accumulate in the soil by pouring it with calcareous tap water. Always water the rhododendron with rainwater if possible.
damage picture
If the rhododendron suffers from chlorosis, the leaves will turn uneven yellow, although the leaf veins may remain green. In the case of chlorosis, also called bleaching or jaundice, the plant cannot produce enough chlorophyll, or it is even broken down. As the disease progresses, the edges of the leaves turn brown and eventually fall off.
chlorosis/calcareous chlorosis
Chlorosis usually begins on young leaves and eventually spreads to older leaves. The plant begins to worry. It is also extremely susceptible to fungal infections.
Common causes of chlorosis in rhododendrons are:
- soil pH too high
- a lot of lime in the irrigation water or in the soil
In order to heal or save the rhododendron, the aim must be to enable the bush to absorb iron as quickly as possible.
Rhododendron with yellowing leaves
First aid for rhododendrons are iron fertilizers:
- Dissolve iron fertilizer in lime-free water
- Spray leaves with the solution
You can also water the rhododendron with a solution of iron fertilizer and water. However, the bush cannot absorb the iron very well or not at all, since lime and a high pH value in the soil prevent absorption. Since the causes of chlorosis lie in the soil, this must also meet the requirements of the rhododendron. To lower the pH value, you should work peat soil or special rhododendron soil into the soil. In addition, you should cover the bottom with pine litter.
improvement of the soil
Often the saving earth cannot be worked into large shrubs because the soil is too deeply rooted. In this case, a watering rim can be used to improve the soil.
- at some distance, dig out the pouring edge around the trunk
- Fill the pouring rim with rhododendron soil
When watering, the pH-lowering substances then get into the soil. However, the effect can only set in after a few weeks. On the other hand, the process can be repeated more often without damaging the rhododendron.
Tip: If a soil sample shows that the location is no longer suitable for the rhododendron, you should consider moving the plant to a more suitable location.
nitrogen deficiency
If the leaves of the rhododendron, including the leaf veins, are yellow, the shrub is suffering from a nitrogen deficiency. Another sign of this disease is a stunted growth of regrowing leaves.
This form of chlorosis often occurs when the soil has been covered with bark mulch without an underlying layer of horn shavings. In this case, it is a matter of nitrogen fixation, i.e. the plant cannot take up any nitrogen. The best way to combat the disease is with a fast-acting mineral fertilizer.
Rhododendron with brown discolored leaf
Tip: The last fertilization should be done at the end of May or mid-June. Nitrogen stimulates growth in the plant. In order for the shoots to survive the winter, however, they must be lignified, which takes a certain amount of time.
In order for the rhododendron to absorb enough nitrogen, you should fertilize it with horn shavings in the long term. This is particularly recommended for rhododendron species that require a lot of nitrogen, such as rhododendron Catawbiense hybrids.
magnesium deficiency
A lack of magnesium also causes the leaves of rhododendrons to turn yellow. This deficiency can also occur in soils with an optimal pH value for the rhododendron, since the magnesium is washed out by rain in permeable soils. Epsom salt fertilizer is a good first aid for the rhododendron.
- Spray the plant with a 2 percent solution of lime-free water
- Water the plant with a solution of water and Epsom salt
Tip: 150 grams of Epsom salts dissolved in 10 liters of rainwater lower the pH value of a square meter by one point.
care mistakes
In addition to the deficiency disease chlorosis, there are other causes of yellow leaves. They are often the result of care mistakes. However, unlike chlorosis, the consequences of care mistakes can be more easily corrected.
lack of water
In hot, dry summers, the rhododendron develops yellow to light brown leaves that curl up relatively quickly. This is a plant's response to a lack of water. Spray the rhododendron with lime-free water and give it generous waterings. After that, the shrub should recover. If the leaves of the rhododendron have brown spots after hibernation, it was not watered enough in winter. The only thing that helps here is waiting and hoping that he will recover over the summer.
Rhododendron with drooping leaves
sunburn
If the rhododendron was deprived of the protective shade, for example by omitting or thinning out the shady tree, too much sunlight can lead to yellow leaves or brown spots.
Sunburn occurs:
- on leaves exposed to direct sunlight
- for leaves that are not shaded by other leaves
Since the leaves of the rhododendron curve slightly downwards at the edges, sunburn often only occurs in the middle of the leaves.
Tip: Sunburn can also occur in sunny winters.
Quick help for sunburn is:
- Give rhododendron enough water
- Shade rhododendrons
Tip: A shading net or garden fleece is suitable for shading.
Also make sure that the shrub is very well supplied with water in the future.
Tip: If there is no way to naturally shade the rhododendron, it should move to a more shady spot.
Rhododendron with dried up flowers
frost damage
In and of itself, rhododendrons tolerate the German winter very well. However, if the shrub is in an exposed position, wind and cold can bother it and its leaves develop brown spots. Cut off the frozen parts of the plant in spring and protect the rhododendron from the cold in the coming winter.
Yellow autumn leaves
With a few exceptions, rhododendrons that are planted in the garden belong to the evergreen plants. However, sometimes the leaves begin to turn yellow in the fall. And a little later they fall off. This does not have to be a deficiency symptom, but the shedding of some of the leaves can also be a natural process. Even evergreen rhododendrons shed some of their oldest leaves every two to four years.
Tip: While this may be a natural process, be sure to check the plant for signs of deficiency or pest infestation.
fungal infections
Brown spots on the leaves of rhododendrons are usually caused by fungal infections. You can recognize an infection with the Phytophthora fungus by brown spots that have dried up light in the center. Other symptoms of the fungal infection are dead terminal buds and wilted shoots. Leaf spot diseases are understood to mean the infestation of various fungi. Infection mostly takes place in humid and warm summers, which is an ideal climate for fungi.
Rhododendron with infected bud
The fungi that cause it include:
- Glomerella
- Pestolotia
- Cercospora
- colletorichum
Depending on the fungus, red-brown to brown-black spots form. Their shape is round or irregular. At the beginning of the infection, the spots are relatively small. Only as the fungal infection progresses do they grow together.
For quick help:
- pick off affected leaves
- Treat rhododendrons with fungicide
Tip: To prevent the fungus from spreading further, the leaves must be disposed of with household waste.