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The rhododendron is loved by many hobby gardeners. Some gardeners are seized by the many beautiful flower colors of the rhododendrons of a collecting passion. Some, however, have little luck with the evergreen flowering shrubs. The leaves are often yellow, brown or spotty or the buds of the rhododendrons turn brown. Sometimes whole shoots wither away. Such problems can have complex causes that can be found either in a wrong location, wrong care or in a pest infestation.
Yellow leaves with dark veins (chlorosis)
The most common appearances on rhododendrons are yellow leaves. If yellow leaves with green veining appear on your rhododendron, and if it hardly forms flower buds, then it is a sign of iron and magnesium deficiency. Possible causes for this are calcareous soil or calcareous irrigation water. If your rhododendron bushes are too close to fresh foundations, limescale leaching from fresh mortar or concrete can be a cause of chlorosis. Because then the entire soil becomes too alkaline. However, rhododendrons require an acidic soil with a pH of 4.5 to 6.0.
With a special rhododendron fertilizer, which mainly contains iron and magnesium, you can help. In addition, the fertilizer also contains all the other nutrients, such as phosphate, nitrogen, potassium and sulfur, needed by rhododendrons to stay healthy and produce many flower buds. It is also important to improve the soil, for example with rhododendron earth. Measure the pH of the soil regularly. If it is too alkaline, it is advisable to lower the pH using elemental sulfur. You can do this by distributing one tablespoon of sulfur powder in the area of the crown of plants with strong chlorosis. If you live in a region where the surface water has a high pH value, a permanent rhododendron culture is only possible in raised beds that are completely filled with special rhododendron earth.
Tip: Never use aluminum sulphate to acidify the floor! Aluminum ions have a toxic effect on plants!
Young leaves brown and deformed
Brown and deformed, young leaves are usually signs of frost damage and can occur especially in less hardy Rhododendrenarten. If the leaves are brown or lightened in the summer, then it is usually a sunburn. Likewise, leaves and shoots in the winter, for example, from the winter morning sun burns.
Already during the planting of the rhododendrons the right location has to be taken care of. It should be sheltered from the wind and not too sunny. Frost-sensitive rhododendrons should be protected in extreme locations in winter, for example with a mulch layer of pine needles and oak leaves on their shallow roots. The mulch cover, however, should not involve the trunk too much. The leaves are protected with shading tissue or conifer branches.
Discolored, deciduous leaves in autumn
Some rhododendron plants get yellow to reddish leaves in autumn, which later fall off. This is usually a completely natural process, because even rhododendrons repel the oldest leaves in a two- to three-year cycle.
Yellow leaves and stunted growth
If the leaves turn yellow and the rhododendron cares, then it is nitrogen deficiency. In particular, the rhododendron Catawbiense hybrids are often affected, which require a lot of nitrogen. They are so-called nitrogen eaters.
Horn shavings are very well suited as nitrogen fertilizers for rhododendrons and are preferably incorporated in the soil as long-term fertilizer in spring. If you like, you can of course also work with artificial fertilizer. But then only in March and again in May. In later fertilization, the shoots no longer have a chance to mature properly and lignify. Because only mature wood is frost resistant.
Dried shoots often indicate the rhododendron wilt, which is caused by the fungus Phytophthora cactorum. The fungus clogs the pathways. With a very strong infestation, the plants can even die off completely.
- Cut out affected shoots
- remove cut shoots from the garden
- Spraying with 1% copper lime broth
- at intervals of 10 to 14 days
- possibly choose a better location
If the upper leaf surface of the rhododendron leaves is speckled yellow-green to yellowish-white, the underside of the leaf is noticeably blackish-brownish and some leaves curl up, later dry up and fall off, then the plant is attacked by the rhododendron bug (Stephanitis rhododendri).The dark plaque on the underside of the leaves are the faeces of the bugs and their larvae, moulting remains and the larvae themselves. Particularly prone to this bug is the rhododendron Flava. Similarly, rhododendrons are often affected at dry and too sunny locations.
In the month of May, June and July, it is necessary to preventively control the undersides of the rhododendrons for infestation. If necessary, spray with a neem-containing insecticide.
In case of severe lack of water, the leaves of the rhododendron curl up to protect against evaporation. These symptoms can manifest in both summer and winter, depending on the weather.
- Apply a layer of mulch so that the water does not evaporate so quickly
- pour regularly
- pay attention to soil moisture even in winter
- better location selection
- transplant if necessary
Rhododendrons often show round, brown spots on the leaves at unfavorable locations, which indicate a fungal attack. Then you may be too close together with shallow rooting big trees that rob them of nutrients and thus weaken them.
The fungus-infested parts of the plants must be cut off and burned or disposed of with household waste. They must not reach the compost or organic waste. If the rhododendrons are too close to shallow roots, they should be transplanted. Rhododendrons, for example, fit well under trees of life, pines and cypress trees, as these trees have long roots.
Withered, brown buds
Withered brown buds with small black spines indicate the rhododendron bud rot (Pycnostysanus azaleae) spread by the rhododendron cicada. In autumn, the rhododendron cicadas lay their eggs in fresh buds. In May of the next year, the yellowish larvae, which feed on the sap, hatch by sucking on the undersides of the leaves. A very strong infestation with rhododendron cicadas then manifests itself in the mottling of the leaves. From about July, the adult animals are seen, the very fast jump or fly as soon as they are disturbed. Due to its two sloping orange lines on its green forewings this cicada is very easy to recognize. Already during oviposition it comes to the fungal attack. Because then cut the cicadas slots for the eggs in the bud scales. As a result of these injuries, fungal spores adhering to the cicadas can penetrate into the plant. Only next spring the damage becomes visible.
Already in April you should remove suspicious buds and burn large or dispose of them with the household waste. As a preventative measure, you can apply yellow boards in the summer. However, it can also stick to beneficial insects, such as bees, dragonflies, butterflies and bumblebees. Also advisable are biological insecticides from neem extract to control the cicada population.
Tip: Unfortunately, as there is no effective fungicide to completely eradicate the fungus, you should resort to natural measures such as regular break-out of the affected buds, natural predators such as songbirds and beneficial insects in the garden and regular rhododendrons with homeopathic remedies.
Copper colored leaves
This discoloration is a natural autumn and winter coloration of the rhododendron variety 'winter purple'. In the cold season, their leaves turn beautifully coppery.
There are rhododendron varieties that have a felty coating (indumentum) on their leaves, for example, Yakushimanum selections such as 'snow pillow', 'edelweiss', 'koichiro wada' or 'silver lady'. Due to the fine hairiness of the leaves, these varieties should not be attacked by bugs and cicadas, according to experts.
Rhododendrons need acidic soil and regular fertilization to help them thrive and be protected against fungi and other diseases. Only with a good immune system can they defend themselves against noxious insects, fungal diseases and other adversities. Likewise, the right location and a good water supply are crucial to keeping the plants healthy and strong. However, regular checks on the leaves and buds of the rhododendrons are always appropriate to avoid possible damage at an early stage or to stop spreading.