When something is wrong with a plant, many people assume that something like an insect or fungus has caused the damage. But most plant problems in Washington are not caused by disease or insects. Usually the problems are caused by cultural stress such as too much watering or too little watering, recent transplanting or construction damage.
Yellow, faded or off color Rhododendron leaves can be caused by many cultural practices. Seldom do we see the cause by disease or insects.
Here are some of the things to consider when this problem occurs: 1. Roots were buried too deeply -- Rhododendrons are shall rooted shrubs.
2. Insufficient air spaces in the soil -- this often results from using heavy soils, clay soils, soils with poor drainage or soils that have been overly compacted.
3. Nitrogen levels may be too low -- with our variation of rainfall and sandy soils, nitrogen can pass through the rooting zone fairly quickly. There are several companies selling fertilizers specially designed for Rhodys.
4. The soil pH may be too high -- Rhododendrons like acid soils ranging from a PH 4.5 - 5.5. Adding lime is a mistake. Instead, add elemental sulfur if you feel this is your problem.
5. Failure to establish due to poor soil or planting practices -- often we don't consider a plant completely established until it has been in place for three years.
6. Lack of water -- this is a real problem and is often overlooked in the summer. Leaf scorch is commonly seen as a symptom when this occurs. Leaf scorch is easy to identify. The leaf margins are dead and often dried out. These signs never show up until well after the period when the water was needed has past. Rhododendrons planted near the house under eave troughs can fail to get enough fall and winter moisture. Plants located on hillsides, mounds or berms need to be watched more closely.
7. Root damage from the above conditions or root weevils can also be a problem -- root weevil larvae eat on the plant's roots. The adult is a nocturnal feeder which eats notches on leaves.
8. Girdling of the crown from winter freeze -- this may be a problem this year because of our excessively cold weather this winter.
9. Planted in full sun, and against south or west facing walls which reflect heat, will cause a greater demand for water.
10. Normal yellowing and drop of older leaves in the spring may be more pronounced if the plants are nutritionally stressed through the winter.
An extra note on Leaf Scorch is worth mentioning. Generally, Leaf Scorch is caused from lack of water at the leaf for transpiration to keep the plant cool. This can come about by, most obviously, lack of watering. Some other reasons can be overfertilization and root rot. Fertilizers are salts and if applied too heavily will prohibit the uptake of water by roots. This applies to all plants. It is commonly called 'fertilizer burn'. In addition, if the roots are in poor drainage conditions, where the roots are resting in excessive water, they will be damaged by root rot. Rotten roots do note take up water either.
Well, what should you do first? Start by checking the exposure first, then check soil related possibilities. Solve or correct the simplest potential problem first, then the next simplest. When all else fails, dig up the plant to look at the roots. They should have a thin brown outer layer and be whitish inside. You should also notice there are lots of small root hairs if the roots are healthy.
For more detailed information, you can obtain a copy of Extension Bulletin 1048 ($.25), LEAF SCORCH OF RHODODENDRON by writing or calling Island County WSU-Cooperative Extension, P.O. Box 667, Coupeville, WA 98239 - 679-7327.
Return to Articles Index
Return to Garden Page