Lichens on woody plants are basically harmless at first. Often older shrubs are attacked by them. However, if the lichens spread too much, the bark of the woody plants is attacked. This can lead to increased infestation by fungal pathogens and thus to serious diseases.
How to get rid of lichens
There is no approved pesticide against lichens on plants. However, since lichens are very sensitive, they die after treatment with paraffinized mineral oil preparations or neem products. If the ornamental shrubs are sprayed with them several times, the lichens disappear. The treatment should be made in the weeks before the foliage shoots.
Indicator of a clean environment
If you find blue-green lichens on your woody plants, take a deep breath: lichens only grow in really clean air. In Europe, leaf lichens are being found more and more frequently in parks and forests. This is a good sign, because 20 years ago there was a sharp decline in lichen vegetation due to increased sulfur dioxide concentrations in the air. Pollution has decreased, and the air is becoming increasingly pure.