Master Gardener’s: Lichen’s: A tree’s friend or foe?
By Sheri Bethard
Certified Texas Master Gardener, Orange County Master Gardeners
Have you ever seen a crusty object or a thin layer growing on your trees or rocks? We’re here to tell you NOT to panic. It might just be lichen, which is not completely a fungi or a bacteria. It is one of the most common mutualistic relationships in the plant world made up of one-part filamentous fungi and one part algae or blue-green bacteria (cyanobacteria) or even all three! Lichens are not considered a “true species.” The unique combination results in a very hardy, weather-tolerant, and genetically diverse group of Nitrogen fixers that is practically self-sufficient. The fungal partner cannot survive alone, but instead thrives on the availability of photosynthetic products provided by the algae or bacteria. The alga feeds the fungus through photosynthesis, while the algae receives some food and support from the fungus
Lichens are common pioneers on trees, shrubs, soil, and even rocks, but do they kill the host that feeds them? The truth is, whichever host the lichen colonizes, it benefits from the added moisture and environmental protection, while also providing a place for the lichen to take root and establish a strong residence. Lichens commonly grow on limbs, branches, and tree trunks of certain tree species. Since they are not pathogenic or considered pests, there is no control method available today.
Lichens appear as surface growths that are usually grey or grey-green in color. Three forms of lichens exist – crustose (flat type of growth), foliose (leaf-like but with a prostrate growth), and fruticose (bush-like and erect, or hanging growth).
Lichens grow under conditions of high light intensity. Heavy lichen growth is often an indication of poor tree vigor as a result a cultural problem or stress. Lichens are NOT parasitic to the tree itself, though heavy growth may begin to restrict gaseous exchange from the limb or twig, and can start to block light from reaching the plant’s surface.
While they are sometimes unappealing, removing the lichens mechanically can create fresh wounds that invite more severe plant pathogens to inhabit your landscape.
If a tree or shrub seems to be declining AND is covered in lichens, there is some other cause for the demise such as insect infestation, watering practices, or perhaps even legitimate plant pathogens. Overall, the relationship is mutually beneficial and should not be a cause of worry.
There are currently no chemicals to control lichen growth. The best way to reduce lichen growth is to focus on improving the tree’s vigor and encouraging the growth of a dense canopy, which will reduce light penetration to the surfaces. Fertilization and timely irrigation can be considered strategies for tree vigor improvement. As the canopy density increases, shading will reduce the photosynthetic capacity of the alga, and, over time, the presence of the lichens should be decreased.
So, to recap, lichen are NOT plant parasitic fungi OR bacteria but can be beneficial to your tree and give you warning signs if there is something wrong with your tree. They are just a little reminder to give your tree a little TLC.
References: The Texas Plant Disease Diagnostic Laboratory at Texas A&M AgriLife Extension Service