Lichen and Trees

When you see tree bark covered in crusty green stuff you may wonder, does lichen impact trees? Don’t panic! It’s unlikely that the ‘green stuff’ is harming the tree.

Lichens are non-parasitic organisms that can grow on tree branches and trunks. They consist of a symbiotic partnership between a fungus and a green alga or cyanobacterium. The fungus collects water and minerals from the air and the alga/bacterium help produce food through photosynthesis as well as other nutrients.

lichen on tree branch
Lichens can grow on tree trunks or branches, but do not cause damage to the host tree.

Where Do Lichens Grow?

The lichen itself grows on the surface of organism or objects. Examples include trees, rocks, dead wood, gravestones, etc. Though it grows on these host surfaces, it does not cause any harm or negative impact to that host.

Lichens grow best in sunny locations with good air quality. As such, you’ll often see them on tree trunks where there is ample light. Sometimes, a tree that is already in decline will have a thin canopy with less foliage coverage. Less leaves means more light is able to get through to the branches and trunk and lichen can grow more abundantly. However, that lichen is not the underlying source of the tree’s decline.

What Should You Do if There is Lichen on Your Trees?

As mentioned previously, a tree with a thin canopy that is letting in more light than normal will foster growth of lichen. While the growth itself does not harm the tree, the loss of canopy is a concern. If you have a tree or shrub with excessive lichen growth and a thin canopy, evaluate the tree and the surrounding environment. Stress, disease and insect problems may need to be corrected to improve tree health and reinvigorate the canopy. Stress in particular can come from environmental and cultural problems such as compacted soil or issues with lack of nutrients or incorrect soil moisture. Proper soil care and root invigoration may help improve tree and shrub health in this case.

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