We get quite a few calls about trees and shrubs in decline and requests for advice on corrective actions. The one thing most of these calls have in common is the presence of a gray-green mold growing on the trunk and branches. Callers assume the gray-green stuff is responsible for the problems a tree or shrub is having, and they want to know what can be sprayed to take care of the “problem.”

What is the Gray-Green, Moldy Looking Material on Trees?

The gray-green, moldy-looking material is actually an organism called a lichen. Lichen is an unlikely combination of fungi and algae living in a symbiotic relationship on the plant’s bark. Algae supply food via photosynthesis, and fungi gather water and other needed nutrients.

Types of Lichen

Some have a flattened, crusty appearance. Other lichen spreads across branches and develops wavy folds that resemble a crumpled sheet. Some resemble highly branched balls of fuzzy material with projections going out in all directions.

What Causes Lichen Growth on Trees & Shrubs?

When woody trees and shrubs are under some kind of stress, many of them begin to struggle. Most of these stresses are abiotic and environmental. When the plants struggle, their canopies tend to thin and open up, allowing more light into the interior of the canopy. Lichens begin to grow as they start receiving more light. The combo of moisture and increasing temperatures creates the perfect environment for lichen growth.

How Do You Control Lichen Growth?

So, how can the home gardener control lichen growth? There really isn’t any need to remove them. Lichens are naturally occurring organisms that opportunistically increase their growth as a tree canopy starts to decline. The very best solution is keeping landscape plants in optimum condition by following recommended watering, fertilization and other management practices. A healthy and well-growing plant will have a canopy that discourages lichen growth. Light pruning of damaged branches encourages new branch growth, which, in turn, helps to establish a denser canopy.