When people notice lichens on their trees, shrubs and other plants, they become concerned that they have problems. But they shouldn’t worry, says LSU AgCenter plant scientist Raj Singh.

“Lichens are fascinating creatures,” says Singh, who is often called the “plant doctor” and heads up the AgCenter Plant Diagnostic Center. “They constitute the most questions I receive at all the garden shows I attend.”

Lichens are actually small plants that are composed of two different organisms — a fungal partner and a photosynthetic partner living in a symbiotic relationship. The fungus provides the body and shape while the photosynthetic partner — either a green alga or a cyanobacterium — produces the food through photosynthesis.

These small plants are not parasites but rather epiphytes, which grow successfully under different environments and geographical areas ranging from arctic to desert.

“They can almost grow on any surface, including roofs and walls of buildings, rocks, shrubs, trees and even iron fence posts,” Singh says.

Lichens have several different growth habits. Some grow flat like a crust; others are filamentous like hair, while others are leafy or branched. They come in some of the most vibrant colors, ranging from lime green to bright orange.

Lichens grow slowly and may live a long time.

Actively growing lichens are an indication of good air quality; they don’t do well in polluted environments.

So the question is, are lichens dangerous to plants? “The answer,” Sing says, “is NO! They are not plant pathogens. They use the tree surface as a substrate to grow epiphytically.”

Because they are not parasites, lichens do not derive any nutrients from the host they are growing on. Lichens may grow on healthy as well as stressed trees.

Trees and shrubs can be stressed by drought, improper fertilization, compact soils, disease or insect pressure or other poor cultural practices.

Generally, no chemical control is recommended to manage lichens, but homeowners should avoid any biotic (insects, diseases, nematodes and weeds) or abiotic (nutrients, drought, water logging and compaction) stresses to their trees.

Got a gardening question? Write to GardenNews@agcenter.lsu.edu.