Landscape - Lichens on Tree Trunks

Lichens can be found growing on tree trunks, sometimes causing concerns by homeowners and property managers. However, lichens are generally harmless. The following is an article on lichens.

Homeowners sometimes ask: Is that profuse, greenish, crusty stuff growing on my tree a disease? Is that crusty, green or gray material that covers the bark of tree trunks and branches going to harm the tree? What are those leathery things covering the tree bark? The short answers are that the grayish-green crusty things are lichens and that lichens are not tree parasites.

Lichens often appear as a perennial green or gray coating on the trunks and branches of trees. They are actually two organisms in one, being composed of a fungal body harboring green or blue-green algae, which live together in complete harmony. In the symbiotic relationship, the algae, through photosynthesis, supply carbohydrate food to the fungus and, in turn, receive protection and trapped water and mineral elements from the fungus. In this relationship, the algae and the fungus are not distinguishable except with a microscope, and the lichen persists longer than the alga or the fungus would separately.

Lichens do not parasitize trees, but merely use the bark as a medium on which to grow. In fact, lichens can be seen growing on rocks, weathered lumber, or on dead branches fallen from the tree. Some may consider lichens unsightly, but they are not generally injurious except that, when extensive, they may interfere with the gaseous exchange of the parts they cover. Because of their extreme sensitivity to sulfur dioxide air pollution, lichens seldom appear on trees in industrial cities... They rarely develop on rapidly growing trees, because new bark is constantly being formed before the lichens have an opportunity to grow over much of the surface. Because of this, lichens on certain species may indicate poor tree growth. We have noticed that in some plantings, those trees that are more vigorous have fewer lichens than those of the same age nearby in a state of decline. Few studies have been conducted to verify any correlation between lichen growth and tree vigor.

Lichens on trees take on various forms. Some are closely appressed to the bark surface and are described as crustose. Lichens which are foliose have leaf-like lobes which extend out from the bark surface. Others have hair-like or strap-like forms and are referred to as fruticose lichens. Lichen color may include forms that are green, blue-green, yellow-green, brown, gray, or even red. Increases in lichens are sometimes associated with moist climate - perhaps the relatively moist weather of the past two summers accounts for increases in lichen questions. Lichens proliferate when more light is provided, which could explain why they are more frequently seen on dead, leafless branches.

As a rule, lichens can be eradicated by spraying the infested parts with Bordeaux mixture or any ready-made copper spray. Read the fungicide label to be sure that this use is permitted for the product chosen. However, suppression of lichens with chemical sprays should not be expected to improve tree health.

Reprinted from the article "LICHENS INFESTING TREE TRUNKS AND BRANCHES" By John Hartman in the March 7, 2005 issue of the Kentucky Pest News from the University of Kentucky College of Agriculture.