Horticulture News: What is that growing on my tree?

By Kim Cowherd Published:


I have recently had several calls and some samples brought to the office from homeowners regarding a profuse, greenish, crusty stuff growing on their trees. Is this a disease? Is that ugly, green and gray stuff covering the bark of my tree trunks and branches going to kill the tree? What are those leathery things covering the tree bark? They look like an alien life form! 

The short answer is that the grayish-green crusty things are lichens and that lichens are not tree parasites or killing your trees.

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 – use the tree for food – but merely use the bark as a place 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. 

It can be noted 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 pressed 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 summer 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, or trees that are in poor health and have a thinning leaf crown. 

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 on lichens is permitted for the product chosen. Follow all instructions on the label when applying any chemical spray. However, suppression of lichens with chemical sprays should not be expected to improve tree health.

To also help clear up the lichens and to prevent lichen growth, be sure your tree is in good health. Properly timed fertilization, correct mulching, sufficient moisture, and proper plant placement in the landscape will help insure a healthy, vigorously growing tree that will be less susceptible to lichen growth. 

For additional information on lichens log on to: http://www.aces.edu/pubs/docs/A/ANR-0857/ANR-0857.pdf. For other horticultural issues, contact the Franklin County Extension Office by phone, 502-695-9035 or email DL_CES_Franklin@email.edu.UKy.


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