Lichen on tree bark: what extension services say it is

This page describes what the named extension services say. It does not diagnose a tree, say whether any tree is healthy or failing, or tell anyone what to do about growth on bark. Where a source gives management advice it is mentioned in general terms and attributed.

What a lichen is

The University of Florida IFAS Extension publication ENH1224 (Sewards and Park Brown, original date September 2013, revised August 2016 and December 2023) calls lichens organisms made of a fungus together with an alga or cyanobacteria, living in a mutually beneficial relationship. The fungus takes water and minerals from the environment and gives the alga structure and protection, and the alga supplies carbohydrates and mineral nutrients. The paper says the combined body, the thallus, is so different from either partner that a microscope is needed to tell them apart, and that nitrogen comes from bird droppings, organic debris or plant leachate. Clemson Cooperative Extension (Home and Garden Information Center factsheet 2354, updated November 13, 2013) adds that lichens quite commonly include a yeast as well.

On looks, Florida says lichens may be flat, leafy, crusty, powdery or hair-like, in colours from white and gray to red, green, yellow or black, and that they grow on rocks, soil and fence posts as well as bark. Clemson describes green to gray-green leafy or crusty growths on trunks or branches. Washington State University (Hortsense, last review March 26, 2026) groups mosses and lichens as primitive plants often found on the bark of older trees and shrubs, typically grayish, greenish or orange, flat or branched.

Whether the sources call them harmful

Source	What it says
Clemson (2013)	Not harmful to the plant, only using it as a place to anchor. Typically found in abundance on plants declining in health or vigor, and in no way responsible for the poor health.
Florida IFAS (2023)	Commonly blamed for decline and death of shrubs and trees, but they do not cause harm.
Washington State (2026)	Not parasitic and will not harm the trees. Often associated with shaded or declining trees, but do not cause decline.

Why they show up on weak trees

Clemson says a less vigorous plant tends to be more open, so more sunlight reaches the bark and lichen growth rises. Florida says some lichen is normal on many Florida trees and shrubs, but heavy growth often indicates poor plant growth and decline from environmental stress, damage or poor management, and that defoliated branches let in sunlight that lets lichens thrive. Washington State says regular pruning usually limits development of mosses and lichens.

What the sources say about management

Clemson says controls are not necessary because lichens are not harming the plant, that lichens will gradually disappear if plant health is restored, and that they will probably reappear if the true cause of decline is not corrected. Washington State lists planting in sunny places, proper plant culture and hand removal of heavy buildups when practical, and says fungicides are not usually recommended. Where a pesticide is part of any advice, the guide on who may apply pesticides covers licensing.

Look-alikes and neighbours

Florida lists Spanish moss and ball moss alongside lichens in the same publication, and the page on moss on trees covers them. Parasitic plants behave differently: Florida contrasts these harmless epiphytes with mistletoe, a plant parasite, covered on the mistletoe page. The wider group is on things growing on trees.