Lichen and moss on bark: harmless, and what they say
Whether the gray-green crust or the moss on a trunk hurts it, why they favor slow or thinning trees, what a sudden crop means, and why scraping helps nothing.
On this page
What lichen and moss areWhy slow and thinning trees carry moreA sign of clean airLeave it on the barkCommon questionsSourcesLichen and moss on a tree’s bark do the tree no harm. Neither has roots in the bark, and neither takes food or water from the tree. Both gather where bark stays put and light gets in, so a crop that appears on a thinning tree is a sign to check the tree’s health, not the cause of its trouble.
What lichen and moss are
A lichen is a fungus and an alga living as one body, on rocks and fences as well as bark. The alga makes food from sunlight, and the fungus keeps it from drying out. Fine fungal threads hold a lichen to the bark, but they carry no water or food, and the lichen takes both from rain and air. It takes nothing from the tree.
“Lichen are often blamed for killing a tree or shrub but this is not true.” (University of Maryland Extension, Lichen, Algae, and Moss on Trees)
Moss is a small plant with no true roots, attached to the bark by rhizoids. Moss and green algae grow on any bark that stays moist and shaded, on any side of the trunk, not only the north. Bark that turns green only while wet may be the tree’s own green layer showing through.
Spanish moss, the gray strands on southern live oaks, is an epiphyte, a plant that grows on another without taking water or food from it. Only a heavy load harms a tree, by shading out its lower limbs.
Why slow and thinning trees carry more
Lichens favor bark that changes little over the years, so slow-growing trees such as oaks carry more than fast ones. A fast-growing tree carries little, probably because it sheds its outer bark before a lichen has time to spread. Many lichens grow fastest in full sun, and a thinning crown lets that sun reach the trunk and limbs.
Lichens also grow slowly, so a crop you notice has been there a while. Clemson’s Home and Garden Information Center gets many calls from people who believe lichens are killing their trees and shrubs, and they are not. A crop on a tree that never carried one is the time to look at the tree itself:
“If lichens are found on a tree where they have never been seen before, investigate possible stress sources to the tree.” (Clemson Cooperative Extension, Gardening Myths: Lichens Kill Trees)
The stress may be too much or too little water or light, a tree planted too deep, or an insect or disease. Each has its own signs, from a crown dying back from the top to leaves scorched in a dry summer.
A sign of clean air
Lichens are extremely sensitive to air pollution such as sulfur dioxide, and are seldom found in industrial areas. Their absence is one measure of how polluted the air is, and plenty of them on the trunks is a sign of good air. Hummingbirds line their nests with strips of lichen, and the caterpillars of the painted lichen moth eat only lichen.
Leave it on the bark
Lichen and moss need no control, and no chemical is labeled for lichens. Scraping or spraying them off does nothing for the tree, and lichens come back unless the tree’s real trouble is put right. Once the tree’s health is restored, they gradually disappear.
Blue Whale Trees answers questions about a tree you already have through its support page. If lichen arrived along with thin leaves or dying limbs, say when the trouble began. A photo of the whole tree and a close-up of the bark can follow by email once we reply.
Common questions
Is lichen on a tree bad for it?
No: lichen takes nothing from the tree and does it no harm. A heavy crop on a tree that is also losing leaves or limbs points to some other trouble worth finding.
Should I remove moss from my tree trunk?
No: moss does the trunk no harm, and no control is needed. It grows wherever bark stays moist and shaded.
Why is there green stuff growing on my tree bark?
It is usually moss, green algae or a gray-green lichen, all harmless and living on the surface. On young, smooth bark, a green tint after rain can be the tree’s own green tissue showing through.
Sources
(5 references)
- Lichen, Algae, and Moss on Trees: University of Maryland Extension, Home and Garden Information Center (updated February 27, 2023): lichen as a fungus and alga, the alga’s photosynthesis and the fungus that keeps it from drying out, lichens on trunks, rocks and fences, the quotation, moss and algae on any moist, shady bark and not only the north side, living on bark without harm, no control needed, sensitivity to sulfur dioxide and good air, lichen rare on fast-growing trees that shed their bark, more lichen on slow or declining trees, and hummingbirds and the painted lichen moth.
- Gardening Myths: Lichens Kill Trees: Clemson Cooperative Extension, Home & Garden Information Center (2021), with its post Lichens and its factsheet Spanish Moss, Lichens & Slime Molds (HGIC 2354): lichens taking no nutrition from their host, lichens on rocks, more lichen on slow-growing trees such as oaks and on trees with a declining canopy that lets in sunlight, slow growth, the quotation, the stresses behind it, no chemicals labeled for lichens, the many calls from people who believe lichens kill trees, Spanish moss as an epiphyte that takes no water or nutrients and shades the lower limbs only in a heavy load, and lichens that disappear once a plant’s health is restored.
- Lichen Biology: US Forest Service: lichens taking water and nutrients from air and rain, and rhizines that hold a lichen down and move no water or nutrients.
- Lichens: Iowa State University Extension and Outreach, Yard and Garden (Paula Flynn, reviewed 2024): many lichens growing fastest in full sunlight, which puts them on dead or dying trees, and their absence as a measure of air pollution.
- Writers Guild Article: The Wild World of Moss: University of Minnesota Extension: moss as a plant without true roots, held to its surface by rhizoids.
