Is Lichen on Tree Bark Harming Your Tree?
No, lichen does not harm your tree. Penn State Extension's Allegheny County Master Gardeners explain that lichens have no roots, are not connected to the tree's vascular system, and take no water or nutrients from the tree, and the University of Maryland Extension says lichens, algae, and mosses grow harmlessly on tree trunks and no control is necessary. The gray-green crust or leafy patches on your maple or oak are using the bark as a place to live, nothing more.
Heavy lichen can still tell you something useful about the tree above it. Rutgers Cooperative Extension says the main reason lichens are more common on trees and branches with reduced foliage is the extra sunlight that reaches the bark, and Penn State's Center for Private Forests makes the same point about struggling trees. That makes a lichen-covered trunk a reason to look up at the canopy, not to scrape the bark, which the Allegheny County Master Gardeners warn can permanently damage it. This post explains what lichen is, why it spreads, and what is worth checking instead.
What Is Lichen?
A lichen is a combination of organisms living as one: Penn State's Allegheny County Master Gardeners describe it as a symbiotic partnership between a fungus and an alga or a specialized bacterium. The fungus provides structure, and its partner makes food from sunlight. Rutgers notes that lichens produce their own food using sunlight energy and do not feed on the tree bark.
Rutgers and the University of Maryland Extension group lichens into three growth forms:
Crustose: flat, crust-like patches pressed tight to the bark
Foliose: leaf-like lobes that lift slightly at the edges
Fruticose: tube-like or beard-like strands
Colors run from gray-green to bright orange-red, according to the University of Maryland Extension. One of the most common in our area is common greenshield lichen, which the Allegheny County Master Gardeners describe as one of the most widespread lichens in eastern North America, pale yellow-green with concentric circular patterns.
Does Lichen Damage Bark or Kill Trees?
The research answer is consistent across extension services. Rutgers says the structures lichens use to attach typically do not penetrate deep enough into the inner bark to cause harm. University of Maryland Extension states that lichens are often blamed for killing a tree or shrub, and that this is not true.
The confusion comes from where lichens show up. The Allegheny County Master Gardeners note that lichens prefer older trees with ridged bark or trees that are stressed or declining from poor drainage, too much or too little water, insects or disease, and then add that lichens are never the cause of that decline. A homeowner sees a dying branch covered in lichen and connects the two, when both are responding to something else.
Why Is Lichen Spreading on My Tree?
Two explanations cover most cases.
More light is reaching the bark
Lichens need sunlight. When a tree loses leaves or branches, more light hits the trunk and limbs, and lichens grow faster there. Rutgers calls this the primary reason lichens are more common on trees with reduced or partial foliage, and Penn State's Center for Private Forests explains that lichens become more visible on struggling trees because thinning foliage lets in more sunlight.
The tree is growing slowly
University of Maryland Extension points out that a mature, declining or less vigorous tree may carry more lichen because of its slower growth rate. Older trees with deeply ridged bark also give lichens more places to settle, according to the Allegheny County Master Gardeners.
Neither explanation means the lichen is doing harm, and neither is a reason to reach for a scraper or a spray. Both mean the tree's condition deserves a second look if the lichen is new or spreading fast.
Is Lichen a Sign of Clean Air?
Often, yes. University of Maryland Extension calls lichens an indicator of good air quality and says they are extremely sensitive to sulfur dioxide. The Allegheny County Master Gardeners add that their presence is a sign the local environment is clean enough to support them, which is one reason they are less common in urban areas.
Penn State's Center for Private Forests notes that lichens reestablished after the Clean Air Act, and that ash trees alone host more than 500 different lichen species. For homeowners in Westmoreland and Allegheny Counties, lichen on the trees is a reasonable sign of healthy air rather than a problem to fix.
Should You Remove Lichen From a Tree?
No. University of Maryland Extension says no control is necessary, and the Allegheny County Master Gardeners warn that attempts to remove lichen risk permanent damage to the bark. Scraping, pressure washing or brushing a trunk can injure the bark, and the lichen was never taking anything from the tree in the first place.
If lichen on a small ornamental bothers you, pruning to open the canopy will not reduce it, because more light usually helps lichen grow. Leaving it alone is the right call for the tree.
What Should You Check on the Tree Instead?
Because lichen tends to thrive where foliage is thin, use a heavy or fast-spreading patch as a prompt to inspect the whole tree. Things worth checking:
Dead or leafless branches in the upper crown
Leaves that are smaller, fewer, or dropping early compared with past years
Shelf-like fungi or mushrooms growing from the trunk or roots, which are a separate issue from lichen
Cracks, cavities, or soft wood in the trunk
Our guides to signs your tree is dying, fungus types that commonly affect trees in PA and signs a tree is rotting inside cover each of these in more detail. If you are not sure how often to take a close look, how often trees should be inspected in PA gives a schedule.
FAQ
Is lichen a fungus?
Partly. Penn State's Allegheny County Master Gardeners describe lichen as a partnership between a fungus and an alga or specialized bacterium, which makes it a different organism from the decay fungi that rot wood.
Does lichen mean my tree is dying?
Not by itself. Lichen grows on healthy trees too. Heavy growth on a tree with a thin crown is a reason to check the tree's health, since Rutgers notes lichens thrive where reduced foliage lets in more sunlight.
Is moss on a tree trunk harmful?
University of Maryland Extension says lichen, algae, and mosses grow harmlessly on tree trunks and no control is necessary.
Can I spray something to kill lichen?
There is no need. University of Maryland Extension says no control is necessary, and removal attempts can damage the bark, according to the Allegheny County Master Gardeners.
Why is lichen on my dead branches but not the living ones?
Dead branches have no leaves shading them, so more sunlight reaches the bark. Rutgers identifies that extra light as the main reason lichens are more common on branches with reduced foliage.
Is orange lichen different from green lichen?
Only in species. University of Maryland Extension notes lichen colors range from gray-green to bright orange-red, and none of them feed on the tree.
Tree Inspections, Pruning and Removal From Brinko Tree Service
Brinko Tree Service is based at 1010 Sunset Blvd in Harrison City and brings nearly 20 years of experience to homeowners and businesses across Westmoreland and Allegheny Counties. If lichen led you to spot dead branches or a thinning crown, the team offers pruning and trimming to remove dead or hazardous limbs and tree removal when a tree is beyond saving, followed by stump grinding and cleanup. Brinko is fully licensed and insured, follows OSHA and ANSI safety regulations with an OSHA safety site supervisor on every job site, and offers free estimates.
Visit Brinko Tree Service or call 412-373-7450 today. Contact us for more information.
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