If you’ve noticed pale green, grey, or even orange patches on your tree’s bark, you’re probably looking at tree lichen. It can show up on trunks, branches, fence rails, and rocks, so it’s easy to wonder if it’s a sign something is wrong with your tree. In most cases, lichen is more of a clue about light and moisture than a sign of disease.
What Lichen Actually Is
Lichen isn’t a single organism. It’s a close partnership between a fungus and a photosynthetic partner, usually a green alga or a cyanobacterium. The fungus provides the structure and helps with moisture and nutrients, and the photosynthetic partner makes sugars using sunlight.
Does Lichen Harm Trees?
For healthy trees, lichen is generally harmless. Lichens growing on bark are not parasites and do not feed on the tree’s tissues. A common misconception is that lichen blocks the bark from breathing. Tree bark doesn’t function like lungs, and lichens don’t invade the tree’s vascular system. Additionally, lichens do not penetrate into the inner bark or take nutrients and water from the tree.
That being said, lichens can make early pest or damage signs easier to miss, especially small things like scale insects on twigs or fine cracks in bark, because the surface already looks busy. If anything seems off beyond the lichen, it’s worth a closer look.
Why Lichen Shows Up On Some Trees More Than Others
Lichens are picky about two things: light and periodic moisture. They tend to do well where air movement and sunlight hit the bark, and where there’s enough dew, fog, rain, or snowmelt to rehydrate them now and then. Their ability to tolerate drying is one reason they can thrive even in places with long dry stretches.
They also cycle between inactive and active states. When they dry out, they can essentially pause, then resume activity when they get wet again, with photosynthesis closely tied to hydration.
When Lichen And Tree Decline Show Up Together
Sometimes people notice lichen at the same time they notice a tree looking thin or tired, and it’s easy to connect the two. What’s usually happening is correlation, not cause. Lichens can be more noticeable on slow-growing branches, and they may be easier to spot when a canopy has thinned and more light reaches the trunk.
That’s why you can see plenty of lichen on a stressed tree and on a perfectly healthy one. Lichen often just happens to be there. If the tree is declining, it’s usually not the cause, it’s a sign something else is affecting the tree.
Should You Remove Lichen From Your Tree?
Most of the time, no. Trying to remove lichen by scraping, pressure washing, or with harsh products can do more harm than the lichen ever would, because you can tear or bruise the bark and create openings for real pests and pathogens.
If you really dislike the look, the better approach is to focus on the tree’s overall health: consistent watering during dry periods, keeping mowers and trimmers from damaging the trunk, and pruning deadwood properly so the tree is not carrying unnecessary stress. Lichen may persist anyway, and that’s fine.
What About Invasive Lichens?
In BC, researchers are tracking orange wall lichen (Xanthoria parietina), a bright yellow-orange species often seen on bark, rocks, and manmade surfaces in sunny, nitrogen-rich spots.
If you think you’ve found it, reporting it is usually more helpful than removing it. Take a clear photo and submit the location to the project, and skip scraping or pressure-washing, which can damage bark. If you prune lichen-covered branches, keep the prunings on-site or bag them for landfill.
When To Call An Arborist
Lichen usually isn’t the problem, but a few lookalikes can be. If you see any of the following, it’s worth getting an arborist’s opinion:
- Black, wipeable coating (sooty mould from sap-feeding insects)
- Many tiny shell-like bumps on twigs (scale insects)
- Cracked, sunken, or oozing bark, or fungal conks
These point to a real health or safety issue, not the lichen.
Summary
Lichen on bark is common and, in most cases, harmless. It’s not a sign your tree is infected, and it usually doesn’t need to be removed. If a tree is struggling, don’t blame the lichen. Use it as a reminder to check the basics like canopy health, deadwood, trunk condition, and overall stress, and bring in a trained set of eyes if anything looks off.
