Do Lichens Harm Trees? What Extension Sources Say

ByKen

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Do lichens harm trees? The extension sources checked for this article say no. Lichens are the greenish or gray, frilly growths that often appear on bark, and they are frequently blamed when a tree starts to look unhealthy. The university and extension pages reviewed here describe lichens as harmless to trees. They explain how lichens get food and water, why they tend to appear on slow-growing or declining trees, and which problems the sources point to instead. This article answers the question directly and explains the mechanism behind it.

Quick answer

  • The sources checked say lichens do not kill trees and do not cause tree disease.
  • A lichen makes its own food through photosynthesis and does not take nutrition from the tree it grows on.
  • Lichens look most noticeable on trees that are already slow-growing or declining, because the slow growth gives lichens time to build up.
  • The decline itself is attributed to other stresses, such as light, water, planting depth, insects, or disease.

What a lichen is

A lichen is not a single organism. The Clemson Home and Garden Information Center describes lichens as made up of two different types of organisms, a fungus and an alga or cyanobacterium, that form a symbiotic relationship in which both partners benefit. The fungus provides structure and protection from the elements, while the photosynthetic partner produces food. Iowa State University Extension makes the same point from a different angle: the two components exist together and behave as a single organism. Because of this partnership, the growth that people see on bark is one combined body rather than a plant attached to a tree.

The University of Maryland Extension describes lichens as non-destructive, living organisms made of a fungus and algae living in a symbiotic relationship. In that relationship, the algae derive nutrients through photosynthesis, and the fungi protect the algae from drying out. The same source notes that this arrangement lets lichens live and grow in extremely barren areas. Lichens are therefore not limited to trees. They also grow on rocks, soil, and other surfaces, which matters for the question of whether a tree is their source of food.

How lichens get food and water

The photosynthetic partner does the food-making work. Iowa State University Extension explains that the alga, because it is a green plant, can photosynthesize and provide energy for the lichen. The fungus contributes by obtaining water and minerals and by protecting the algal cells from desiccation. Clemson adds the key point for tree owners: since the photobiont photosynthesizes like a plant, lichens do not obtain their nutrition from their host. Clemson contrasts lichens with plant parasites such as mistletoe, which do need a living host for nutrients.

Penn State Extension describes a different structural limit. Lichens do not have roots, stems, leaves, or any sort of vascular system to move nutrients and water. They rely on rain and the air in their environment to get the water and nutrients they need. Because of that design, a lichen on bark is not drawing water or food from the tree’s living tissue. The same source notes that lichens cannot conserve water during dry periods, so they become dormant when the weather is dry and resume when moisture returns.

The University of Maryland Extension explains how lichens handle dry spells in a way that is visible to anyone looking at them. During dry spells, lichens shrivel and their surfaces become opaque and faded. This protects the internal, photosynthesizing algae from drying out. When re-moistened, lichens expand and their surfaces become transparent again, so light and moisture can reach the algae and photosynthesis resumes.

How lichens arrive on a tree

Lichens spread through reproductive structures that wind carries. Clemson states that the wind disseminates these structures and that even a slight breeze can disperse them for miles. Once a fragment lands on a suitable surface, such as a tree, a large rock, or a brick wall, it begins to grow. The same source says that lichens prefer a surface that does not change much over time. This is why Clemson sees lichens more often on slower-growing tree species like oaks or pecans than on faster-growing pines.

The University of Maryland Extension makes a related observation about bark. Lichen rarely develops on rapidly growing trees, probably because the bark is shed before the lichen has time to spread.

Why lichens show up on weak trees

Lichens are easy to notice on trees that are already struggling, and that timing is a large part of why they get blamed. Clemson says lichens are more noticeable on stressed trees that grow slower than normal or have a declining canopy, because the thinner canopy lets more sunlight reach the organism. The University of Maryland Extension says there may be more lichen on a mature, declining, or less vigorous tree, due to a slower growth rate.

Both sources also make clear that the lichen is a symptom rather than the cause. The University of Maryland Extension states that lichen themselves do not cause the decline. Clemson lists the possible stresses behind a declining tree: too much or not enough light or water, improper planting depth, or an attack by an insect or disease. Clemson also notes that lichens grow slowly, so by the time the growth is noticed on a branch, it has usually been present for a while. Lichens often go unnoticed until a tree starts looking bad.

Penn State Extension states the same conclusion in its own words. When the way lichens get nutrients and water is considered, they pose no danger to the trees on which they grow. It also notes that lichens are oftentimes simply not noticed until the tree appears unhealthy.

What lichens do not do to a tree

Several sources address the common belief that lichens kill trees directly. The University of Maryland Extension says lichen are often blamed for killing a tree or shrub, but that this is not true. It also says lichen, algae, and mosses grow harmlessly on tree trunks and that no control is necessary. Iowa State University Extension states that these organisms do not cause disease problems. It describes them as living and gathering sunlight on twigs or branches without infecting the tree.

Clemson offers a simple test of the idea. Lichens also grow on rocks, and a rock provides no nutrition to gain. That observation is hard to square with the idea that lichens are feeding on the tree. Clemson concludes that lichens are not actually harming the trees on which they are growing, and it describes the belief as one of the gardening myths in its title.

Lichens can even be useful as a signal about the air. Iowa State University Extension notes that lichens are extremely sensitive to air pollution, so their absence can be used as a measure of how polluted an area is. The University of Maryland Extension notes that lichen absorb nutrients from the air. This is a different use from the tree question, but it shows why lichens are often studied as indicators rather than as pests.

How to read a lichen on a tree

The practical reading from these sources is that a lichen on a tree is a sign of the surface and the growth rate, not a cause of decline. The University of Maryland Extension says lichen rarely develops on rapidly growing trees, and Clemson ties more lichen to slow growth and declining canopies. The sources point to the stresses that slow growth in the first place as the thing to check. For pruning or mulching decisions, the related guides on how to prune a tree branch and how to mulch trees cover the tree-care side of that question.

The sources do not suggest that lichen should be scraped, sprayed, or removed from a healthy tree. The University of Maryland Extension says that no control is necessary for lichen, algae, and mosses growing on tree trunks. A tree that is declining should be checked for the stresses Clemson lists, rather than being treated for the lichen alone.

How we chose

We chose four university and extension pages that address lichens on trees directly. Each page was opened on 2026-10-27 with a direct HTTP 200 response. None of the sources is a retail page, and none of them is a product guide. The sources were chosen because each one covers a different part of the question: what lichens are, how they get food and water, why they show up on weaker trees, and whether they harm the tree. The verbatim quotes behind each claim are kept in the research file so the paraphrases can be checked.

What we could not verify

The checked sources answer the core question, but they leave several details open. They do not give a single threshold for how much lichen cover is too much on a tree, and they do not compare lichen levels across all tree species. Those gaps are noted here rather than filled with estimates.

What the sources do not say

The sources do not give a measured growth rate for lichen on any particular tree. They do not provide a removal method for lichen on a declining tree, because they say lichen is not the problem. They also do not explain the full chemistry of the symbiosis beyond the food and water exchange described above.

Frequently asked questions

Do lichens kill trees?

The University of Maryland Extension says lichen are often blamed for killing a tree or shrub, but this is not true. The source describes lichen as growing harmlessly on tree trunks.

Do lichens cause tree disease?

Iowa State University Extension states that lichens do not cause disease problems. It says they live on twigs or branches without infecting the tree.

Should I remove lichen from a tree?

The University of Maryland Extension says no control is necessary for lichen, algae, and mosses growing on tree trunks. Penn State Extension says that, given how lichens get nutrients and water, they pose no danger to the tree.

Why do lichens grow on some trees and not others?

Clemson says lichens prefer surfaces that do not change much over time, which is why they are more common on slower-growing species such as oaks or pecans. The University of Maryland Extension says lichen rarely develops on rapidly growing trees, probably because the bark is shed before the lichen can spread.

Are lichens parasites?

Clemson says lichens do not obtain their nutrition from their host, which sets them apart from plant parasites such as mistletoe. Iowa State University Extension describes lichen as a symbiotic partnership between a fungus and an alga.

Sources

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