Moss and lichen on trees: harmless, or a sign of trouble?

Is moss bad for trees?

No. The Pacific Northwest Plant Disease Management Handbook, in an article by two Washington State University plant pathologists, says algae, lichens and mosses all contain chlorophyll and make their own food, so they do not directly injure the plants they grow on. Botanists call them epiphytes: organisms that use another plant as a perch.

Lichens get the same verdict. Oregon State University botanist Bruce McCune, writing in the same handbook, notes that lichens extract nothing from their woody host and simply use it as a growing surface. Most mosses take their water and nutrients from dust and rainwater, not from what they grow on.

The handbook’s one caveat is that heavy growth can have drawbacks for a homeowner, mainly that some people find it unsightly. The cases where it can matter for the tree, from hidden decay to waterlogged limbs, are covered further down.

Why there’s so much moss on trees west of the Cascades

Water. The handbook article puts it plainly: the cool, cloudy, wet weather of western Washington and Oregon is ideal for these organisms. NOAA’s 1991–2020 climate normals show the gap across the mountains. Olympia averages 50.62 inches of precipitation a year and Corvallis 43.04; on the dry side of the Cascades, Yakima averages 8.01 and Kennewick 7.87.

Timing matters as much as the total. McCune notes that Northwest mosses flourish in winter and spring, when water is plentiful, light is low to moderate and temperatures are mild, and that most mosses can dry out completely in summer and pick up again when the rain returns. In the same NOAA normals, Olympia gets 43.37 of its 50.62 inches from October through April and Seattle 32.83 of 39.34. Yakima’s October-to-April total is 6.13 inches.

At the extreme, the Forest Service’s silvics manual describes bigleaf maples on the west side of the Olympic Peninsula carrying epiphytes that weigh nearly four times as much as the trees’ leaves. The same manual lists mosses, lichens and licorice fern among the common epiphytes on the species; more on the tree at bigleaf maple.

Moss vs. lichen on trees: how to tell them apart

People use the words interchangeably, but they are different organisms that happen to share the bark.

Sources: PNW Plant Disease Management Handbook (Davison and Byther; McCune and Whitbeck); USDA Forest Service lichen pages; checked 2026-10-02.

A third tenant, algae, often coats bark too. The handbook describes algae as the simplest green plants, present almost anywhere with enough moisture, and treats all three the same way.

Lichens on trees and air quality

Because lichens take their water and nutrients from the air and rain, they also take in what is in the air. The Forest Service’s lichen pages note that heavy metals, sulfur and other atmospheric pollutants are absorbed into the lichen body, where scientists can measure them, and that very few lichens survive near factories, highways and other pollution sources.

The agency keeps a National Lichens and Air Quality Database because lichen surveys help land managers detect, map and track the effects of air pollutants. It describes one species, Usnea longissima, as a strong indicator species of air pollution. McCune adds that mosses work the same way: relying on the atmosphere for water and nutrients makes them susceptible to most forms of air pollution.

Tip: A trunk crusted with several kinds of lichen is not a problem for the tree. It is one of the things scientists read when they check how clean the local air is.

When moss or lichen is a sign of trouble

The growth itself is not the problem, but it can point to one or hide one. Read it alongside the rest of the tree.

Sources: PNW Plant Disease Management Handbook; USDA Forest Service urban tree risk manual and silvics manual; checked 2026-10-02.

A thinning crown under the lichen. The WSU authors note that algae, lichens and moss often become established on slow-growing plants. So when lichen is spreading over a tree that is also leafing out poorly, look at the tree, not the lichen. ISA’s mature-tree checklist names four vitality signs: new leaf or bud formation, leaf size, twig growth and no crown dieback. If those look bad, how to tell if a tree is dead walks through the next checks.

Decay hidden under moss. The Forest Service’s urban tree risk manual lists the few outward clues to decay: rotten wood, cavities, cracks, bulges and fungal fruiting bodies. A thick moss mat can cover any of them, especially at the root flare. If you find mushrooms or conks, mushrooms on trees explains what they mean.

Weight on the wettest sites. The silvics manual notes that some Olympic Peninsula bigleaf maples heavily laden with rain-soaked epiphytes are more prone to windthrow than maples with less growth. That is a rain-forest extreme, but a long, moss-heavy limb over a roof is worth an arborist‘s look before the winter storms.

How to get rid of lichen and moss on trees, and why you usually shouldn’t

You don’t need to remove it for the tree’s sake. If you want less of it, change the conditions: the handbook recommends opening up the canopy for better air circulation and light, pruning shrubs and overstory trees where needed, and keeping plants vigorous. In a climate as wet as the west side, it adds, those steps will reduce moss and lichen but probably not eliminate them.

Don’t scrape or pressure-wash bark. The Forest Service’s tree risk manual puts it simply: wounds start the decay process. Bark you gouge or blast off to clean away moss is a wound, traded for a cosmetic gain that the rain will undo.

If you still want it gone for looks, the handbook describes dormant-season sprays of copper-based fungicides or lime sulfur, used only on plants listed on the product label; the label is the law. These sprays can injure leaves and nearby evergreens, lime sulfur can stain painted surfaces, and the dead moss and lichen stay attached and visible until weather and growth slough them off. None of it does anything for the tree’s health.

Moss on the roof under the trees

Here the trees are part of the cause. McCune notes that a rooftop shaded by trees stays moist long enough to be ideal moss habitat, that wood and composite shingles suit moss while slick metal roofs are harder to colonize, and that the idea of moss making roofs leak is overblown: moss does not pry shingles apart, though plants that root in thick moss mats can damage the roof.

For the roof itself, the University of Washington’s Miller Library calls physical removal the most environmentally sound method, and warns that runoff from zinc strips should not drain onto plants, ponds or water supplies because some zinc contamination is likely. For the trees, thinning the limbs over the roof lets light and air reach it; the methods are on crown thinning and reduction, and timing is on best time to trim trees in the Pacific Northwest.