06/20/2026
Usnea is my favorite lichen!
Most people walk past lichen every single day without giving it a second thought, assuming it is some kind of moss or crusty plant growth on rocks and tree bark. The reality is considerably more interesting than that, and understanding what lichen actually is changes how you look at every tree branch, stone wall, and garden rock you have ever noticed it on.
Lichen is not a plant. It is not a fungus. It is not algae. It is all of those things simultaneously, living in such intimate partnership that they function as a single organism with its own structure, its own growth form, and its own ecological identity. The fungus provides the physical body of the lichen, creating the structure, attachment, and protection. Algae or cyanobacteria living within that fungal tissue perform photosynthesis, producing sugars that fuel the entire partnership. Neither organism could survive in the same way independently. Together they can colonize bare rock in Antarctica, volcanic lava fields, and desert boulders, in environments where almost nothing else can gain a foothold.
This photo shows three of the four main lichen growth forms beautifully on a single branch.
Leprose
The powdery, paint-like coating on the left side of the branch is leprose lichen. It genuinely does look as though someone applied a wash of pale gray-green pigment directly to the bark. Leprose lichens lack the organized internal structure of other growth forms, appearing instead as a granular or powdery layer that is essentially impossible to remove without taking bark with it. They are among the most common lichens on tree bark and are entirely harmless to the trees they colonize. They are using the bark surface as an anchor, not as a food source.
Foliose
The lobed, leafy growth on the right is a foliose lichen, and it is the form most people find most visually striking up close. The ruffled, rounded lobes genuinely do resemble small leaves, and the upper and lower surfaces are distinct from each other, a characteristic that separates foliose lichens from the more tightly fused crustose forms. Foliose lichens attach to their substrate more loosely than crustose types and can sometimes be gently peeled away without damaging the surface beneath. Many foliose lichens are sensitive to air quality and their presence on urban trees is considered a reliable indicator of relatively clean air. According to the United States Forest Service, foliose lichen diversity is used by ecologists as a bioindicator when assessing air quality and ecosystem health in forested areas.
Fruticose
The hanging, branching growth dangling from the underside of the branch is a fruticose lichen, and it is the most three-dimensional of all the growth forms. Rather than lying flat against a surface, fruticose lichens grow outward in all directions, forming upright tufts, pendulous hanging structures, or miniature shrub-like colonies. The one in this photo resembles a small cascade of pale green threads, and in a forest setting these hanging lichens can drape entire branches in a way that transforms the visual character of the landscape entirely. Old man's beard, the common name for several Usnea species, is one of the most recognizable fruticose lichens in North America and has been used by Indigenous peoples across many regions as emergency tinder, wound dressing, and even insulation material. According to ethnobotanical research compiled by the Native American Ethnobotany Database, Usnea species had documented uses among dozens of North American Indigenous cultures.
Why Lichen on Your Trees Is Not a Problem
This is the question gardeners ask most often when they notice lichen on their fruit trees, ornamentals, or garden shrubs. The short answer is that lichen is not harming your tree. It does not pe*****te bark, does not extract nutrients from the tree, and does not weaken branches. It is simply using the bark surface as a stable platform to anchor itself and access light.
The correlation people notice between lichen-covered branches and declining trees runs in the opposite causal direction from what they assume. Declining trees grow more slowly, which means their bark surface changes more slowly, which makes it a more stable substrate for slow-growing lichens to colonize. The lichen did not cause the decline. The decline created conditions that allowed lichen to establish. According to the Penn State Extension, removing lichen from tree branches provides no benefit to the tree and is entirely unnecessary from a plant health standpoint.
Finding lichen in your garden is genuinely something to appreciate. It means your air is clean enough to support them, your garden has the kind of stable, undisturbed surfaces they need to establish, and you are sharing your space with one of the most ancient and ecologically fascinating organisms on the planet. 🌿
