Moss grows on trees and is a common sight in humid, temperate environments. Moss is a non-vascular plant, meaning it lacks the internal structures (xylem and phloem) that higher plants use to transport water and nutrients. Instead, this primitive plant absorbs everything it needs directly through its external surfaces, requiring it to live in consistently damp locations. The presence of moss on a trunk is generally an indicator of a thriving, moist microclimate rather than a sign of a sick or dying tree.
Moss is an Epiphyte, Not a Parasite
The relationship between moss and the tree is best described as an epiphyte, a plant that grows upon another merely for physical support. Unlike a parasitic plant, moss does not draw water or nutrients from the host’s vascular system. Mosses use the tree’s bark for height and anchorage, utilizing simple, root-like structures called rhizoids instead of true roots that penetrate tissue. This means the tree is not actively harmed or drained of sustenance by the moss itself.
Moss obtains all moisture and mineral nutrients directly from the environment, absorbing them from rainfall, dew, and atmospheric dust. The debris and organic matter accumulating on the bark also provide decaying nutrients. While moss is not a parasite, a very dense layer can hold excessive moisture against the bark, which may concern trees already weakened by other factors. A thick mat of moss on branches could also contribute to minor limb breakage during heavy ice or snow storms by adding extra weight.
Why Moss Prefers Certain Locations
The location of moss on a tree trunk is determined by the physical microclimate surrounding the tree. Moss requires consistent dampness to survive and reproduce, making shade the most important factor. In the Northern Hemisphere, the north side of a tree receives the least direct sunlight, allowing the bark to stay cooler and retain moisture longer.
While moss growth is often more prevalent on the northern side, this is not an absolute rule. Any part of the trunk consistently shaded—such as a side facing a dense forest, a hill, or a large building—can create the perfect humid microclimate. The distribution of moss is an indicator of local light and moisture conditions, not a directional guide. Improving air circulation and increasing sunlight penetration can naturally reduce moss growth by drying out the bark.
Differentiating Moss from Lichen
People often confuse moss with lichen, which also grows on tree trunks and branches. Moss is a soft, three-dimensional plant that grows in dense, cushion-like mats of tiny green stems and leaves. It is a member of the bryophyte division and is capable of photosynthesis. Its texture is typically velvety or spongy, feeling wet to the touch after rain.
Lichen, conversely, is a symbiotic partnership—a composite organism made up of a fungus and an alga or cyanobacterium. Lichens do not have stems or leaves, but instead grow as a flat, crusty layer (crustose), a leaf-like flap (foliose), or a shrubby, branching growth (fruticose). They appear in a wider range of colors, including gray, yellow, white, and orange, and their appearance is often dry or brittle. Like moss, lichen is an epiphyte and poses no threat to the health of the tree.