The concern about whether moss is killing a tree is common for many homeowners. The simple answer is no: the green, velvety growth on the bark is not a threat to the tree’s survival. Moss is a non-flowering plant that grows on the bark, using the surface merely as an anchor. Understanding this relationship helps determine if any action is needed.
Moss and Lichen: Understanding the Difference
Many people confuse moss with lichen, but they are biologically distinct organisms. Moss is a simple, non-vascular plant with tiny stems and leaves, lacking the internal system to transport water and nutrients. It appears as a soft, dense, green mat that needs a consistently moist environment to thrive.
Lichen is a symbiotic partnership between a fungus and an alga or cyanobacterium. Lichens come in a wider variety of colors, often gray, yellow, or pale green, and can have crusty, leafy, or hair-like textures. Neither moss nor lichen penetrates the tree’s tissue or draws nourishment from it.
The Biological Truth: Why Moss Is Not a Parasite
Both moss and lichen are classified as epiphytes, meaning they grow upon another plant for physical support. Epiphytes are entirely self-sufficient, relying on the environment for sustenance. They absorb moisture and nutrients from the air, rain, fog, and debris that collects on the bark surface.
The small, hair-like structures moss uses to attach to the bark are called rhizoids. These function only as anchors and do not penetrate the tree’s vascular system. Since the bark is dead tissue, moss cannot extract resources. This relationship is a form of commensalism, where the moss benefits while the tree is neither helped nor harmed.
Heavy moss often indicates a tree that is growing slowly or is in an environment with high moisture and shade. Moss is opportunistic; it colonizes surfaces where the canopy is dense and air circulation is poor. A thick covering of moss is generally a symptom of reduced tree health or specific site conditions, not the cause of its decline. Removing the moss without addressing underlying issues, such as poor drainage, will not improve the tree’s long-term health.
When to Consider Removing the Growth
While moss is generally harmless, there are limited exceptions where its presence can create an issue. Excessive, dense growth on small or weak limbs may hold too much water during a rainstorm. This added weight increases the strain on branches, potentially leading to breakage during high winds or winter ice events.
Removal may also be considered if the moss is so thick that it covers new growth buds. This dense layer can interfere with the tree’s ability to produce new shoots or hinder visual inspection for pests and diseases. In these rare cases, improving air circulation by selectively pruning the tree’s canopy is often a more effective solution than directly removing the moss.