Mosses are small, non-vascular plants belonging to the division Bryophyta. They lack true roots, stems, and internal water-transporting tissues. Their presence on the bark of trees is a common natural occurrence, particularly in environments with high moisture levels. Mosses grow on trees not to seek resources, but because the tree provides a stable surface. Understanding the specific environmental needs of these organisms explains why they colonize certain trees and not others.
Defining the Relationship Between Moss and Trees
Moss on a tree is generally harmless because the relationship is non-parasitic, meaning the moss does not draw nutrients or water from the tree itself. Mosses are classified as epiphytes, organisms that grow upon another plant only for physical support. They absorb all necessary water and minerals directly from the atmosphere, primarily through rainfall and the air, using specialized structures called rhizoids only for anchoring to the bark.
The appearance of moss does not indicate a tree is dying, but it can suggest an underlying environmental condition that favors the moss. Heavy moss growth can retain excess moisture against the bark, which may occasionally promote fungal or bacterial growth in certain species. Very dense moss may also slightly conceal pests or diseases already affecting the tree, but the moss itself is not the cause of the decline. In rare cases, excessive moss on branches could potentially shade lower leaves, intercepting light needed for photosynthesis.
Key Environmental Conditions That Encourage Growth
The primary cause of moss growth is the presence of specific microclimates that satisfy the unique requirements of bryophytes. Mosses must have consistent moisture because they are non-vascular and lack the internal structure to efficiently transport water throughout their bodies. They absorb water directly through their simple, single-cell-thick leaves from the surrounding environment.
Shade and poor air circulation contribute significantly to maintaining the necessary damp environment for moss colonization. Moss cannot tolerate direct, intense sunlight because it causes the plant to dry out too quickly, hindering both growth and reproduction. The dense canopy and lower trunk areas of a tree often provide the required deep shade, especially on the north-facing side in the Northern Hemisphere where sunlight is minimal.
Furthermore, the texture of the tree bark plays a role in establishment. Rough, fissured bark provides an ideal, stable substrate for the moss rhizoids to take hold. The presence of moss often acts as an indicator that a tree is slow-growing or mature, as the undisturbed, stable surface of older bark allows the moss colonies time to develop into thick mats. The three most influential factors remain moisture, light availability, and the chemical nature of the bark.
Practical Guidance for Management
Since moss thrives in damp, shaded conditions, management strategies should focus on altering the environment to make it less hospitable. Improving light penetration and air circulation through selective pruning is the most effective long-term solution. Removing overcrowded branches and any overhanging vegetation increases sunlight and airflow to the trunk and limbs, helping the bark dry out more quickly.
Physical removal of moss is necessary for young, vulnerable trees or when the growth is extremely heavy. A soft brush can be used to gently scrub the moss off the bark without causing damage to the tree’s surface. Alternatively, a very low-pressure washer can be used carefully to dislodge extensive growth.
A mild mixture of baking soda and water can be sprayed onto the affected areas to create an unfavorable growing environment for the moss. However, focusing on correcting the underlying environmental factors—excessive shade and poor air movement—is the most sustainable way to prevent the moss from returning.