Algae, lichens, and mosses
Bark of woody plants can host algae, lichens, mosses, or a combination of these. These growths can be increasingly common as trees and shrubs age.
Gray, green, orangish, or whitish tissue growing on or hanging from bark is commonly an algae, lichen, or moss. Heavy growth is common on older trees in part because the more rapidly expanding bark on younger trees spreads and “dilutes” the appearance of these surface growths. These growths can also occur on moist, shady locations such as on decks, pavement, and roofs.
Many of these plants are epiphytes, green plants that derive their nutrients and moisture from the air and grow on the host surface only for support. Algae and mosses are relatively simple (primitive) green plants. Lichens are an association between certain algae and fungi. Some are saprophytes, absorbing nutrients from soil and dead organic debris that lodge in bark crevices and minimally consuming the outer, dead bark surface.
These organisms are generally harmless to trees. Some people consider epiphytes and saprophytes in landscapes to be interesting and desirable. However, profuse epiphytes or saprophytes on bark may indicate that a host plant is growing more slowly than is desirable for its long-term health and survival, and that the host could benefit by improving the growing environment and cultural care practices.
For example, abundant algae and moss on plants sometimes indicate that soil drains poorly, landscapes are being overwatered, or both. These poor growing conditions can seriously damage plants. Algae and mosses on sidewalks or other hard surfaces make them slippery, perhaps unsightly, and unsafe for walking. These situations may warrant management action.
Algae, lichens, and mosses on bark are generally innocuous and no control is needed to protect plant health.
Since algae and moss thrive under damp conditions, to potentially reduce their growth on bark
- Reduce humidity, such as prevent sprinklers from hitting bark.
- Improve air circulation around host plants, such as by removing or trimming some branches.
To reduce growth on soil
- Avoid or reduce the use of quick-release nitrogen fertilizer.
- Improve soil drainage, such as incorporate sand.
- Increase the frequency between irrigations to the extent compatible with healthy plant growth.
Where growth on hardscapes is a problem
- Minimize the amount of eroding soil and water flowing over concrete or other impermeable surfaces. For example, redirect downspout water away from hardscapes.
Moss prefers partly shaded locations, so its growth may be reduced by increasing light around plants, such as by thinning branches and nearby plants. However, increased light can also stimulate growth of lichens and sometimes algae.
Herbicidal soaps developed specifically to control algae, lichens, and mosses can be applied where these growths are not tolerable or are creating a slipping hazard. Certain copper sprays, iron HEDTA, or certain other algaecides/herbicides (pesticides) may be applied to provide control according to product labels. Keep these pesticides out of waterways and away from desirable, succulent plant parts, which they may damage.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Algae on a tree trunk.
Moss on a tree limb.
Lichens on a tree trunk.
Lichens on bark close-up.