Frequently I get inquiries about whether the light green lichens growing on trees and shrubs in our area is a sign of plant decline or if they are killing the plants they reside on.
The answer to the first question is: Possibly
Lichens area slow growing organism that require a static surface to establish themselves on. Often found growing on rocks, decks, undisturbed soil and anything else inanimate and outdoors around here. Plants and trees that are slow-growing by nature and are also in a chronically moist and shaded environment will most likely be colonized by lichens. In this case, there is nothing to worry about in terms of the health of the plant. If the presence of lichen is unacceptable on these otherwise healthy plants, pruning to improve sunlight penetration and air-flow will help discourage lichen colonization.
Conversely, if you are looking at a young, normally fast-growing plant or tree that is being colonized by lichen, it is possible that the health of that plant was previously compromised by some other cause, whether poor soil conditions, lack of nutrients/water or mechanical damage to roots or stem. This compromised plant would make for a more static surface for lichen to grow on. The focus should then be to address the lack of vigor of the plant itself.
To the second question:
Most importantly, rest assured that lichens are not parasitic or invasive to their host plant. Although I do believe that a heavily colonized plant that is in decline would benefit from lichen removal to give every chance for sunlight to reach newly emerging leaf or needle buds, provided all other nutrient and environmental corrections are addressed.
Arborist’s recommendations for removing lichen include hand pulling (gentlest method), scrubbing gently with a soft bristle brush or using a garden hose with a focused nozzle. Power washing is generally not recommended for obvious reasons but I think with the right nozzle and low pressure, one could carefully scour off lichen without bark damage. Lichen is best removed in the dormant season when the risk of damaging tender buds and leaves is at its lowest.
A good article on lichens and air quality:
Wolberts Plant Essentials
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