The phone call often goes like this: “My tree is dying and it has this blueish green ‘stuff’ growing on the trunk and branches that is killing it! What can I spray it with?”
Indeed there are many trees suffering in central Indiana. Even if we rule out the plight of the ash tees, there are still many others struggling for one reason or another, including poor planting practices, excessive mulching, too much water and heavy soils, the residual effects of the 2012 drought and a variety of pest-related issues that may be specific to certain species.
And it is true that many of these declining trees (and many others) may well have blueish green “stuff” or “crust” growing on them.
In this case, though, the simple fact that something exists does not imply that it is the cause of injury any more than an innocent bystander should be held accountable for a crime.
What the caller sees growing on the trunk and limbs is often nothing more than a lichen — a harmless and interesting lifeform that is actually a variety of algae capable of photosynthesis and a (non-pathogenic) fungus. These two very different organisms have evolved to grow together in a form that benefits both and allows them the ability to survive where alone they could not.
The important thing to remember is that despite their exotic names, such as rock pimples, earth wrinkles, angel’s hair, freckle pelts, fog fingers, dragon’s funnel, tar-jelly and old man’s beard, lichens do not damage plants any more than the rocks that they can also be found growing on.
Lichens can appear in a variety of forms from the firmly-attached “crusty” forms often found on rocks, trees and sidewalks; the “scaly” shingle-like form; to those that look almost like paper thin blue green flattened “leaves,” which in some environments can be quite large.
Lichens are located on every continent on Earth, including both the Arctic and Antarctic. They survive in all climates and altitudes. In general, they need three things: undisturbed surfaces, time and clean air.
Lichens will make themselves at home on most any undisturbed surface. Bark, wood, mosses, rock, soil and peat are all natural substrates. They also can establish on glass, metal, plastic and cloth. Interestingly, most lichens are restricted to certain types of surfaces; lichens normally found on tree bark, for instance, are rarely found on rock, and vice-versa.
Lichens are early colonizers that re-establishes life on rock and disturbed sites. They play an important role in soil formation over much of the earth. When they die, they contribute decayed organic matter to the area they inhabited which enables mosses and seeds from vascular plants to begin developing among the pockets of new soil.
Hummingbirds and many other bird species regularly use lichen as their preferred nesting material.
But let’s get back to that tree dying in the landscape. Is there even a remote chance that the lichen is harming it?
The short answer is no — lichens do not cause plant damage. The lichen is not damaging bark in any direct way. It doesn’t rob bark of significant amounts of moisture, does not damage living plant cells, and lichens do not appear to be associated with providing entranceways for pathogens into plant tissue.
So why do so many people, including many horticulturists, think lichens damage plants?
I think we call this guilt by association.
Perhaps it is because when branches begin to dip toward the ground, the amount of sunlight penetration increases and lichen growth proliferates. The lichens did not cause the branch decline but rather one of the effects of the plant decline was an increase in lichen growth.
Discovering a lichen growing on your tree is not a bad thing. In fact, it should be celebrated by giving you peace of mind knowing the environment in your neighborhood is clean enough to support this amazing dual organism.