Lichens are often found on trees and are not harmful. Photo: Neil Sperry, theeagle.com
When trees lose their leaves in the fall, you sometimes discover crusty or fibrous grayish or milky green growths on the branches and trunks of your trees. These are not fungus, but lichens: composite, symbiotic organisms formed by the association of algae or cyanobacteria and a filamentous fungus. You’ll also see lichens on rocks and sometimes on the bare ground. And while ghostly gray and muddy green are their main colors, they can come in all sorts of shades, even bright oranges, yellows, yes, even pink!
In no way are tree lichens harmful to the bark on which they grow. They are strictly epiphytes, that is, organisms attached to the outside of the bark. They are not parasites (which, by definition, harm their host) and seek nothing from the tree other than a support on which to grow.
Benefits of Lichens
Lichens are even considered beneficial.
First of all, they make interesting pollution indicators, as they won’t grow in polluted air. If lichens start to grow on your trees, that’s good news: a sign that the air is fairly pure! In fact, lichens are moving back into cities after decades of absence as air quality improves compared to what it was in the 1970s and 1980s, back when leaded gas and industry chimneys belching toxic smoke were the norm.
Also, lichens have the capacity to absorb nitrogen directly from the air. When it rains, some of this nitrogen trickles down to the roots of the host tree, helping it grow better.
Many birds—including hummingbirds!—use lichens to build their nests.
Also some animals feed on lichens, including reindeer and caribou (caribou moss, the main winter food of caribou, is actually a lichen: Cladina spp.), but also many others.
Finally, indigenous peoples all around the world use lichens in medicinal treatments and in preparing dyes. Some lichens (but not all!) are even edible!
Disadvantages of lichens
There aren’t any.
The belief that lichens kill tree branches is an old myth. True enough, lichens are often numerous on old, weak or dying branches, but it’s not the lichens that are killing them. Lichens have just settled in because there is less leaf coverage on those branches and they need sunlight in order to grow.
The presence of abundant lichens on a tree could indicate that it’s in decline, with a less dense leaf covering than it should normally have. If so, don’t blame the lichens, but give the tree better care. You could try fertilizing it modestly and watering it in times of drought. Or if there is a drainage problem, fix that. And sometimes trees just die and nothing can be done to save them. That’s just something that you, as a gardener, have to learn to accept.
But while lichens may warn of a problem. they aren’t the problem. So, don’t shoot the messenger!
What Should You Do?
Normally, if you find lichens growing on one of your trees, you should simply leave them alone. Think of their presence as a sign to visitors that nature is welcome in your garden.
If you simply cannot tolerate the presence of lichens, just rub the bark with a soapy brush to knock them free. Rub gently, being careful not to damage the bark or dormant buds.
And no, fungicides won’t kill lichens, so don’t waste time spraying them. After all, lichens aren’t fungus, are they?
Article adapted from one published on October 17, 2015