When the leaves begin to fall, sometimes you discover crusty grayish growths on the branches and trunks of your trees. These are lichens: composite, symbiotic organisms formed by the association of an algae or a cyanobacteria and a filamentous fungus. You’ll also see lichens on rocks and sometimes on the bare ground.
In no way are tree lichens harmful to the bark on which they grow. They are strictly epiphytes, that is, organisms fixed 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 pure!
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 caribou (the so-called “caribou moss” 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 rumor that they kill the branches of trees is an old wive’s tale. They may start to grow on older, dying branches, but it’s not the lichens that are killing them. Older, shaded branches just tend to die. Get used to it! Lichens are, I repeat, harmless.
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 loose. Rub gently, being careful not to damage the bark or dormant buds.