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
It is early December in upstate NY and as I look around my 25 acre wooded parcel of land, it seems that all of the trees that have lost their leaves are covered in lichens! Should I be worried?
if the lichens fall onto the soil when brushed off the tree, do they need to be removed? will they cause harm to the soil?? and will they grow in the soil?
They’re harmless. Most will die, decompose and enrich the soil. Very few will be able to live on there.
Hi my 28 year old tree that produces very small crab apples has masses of huge lichen lumps on its lower trunk and higher branches are full of the growth laid on them. Trouble is that our new 4 year old natural sandstone patio is now covered in hideous black spots… is it from this tree? Wondering if I cut it down it would solve both problems? I do have healthy Roman and lilac trees too. Help please
The black spots could be a few things: from sooty mold due to honeydew dripping from aphids or other insects hiding on the upper branches to spores launched by artillery fungus (https://laidbackgardener.blog/2017/06/07/black-spots-on-a-white-wall/), but has nothing to do with lichens.
Thanks will investigate further on the trees and in the mean time use the 15% bleach bought on Amazon to try and remove them as the product says.
Exactly! So many people believe that lichens are causing the distress in the trees that are overloaded with them. Even for healthy trees, an abundance of lichens indicates that the tree trunks and limbs are not actively expanding and exfoliating bark. Well pruned apple trees that grow more vigorously tend to sustain less lichens, not only because of the shade of the healthy foliage, but also because some of the smaller limbs last only a few years, and those that last longer are so active.
That’s a great hummer picture! I love the way lichens decorate tree bark, a different spin on winter interest. Your post nicely summarizes lichens’ ecological role and will lay to rest any fears of those who first view them with alarm.
Having lichens on your tree is very fortunate as it means the air you are breathing is clean!