Question of the Week – Number 3: May 23, 2019

“What is this fungus that is now growing all over the bark of my tree? What can I do to stop it?”

I’ve addressed this before here, but like other questions, this one keeps getting asked and I keep answering it. Please let me do so one more time.

These are lichens. They are symbiotic growths of funguses and algae that co-exist on these tree trunks to nourish one another. They do not derive any nourishment from the tree itself, so they are in no way parasitic. In fact, if you’ve ever bought a “moss-covered boulder,” you’ve been paying extra to get lichens on the rock. They grow there, too. So they’re no more parasitic to that rock than they will be to your trees’ trunks.

The one time that I start worrying about excessive build-ups of lichens is when they’re accompanied by a slow-down in growth of a tree’s trunk or limbs. I need to explain a function of tree growth to tell you why. Bark is a dead tissue. As a tree grows, its bark can do nothing to expand. All it can do is pop and fall to the ground. In doing so it takes the lichens with it. But when they accumulate on the pieces of bark, that tells us that the tree has lost its vigor.

So from that standpoint, lichens do indicate a potential problem. But the lichens themselves are not the problem. You’ll want to track down why the tree has slowed down its growth and why the bark has stopped falling to the ground. It may be nothing more than old age of the tree, but it could also be decay or insect damage, excessive shade from nearby taller trees, poor maintenance or a multitude of other issues.