What is this strange growth on my ash trees?

Thanks to Barbara and Paul Lloyd in Fredericksburg for this great question!

Well, these interesting little creatures are lichens, complex organisms that are composed of a fungus in symbiotic union with an alga or cyanobacterium. They’re unusual among living organisms, but are fairly common on trees, especially in wild or natural areas, and also commonly grow on rocks. If you pay attention while hiking almost anywhere, you’ll see many different lichens, in lots of different shapes and colors. They aren’t, in and of themselves, harmful to trees, but are simply using the trees as a support surface.

We reached out to Guy LeBlanc of Tree Care by Guy, to get the Certified Arborist scoop. He confirmed that the lichens are harmless, since they don’t invade the plant tissue.

But, as with ball moss, another common interloper in our Central Texas deciduous trees, if enough lichens are present, they do stunt growth by covering the surface of the bark and inhibiting branching and the production of new leaves.

Lichens tend to be more problematic on trees that are stressed for other reasons, like our past few years of drought, which caused many trees to lose more leaves and have more of an open canopy than usual.

With more sunlight getting to the lichens, they’re able to thrive, reproduce, and spread. If these are Arizona ash trees, they are a riparian tree, meaning they need more water and tend to be more prone to drought stress, dropping more leaves and struggling more in general, making them more susceptible to secondary issues, than native ash.

The best course of action would be to water your trees well during hot, dry periods, and hope for them to recover and put on more canopy. If they’re too far gone, the trees may continue to struggle to put on new growth, which would, in the end, lead to their early demise.