On a trip this past week to Seattle, I was able to get out and explore some of the area’s natural plant life. Because it is wedged between the ocean and a mountain range, the climate is mild and fairly wet.
In my adventures, I noticed a phenomenon we have here in our area taken, in some cases, to extremes. It is a mostly harmless issue that I receive lots of calls about from concerned gardeners who are worried over a crusty growth on their trees and shrubs called lichen.
Lichens are an interesting life form that is actually a joining of two different types of life. A lichen is a symbiotic joining of a fungus and algae (or sometimes something called a cyanobacteria) into one composite individual. Symbiosis means that they rely on each other for life, and both bring benefits to the relationship.
The algae (or the cyanobacteria) has chlorophyll, so it can make food through photosynthesis. The fungus can break down organic matter into food as well. They coexist peacefully together and cannot live without each other. All the different types of lichens you see are the combinations between different fungi and algae/cyanobacteria.
Lichenologist (yes, there are people who study these things) Trevor Goward is quoted as saying that “lichens are fungi that have discovered agriculture.” That is actually a pretty good explanation.
Lichen can grow on trees, shrubs, rocks, untreated wood and more. In many cases, lichens that form on rocks trap dust and dirt that can collect over time (hundreds or thousands of years) to allow plants to find a little niche to grow.
So if you find lichen all over your tree or shrub, are they causing damage? Most likely not.
For the most part, lichens do not cause any damage themselves. They are saprophytes, which means that the fungus it contains only eats dead tissue. The lichen is not causing harm to your plant.
It can, however, be an indicator that your plant is weakened or diseased.
Lichens occur naturally on older trees and shrubs — the longer these plants live, the more opportunity a lichen spore has to colonize and the larger the outer layer of bark (which is nonliving) grows. It is not unusual to see old trees with a fairly large population of lichen.
Keep in mind, though, that many trees and shrubs have a natural lifespan, and the older they get, the weaker they can get. A rapid increase in lichens can indicate increased weakness toward the end of a tree’s lifespan.
Lichens will find their way to trees and shrubs no matter what their age, so a small amount of these crusty growths shouldn’t cause alarm. In Seattle, the constant moisture makes the perfect conditions for lichen to spread and grow with little indication to the health of the tree.
However, in our drier climate, if a younger tree or shrub (less than 15 years old) exhibits lichens, there could be some potential problems with your tree. A younger tree with health issues may exhibit more lichens as more dead tissue may be available for them to colonize.
If you do have lichens growing on your trees and shrubs, there is no need to remove them. In fact, trying to remove them can do harm if you remove the bark or cause injury to the plant.
There can be several causes for an overall lack of health for your tree or shrub. The first culprit I always look at is incorrect planting location. If a woody plant that prefers good drainage or sunshine is planted in a wet or shady area, conditions could exist to weaken the tree and allow for lichens. It is sometimes hard to find good drainage in the area, especially with our heavy clay soils.
The next potential issue is poor planting technique. Planting at the wrong depth is a common problem for trees and shrubs. Another problem is filling planting holes with high-quality soil or compost that makes the roots avoid growing out into heavier clay soil and eventually wrapping their way around the tree. Roots that grow around will eventually meet the tree trunk and girdle, or strangle, the tree. A tree with this issue can undergo a rapid decline without any other symptoms.
Poor nutrition can lead to poor performance in trees as well, so it is important to always test your soil and fertilize accordingly to keep your tree in good health.
Of course, we can’t forget about the possibility of diseases or insects causing damage to trees. Infestations of insects that bore or tunnel through trees can cause long-term damage. Diseases can also cause rapid declines in trees that may invite lichens to colonize.
If your tree does show a large amount of lichens, just remember to treat it well to keep it in good health. Check your tree out for any signs of insect (holes, insect bodies, etc.) or diseases (fungal growths, dead limbs, etc.) to rule those issues out. Make sure there is good drainage around the area, installing drainage if necessary. And keep your trees fertilized and watered to keep them as strong as possible.