When gardeners see lichen growing on the trunks and branches of trees and shrubs they usually get worried. After all, something is growing on their tree. In fact, most people want to blame these frilly blue-green organisms for any tree health problems. This isn’t surprising because they are often seen growing on plants in poor health. However, lichens are not the problem but rather a sign of poor tree health.
Lichens are organisms composed of a fungus and a green or blue-green alga growing together. The fungus, in this mutually beneficial relationship, absorbs water and minerals from both the air and structure to which they are attached. The alga uses photosynthesis to convert these materials into carbohydrates and vitamins. Lichens are extremely tough and grow in some of the harshest environments on earth; from deserts to the arctic tundras.
Lichens come in three distinct varieties that are common in our area. One grows flat, looking like a white, gray or blue-green splotch with little raised definition. The second variety forms leaf-like folds that are frilly and the third develops branch-like structures, which are often long and hair or coral-like. While most lichens that occur on trees are a gray-green in color they can come in a variety of colors ranging from dark brown to yellows and even bright oranges. In South Alabama, it is not uncommon to see orange and almost salmon-colored lichens on live oaks.
Lichens on trees and shrubs mostly occur on trunks and branches that are exposed to full sun. As a result, trees that have thinning leaves or missing branches will have more lichens on the exposed inner branches.
Here is where many misconceptions about lichens are born. We see declining health in trees, which have thinning leaves and more sunlight reaching inner branches and thus more lichens. We then assume it’s the lichens that are the causing the tree decline and seek out costly chemical controls that are not addressing the tree’s problem. Lichens are not parasitic like mistletoe but are opportunists that are a sign of declining tree health.
Furthermore, trees that are vigorously growing will regularly slough-off bark making it difficult for lichens to become attached. This is especially the case with smaller branches at plant extremities. Lichens on exposed tree trunks are not uncommon and should not raise concerns. It is lichens growing on the younger and faster growing outer branches that should raise concern that our trees are unhealthy and need care.
The problem with your shrubs or trees may be poor soil fertility, root disease or circling and girdling roots. These problems are often below ground and difficult to immediately identify.
In the short term, I would recommend the following procedures:
- If the plant or shrub was container grown and planted within the last 3 years carefully dig around the base of the plant looking for circling or girdling roots. Girdling roots will often choke the main trunk or other large roots. If these are found, use a sharp saw to cut them. If the shape of the pot is still discernable in the soil, remove the plant and start over.
- Take a soil test. Soil test kits are available at your local extension office. Lime and fertilize based on soil test results.
- Look for any fruiting fungus at the base of your tree. Their appearance can range from clusters of mushroom to woody disk-shaped growths called conks attached to the base of your tree. If these are seen, contact a Certified Arborist.
Long-term management of declining tree health should include the following:
- Mulch the trees and shrubs. Mulch should be applied to a depth of 2 to 3 inches to the tree’s drip-line with care to not pile mulch at the tree’s trunk.
- Water trees during periods of extreme drought.
- Consider removing trees that are not responding to the above treatments.