Tales From the Clinic:

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I have had several questions this month about lichens. Lichen is a unique organism that is actually the result of a symbiotic relationship between a fungus and either an alga or bacteria. These two organisms together are able to live in very inhospitable environments that included the side of trees, rock faces, fence posts or bare ground. Lichens can vary dramatically in appearance from green, mossy masses to flat, leafy gray plaques.

Homeowners often wonder if lichens are harmful or damaging their plants. Lichens frequently appear on plants that are growing poorly or declining in health, thus giving the appearance that they are sucking energy from the plant. However, the reality is quite different. Lichens require light to grow. A healthy plant usually has a thick leaf canopy that shades the branches and prevents lots of lichens from growing. When a plant is stressed, the canopy tends to thin out allowing more light to penetrate and giving lichens a chance to colonize the branches. Lichens only use the tree for physical support – they do not damage or harm it in anyway – but they are a symptom of the tree’s declining health.

There is usually no need to treat or try to kill lichen growing on a tree, however you should investigate why your tree is growing poorly. Common causes of poor tree growth include environmental stress, poor management or being situated in the wrong location. Improving the growing environment for the tree will help improve its vigor. Appropriate fertilizing and irrigation schedules should help. However, if the tree is growing in an unsuitable location – consider transplanting it to a better location or replacing it with another plant that is better adapted. If your tree is very heavily infested, light pruning will remove branches with the heaviest lichen load and may stimulate new growth within the tree that will help shade the lichen out.

A few lichens here or there are normal and nothing to be worried about; however if you see a lot of lichens on your tree, evaluate the plant’s health and consider what steps you can take to improve it.