Lichens (pronounced “likens”) are composed of two different organisms: an alga or cyanobacteria (both are microscopic organisms found naturally in water) and a fungus. These two organisms form a complex symbiotic relationship, which means that both organisms mutually benefit from interacting with each other. In this partnership, the algae/bacteria provide nourishment to the fungi in the form of carbohydrates, and the fungi provide protection to the algae/bacteria from the surrounding environmental conditions.
Lichens are often confused for mosses, which are tiny, low-growing, non-flowering plants that grow in many of the same areas as lichens, such as on the bark of a tree or shrub. The difference, however, is that moss is a simple plant, whereas lichen is a mix of two organisms: fungi and algae/bacteria.
When analyzing the growth on your tree, pay close attention to the colour and feel of the organism in question. Mosses are often green and spongy, whereas lichens tend to range in colour (from dark red to green to white), appear dry and vary in their size, texture, etc. In addition, mosses tend to prefer shady locations, while lichens love the sun.
Lichens are diverse in both geographic range and species. They can be found across the globe, living in some of the most extreme conditions, ranging from the Arctic to the desert. Lichens can grow on nearly any substrate (the base on which an organism lives), including bare rock, making them a key pioneer species after environmental damage.
More commonly however, lichens are found growing on the bark of trees. They are self-sustaining and non-parasitic, meaning that they do not harm the tree to survive. Instead, they are very shallowly attached to the outside of the bark and do not interfere with the inner bark, where food, water and nutrients are transported. So, if you see a bit of lichen growing on your tree, you shouldn’t be alarmed!
Lots of lichen on your tree, however, may be a sign of stress or poor health. While lichen itself does not harm the tree, its presence may point to an underlying health concern caused by other reasons such as pests, disease, damage or improper care. Trees that are stressed tend to have fewer leaves in their canopy, which can lead to an increase in sunlight and moisture along the trunk – conditions favourable for lichen development. As such, lichens rarely grow on healthy, vigorous trees, so if your tree has lots of lichen, it could mean that there is an underlying health issue causing your tree some stress.
If this is the case, there are a few things you can do to help reduce stress. First, be sure to water regularly. On average, young trees should be watered twice per week using approximately 6 gallons of water each time. That is the equivalent of using 3 full watering cans twice per week or using a hose with no nozzle on a very slow trickle for approximately 15 minutes twice per week. Second, add a healthy layer of organic compost underneath a layer of natural mulch in a doughnut shape around the trunk. This will give your tree a nutrient boost to stimulate new growth and reduce stress.
If you are still concerned about the amount of lichen on your tree, please reach out to a local ISA (International Society of Arboriculture) Certified Arborist for more information on how to improve the health of your tree!
Jon Curtis is the Residential Planting Programs Field Coordinator at LEAF.
LEAF offers a subsidized Backyard Tree Planting Program for private property. The program is supported by the City of Toronto, the Regional Municipality of York, the City of Markham, the Town of Newmarket, the Regional Municipality of Durham, the Town of Ajax, the City of Oshawa, the City of Pickering, the Township of Scugog, the Town of Whitby, Ontario Power Generation, Ontario Trillium Foundation and GrandTrees/Canadian Trees For Life.