Just what are lichens? It’s really pretty simple

Posted 7/7/19

Among the many calls that we get at the Extension office is a fairly common complaint that my trees and or shrubs have a “crusty or mossy” growth on the limbs or bark that is killing my trees.

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Just what are lichens? It’s really pretty simple


Among the many calls that we get at the Extension office is a fairly common complaint that my trees and or shrubs have a “crusty or mossy” growth on the limbs or bark that is killing my trees. These growths are actually lichens. I have already this year had more lichen calls than I usually get during a normal entire calendar year.

Lichens are very often blamed by homeowners for the decline and even death of numerous shrubs as well as landscape and fruit trees in local landscapes. That isn’t too surprising because lichens are very frequently seen on limbs, branches, and bark of declining or dead shrubs. What is surprising to find out, however, is that the lichens are not actually killing or causing the affected trees or shrubs to decline.

Actually lichens rarely have anything to do with poor top growth or decline of trees and shrubs. Their appearance is actually more related to environmental stresses or poor management. Lichens can be thought of as a symptom and not a problem much the same way as a fever is a symptom of an infection.

Lichens are actually two organisms in one so to speak. They are made up of a type of fungus and a type of blue-green algae living in symbiosis. The algae part of the lichen uses photosynthesis to make food that the lichen lives off of while the fungus supplies water, minerals, and protection from any of several harmful environmental conditions.

There are three types (actually forms) of lichens commonly found in Walker County landscapes. Crustose form lichens form a flat grayish crust on the limb or branch, folicose form lichens produce leaf-like folds above the limb, and fruiticose lichen produce and almost “moss-like” appearance with hairy fingerlike branches. It is not uncommon to see a combination of two or all three forms all growing on the same tree.

As I said earlier, the lichens themselves are usually not the problem to be worried about. The important thing to be worried about is why the tree or shrub is under enough stress or has declined to the point that the lichens can grow on it. The one exception is that when lichens grow in extremely large numbers, they can interfere with some plant processes or can actually shade out buds causing further decline.

Healthy vigorous plants is the best defense against lichens. Heavy infestations of lichens are most common on shrubs and trees that are either declining or are under stress or are in poor general health. Following recommended establishment, watering, and fertility practices will help to promote the development of a thick leaf canopy which will inhibit lichen growth. Many years ago we were on an agricultural study tour in Brazil. One one particular stop, the farmer had a small peach orchard that was in a severe state of decline. The main reason was from stress resulting from the peach trees not receiving enough chill hours ( chill hours is a lesson for another time but different varieties of any given fruit needs a certain amount of winter chill hours). In this case the chill hour requirements of the peach trees were not being met which resulted in extreme stress on the trees – which were of course covered in lichens. It is for these reasons that whatever we plant in our landscapes, orchards, or home lawn – we need to make sure that we plant them correctly, make sure that whatever plant we install is zoned for our climate zone (we are a zone 7 here in Walker County – which makes if very difficult to grow colder climate trees such as many spruce, fir, sugar maple, and many other tree species here). In the case of fruit trees, we also need to make sure that the chill hour requirements for the variety we are planting is appropriate for our area.

I think damage from prolonged drought periods that we have experienced at times over the last few years has placed enough stress on many of our plants that lichen formation has been even worse recently than normal.

Better growing conditions and soil fertility may stimulate plant growth and ultimately suppress the lichens. Light pruning (at appropriate times of the year and not during the middle of a drought of course) will help to remove some lichens and stimulate new plant growth. The unfortunate thing is that many plants that develop lichens are already in such poor condition that it may be too late for improved cultural practices to do much good. These plants will probably end up having to be replaced. Again, the main reason is usually either environmental stresses such as poor soil conditions, drought, or sometimes even simple old age in the tree or shrub but not due to the lichens themselves.

The take home lesson here is do a good job planting (not too deep, not too shallow, and plant in a WIDE hole to allow plenty of root growth) and do a good job of watering, fertilizing, liming, and pruning. You will have much healthier plants and far fewer lichens.

Presently, no pesticides are registered (or even recommended) specifically for the control of lichens on trees and shrubs.

The Alabama Cooperative Extension System (Alabama A & M University and Auburn University) is an equal opportunity educator and employer.