Lichens don't harm plants, but get blamed a lot

Many questions have been asked about lichens. The following is a great article by Dan Mullins, Extension agent in Santa Rosa County, Fla.

Things aren’t always what they seem in the landscape and such is the case when lichens infest shrubs. These gray-green scaly, crusty or hairy structures found on the branches of landscape plants are often unfairly blamed for causing sickly, dying shrubs.

It’s true lichens are most prolific on ailing plants, but they are not to blame. Lichens are unique organisms in the plant world. While they appear to be one plant, each one is made up of an algae and a fungus living symbiotically. The fungus provides moisture, while the algae supplies food through photosynthesis.

Lichens are what are commonly known as “air plants.” Though they stick to plant stems and bark, they do not tap into the plants vascular system and act as a parasite like mistletoe does.

One of my biggest challenges over the years has been in attempting to explain that lichens are not plant killers. I have spent many hours examining lichen covered twigs and discussing the biology only to receive many fish-eyed looks of unbelief. Sometimes gardeners have listened patiently and then replied, “O.K., so what should I spray it with?” I am losing ground on this subject, but refuse to give up.

Lichens can be expected to occur on healthy plants, but the next time that you have an ailing plant with excessive lichen growth, take a look around the landscape. You will also find these same lichen species growing on inanimate objects such as rocks and fence posts.

We have had so many lichen covered twigs submitted lately I started doubting my understanding of the lichen/shrub relationship. After reviewing the literature on the subject again, the answer is the same—lichens do not directly damage plants.

It is easy to see why gardeners could believe otherwise. Upon seeing a shrub in decline while lichen growth is increasing, the reason would seem obvious.

Lichens have two basic environmental conditions, both of which are met by a sickly plant. First, they need a non-moving surface on which to anchor. A plant that has stopped growing is home sweet home, while a vigorous plant that increases its stem diameter constantly would not allow lichens an opportunity to become established as easily.

Second, lichens require bright light. Shrubs that lose excessive foliage allow sunlight to penetrate to branches and lichens to increase. Compare thinning, poorly foliated plants with dense plants. You will find that the healthy plants with plenty of leaves have few, if any lichen growth on their branches.

So as the plant declines, it stops growing and loses leaves. Excessive lichen growth is therefore a sign of an unthrifty plant, not the cause. Controlling lichens is not done by spraying, but by correcting the conditions that caused poor plant growth and development.

Heavy lichen growth on shrubs is most often found where three conditions exist. Excessively wet, poorly drained soils result in poor root development or the death of existing roots and little top growth. Shade loving plants such as azaleas and camellias are grown in locations that are too hot and bright. And, sometimes the plants are suffering from a nutrient deficiency. In this case a little fertilizer can make a remarkable difference.

Send your gardening questions or comments to: Brunswick County Master Gardener Column, P.O. Box 109, Bolivia, NC 28422, or call (910) 253-2610. Enclose a self-addressed stamped envelope if requesting information or a reply. Answers may be printed in this column.