I recently received an e-mail from a reader who questioned the growth of ''blue mold'' on some of her trees. She was concerned that the trees were dying from this and wondered if there was something she could do to combat the problem.
The answer is simply, do nothing. There is nothing that should be done and most importantly, it isn't a problem.
So what is this stuff we are seeing clinging to the trunks of our trees during the summer? It's nothing I haven't written about before but perhaps should address again. It is lichen.
Lichen (pronounced 'like-in') isn't hurting the trees. This simple group of organisms are using the tree as a place to live, but does not use anything from the tree. They are not parasites, but are more like houseguests. There is no choking, stealing of nutrients or feeding off the bark going on. The trees are simply runways, so to speak, for this combination of algae and fungi that instead, live off of each other.
There are hundreds, even thousands, of different types of lichen. If you want to see something truly lovely, get a magnifying glass or a jeweler's loupe and take a close-up look at this blue substance on your tree's trunk. It is as though you are looking at an incredible miniature forest.
Some forms of lichen look like tiny green bubbles, while others imitate leaf-shapes clinging together en masse. Some have tiny cups they use to collect what they need from the air. Some are sky blue while others are gray-green and all sorts of shades in between. LIchen is a plant, but unlike the plants we know, it does not have roots, stems or even real leaves.
Lichen is the result of an algae and a fungus working closely together. You could say, the algae and the fungus are 'liken' each other, but in fact it goes further than that. Algae is great at absorbing nutrients and water from the surrounding air, but can't live on its own because it has no body or structure from which to live. The fungus has a great body, but needs something to provide it with food. Together, the two organisms give each other exactly what they need and the result is like finding a soulmate.
Trees get their nutrients in a number of ways, one of which is in the form of transpiration. Transpiration is the flow of water and nutrients through the tree's cambium layer. If you cut into a tree's bark and find your way to the soft, wet and sometimes sticky part just beneath, that's the cambium layer and the lifeblood of the tree. The inner wood, the part we cut, plane to size and use to make lumber and furniture is made up of several dead cambium layers. Each year, the tree will form a new cambium layer beneath the bark and the old inner layer dies away, forming new wood.
We tend to notice lichen most when we've had a period of wet, rainy weather, which we did have at the end of June and the first week of July. During the dryer periods of late July and August, lichen stop growing and go dormant, waiting for the rainy season to begin again.
While lichen take in small amounts of minerals that are continually flying around in the air, they are sensitive to large amounts. Too many air pollutants can kill lichen. More than a century ago, it was noticed by scientists that lichen growing prolifically on trees outside of large cities could not live on the same kinds of trees inside the cities where the air held more pollutants. Lichen has not only become a symbol of clean air, but also has become the focus of research on air qualities and the effects of pollution.
As if that isn't enough, there are actually lichen hunters and in 1958 in Britain, the British Lichen Society was formed to track lichen as indicators of air-quality, traffic pollution and acid rain. They call themselves, lichenologists and have managed to compile a list of all known British lichens.
There also are U.S. lichen experts who have hunted and classified several different species of lichen.
Some lichen prefer conditions that are acidic while others prefer more alkaline environments. Lichen are not just found on the bark of trees, but also live on rocks or wood and each have their own classification.
Descriptions of the various shapes and growth habits also classify lichen. Crustose is lichen that can't be separated from its host; scale is attached only on one side; coleus or leaf lichen have two distinct sides and a hard attachment to its host; and shrub lichen have multiple surfaces making it difficult to determine the top from the bottom.
However you like your lichen, don't let this amazing little plant escape your notice. You may even consider it a garden in itself.