Two things growing commonly on trees are lichens and mistletoe. They may be common but often misunderstood by homeowners.

Do you have trees or shrubs in poor health with trunks and branches covered with green to gray-green leafy or crusty growths? These growths are lichens.

They are totally harmless and in no way responsible for the poor health of any trees or shrubs.

A lichen is an unusual plant composed of a fungus and an alga living together in the same body. The alga converts sunlight and the carbon dioxide in air to food material just as leaves of green plants do. The fungus surrounds the alga, protecting it from drying, and lives off the food it provides.

In a damp climate, the alga can live without the fungus. The fungi found in lichens, however, cannot usually live without associating with algae.

The relationships are very specific. Only a few fungi can form lichens and a single specific kind of algae is required by each fungus.

Why are lichens found in association with declining plants? As woody plants loose vigor and decline, the number and size of leaves gradually decreases. 

This allows more and more sunlight on the trunks and branches. As soon as enough sunlight is present to support lichen life, these unusual plants will begin to colonize the trunk and branches.

The presence of lichens is often a sign of poor plant vigor, but the lichens are never the cause. They can be brushed off plants with a stiff brush. However, unless the true cause of plant decline is identified and corrected, lichens will reappear.

If plant health is restored by correcting the real cause of decline, leaves will increase in size and number; less sunlight will get to the trunk and limbs; and lichens will gradually disappear.

After all the leaves have dropped in autumn, do your trees still have clumps of green in the top? These clumps are mistletoe, a flowering plant that parasitizes trees.

Although mistletoe does obtain water and minerals from the tree, it does not depend totally on the tree for food. The leathery green leaves of this plant contain chlorophyll and are capable of making their own food from carbon dioxide and water like all other green plants.

Mistletoe does not have a true root system, but forms structures known as haustoria and sinkers, which invade the xylem tissue of the host. These structures extract water and minerals and securely anchor the plant to the host.

This haustorial system grows in association with the limb and may extend up to a foot on either side of the point of attachment.

Mistletoe produces small white berries. The pulp of these berries is a sticky viscous material. The seeds are surrounded by gelatinous sheaths used to adhere to the host. The berries spread by birds and wind, stick to the host plant on contact.

Once the seed has become firmly attached to the host, it germinates, forming a radicle. The radicle grows along the surface of the host until it locates a suitable point of entry.

Many think of mistletoe as a Christmas decoration. When using it in the house, take care to keep it out of the reach of children and pets.  Birds eat the berries without any side effects; however, the berries are extremely toxic to humans.

Mistletoe can be controlled by cutting out infected limbs one to two feet below the point of attachment. No chemicals are labeled for control.

Clark Beusse is the Dawson County extension agent. He can be reached at (706) 265-2442.