Lichen Plants

Biology

• A lichen is composed of a fungus and an alga that come together in a mutually beneficial, complex relationship referred to as symbiosis. The fungus absorbs minerals and water from the air and from the material it is growing on, while the alga helps provide vitamins and carbohydrates. The alga either fixes atmospheric nitrogen for consumption or obtains it from the host plant leachate, organic debris or bird excrement. Lichens use the host plant only for structural support and not for sustenance. This is why they are often seen on rocks, fences and logs as well. The plants do not have roots, stems or leaves and use sunlight as their main source of energy.

Description

• Lichens come in a range of different colors, including various shades of green, gray, brown, yellow, orange or red. Main forms include the crustose lichens that grow flat against the bark, the folicose lichens that produce distinct leaflike folds over their surface and the fruiticose lichens that have fine, hairlike or fingerlike projections emanating from their surfaces. Lichens generally remain firm in texture even when wet. Some black and brown varieties tend to turn gelatinous when wet.

Favorable Conditions

• Growth of lichens is mostly seen on trees and shrubs that are stressed and in declining health. This usually happens when there is poor water or fertilizer management. It is a misconception that lichens are the cause of poor growth. The limbs and trunks of mature trees that are exposed to full sun are favorable sites for lichen growth. Many lichens do not thrive in areas of shade. Trees and plants that are growing in polluted areas with high levels of sulfur dioxide, ozone or acid rain have a low incidence of lichen growth.

Control

• Control of lichens is generally not recommended or advised, since these do not affect plant health in any way. Keeping plants in good health is the best preventative measure against the growth of lichens. Avoid excessive thinning of the canopy, as sun in the inner areas is likely to promote lichen growth. In case of lighter lichen growth, prune out and remove affected areas.