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A more tolerant approach to moss, lichen and algae
February 3, 2009
CORVALLIS, Ore. – Wet winter weather in western Oregon provides prime conditions for growth of lichen, moss and algae on trees and shrubs – as well as on rooftops, decks and walkways.
"What is merely slippery green stuff to some people is actually performing serious ecosystem functions," according to Linda McMahan, an Oregon State University Extension faculty member in Yamhill County who specializes in community horticulture.
Lichens can take many forms but often are grayish-green or silver, lace-like organisms on tree trunks and limbs. They provide home and food for beneficial insects, and some are nitrogen fixers. The presence of certain kinds of lichens is an indicator of good air quality.
"When lichens fall off trees and shrubs, they return nutrients to the soil, and some are nitrogen fixers," McMahan said. "They are not parasitic, but use trees only as a platform.
"I see very little reason to control lichen except perhaps on fruit or nut trees when it interferes with spur production or is heavy enough to break limbs," she said.
If you are concerned that moss will shorten the life span of a roof if not controlled, Ross Penhallegon, horticulturist with the Oregon State University Extension Service, recommends sweeping away as much moss as possible and removing overhanging branches from above the roof to allow direct sunlight and good aeration for faster drying.
Commercial moss removers can keep moss and algae from returning. They are best applied when the moss is actively growing in the fall, winter and spring, Penhallegon said. If possible, apply them during a dry spell. Removers containing zinc or iron sulfate can be toxic to plants, however, and powders can be blown around. Liquid and powder formulations need to be applied directly to the problem areas.
"Do not use table salt to get rid of moss and algae," he warned. "It is corrosive to metal and not effective."
The same moss-control chemicals used on roofs also do the job on slippery walking surfaces, McMahan said. "For heavy moss growth, removing as much as possible with a scraper will reduce the amount of pesticide needed for control. Check the label to make sure it is approved for this use."
"If fruit trees have a moss problem, prune the center of the tree to allow more light," Penhallegon said. Copper fungicides also can be used to control many fruit tree diseases. Always read the label on a commercial product to make sure it controls what you want and that you use the right concentration.
Zinc and copper strips on roofs, although expensive, also work well and are easily installed, usually only once.
"For trees, there is really no disease or ecological reason to remove moss," McMahan said. "It provides habitat, food and shelter, and helps retain moisture in the garden."
If algae is a concern, it should be removed only for safety reasons, McMahan said. "Learning to appreciate it in most places (such as a bright green slash on the tree trunk in winter) is the best approach. It will disappear when the air dries in late spring or summer."
Source: Linda McMahan