Green algae are the simplest of green plants. They are found everywhere and are numerous in any place where sufficient moisture is present to support their growth. They are commonly found on tree trunks, twigs, shrubs, soil, rocks and walls and can cause “scum” on ponds and poorly drained lawns. Their habit of growing profusely in evaporative coolers and on rock houses and sidewalks often makes them very bothersome. Although copper fungicides will do an excellent job for controlling algae, in many instances it is not desirable. Certain forms of copper will severely stain houses and masonry work. Under these conditions, compounds cleared for use in swimming pools may be useful. Swimming pool compounds cannot, however, be used where desirable plants are growing. If all chemicals fail, an increase in the penetration of sunlight and a decrease in the level of moisture should correct the problem.