Moss growth is favored by shady conditions, but removal might not be necessary.
Mosses are simple, non-vascular plants that lack true root and leaf systems. Instead, they have root-like rhizoids and leaf-like structures that directly absorb a considerable amount of moisture. Due to these structural and functional differences, mosses and liverworts are different from all other plants, which are vascular. Mosses live in damp, shady conditions, so anything that increases these conditions -- including patio umbrellas -- can increase moss growth.
Conditions That Favor Moss
Mosses live in very damp spots of diffused light and on anything from rocks to trees to shrubs to soil. On the lawn, they are particularly favored by high humidity and infertile, acidic, compacted soils with poor drainage and excessive thatch. This is largely due to the fact that turf grasses are less likely to grow in these conditions, allowing mosses -- or algae -- to take over. Unlike algae, which favor sunlight, mosses favor shade.
Controlling Moss on the Lawn
Removing objects like patio umbrellas that provide shady areas for moss to grow will help reduce or eliminate the problem. For moss growing on the lawn, improve drainage, air circulation and light to the area and loosen compact soils. Avoid automatic watering systems, which can over-water an area. Allow mossy areas to dry out, then only water when the grass really needs it. Ferrous sulfate monohydrate is one of the few chemical treatments that effectively reduce moss. Apply this as a spot treatment on the lawn only and in the winter or early spring when the moss is expanding.
Controlling Moss on Trees and Shrubs
If mosses are growing on trees and shrubs under the shade canopy of a patio umbrella, remove the umbrella or only open it when it is actually being used. Manually remove moss on shrubs in the winter when the plants are dormant; trying to do this in the spring might harm the buds more than the moss does. You can also power wash moss off of trees, but the same rule applies; only do this in the winter and try to avoid any delicate structures.
Evaluate If Removal is Necessary
Removing moss might not be necessary. Moss is not parasitic and, in some wet regions like the Pacific Northwest, moss and lichen are often regarded as a decorative and normal part of the landscape. In addition, removing conditions that favor moss might only encourage lichen and algal growth; these organisms grow in moist areas and prefer sunlight. While you might be able to modify light exposure by removing patio umbrellas and other shade suppliers, there might not be much you can do about a generally damp climate.
