While a few unique gardeners actually work to collect and cultivate moss, most of us view moss as an undesirable part of the landscape. However, we should recognize that moss is an opportunist; it fills the spaces vacated by other plants. For example, moss does not kill grass, a very common misconception, instead, it fills in the open spaces as the grass dies out.
In essence, moss grows because it can tolerate the kinds of poor growing conditions other plants fail to survive. The key to dealing with moss is in “fixing” those growing conditions in order that more desirable plants will grow and the moss will have no place to get started.
In general, mosses are more tolerant of shade than other garden plants, especially grass. A common problem involving moss occurs in yards where there are lots of old trees. As these trees have grown they’ve blocked out more of the sun and slowly but surely the grass has died out. Once the grass is gone the moss is free to move in. If this seems to be the cause of your moss you have two alternatives: (1) Have the trees thinned out to allow in more sun, or (2) Instead of grass, plant shade-tolerant ground cover such as pachysandra or myrtle.
Grass can also fail because its roots have difficulty penetrating compacted soil. Heavy clay soils restrict grass root growth and the grass usually fails during the dry summer weather. Once again, once the grass has vacated the moss moves in. Moss is very shallow rooted and can easily grow in heavy, compacted clay soils. Soil compaction can be improved by annually having the lawn core aerated. This usually requires that you hire a lawn service.
Going hand-in-hand with soil compaction is poor soil drainage. Soils that stay wet for long periods of time and are low in oxygen restrict the growth of grass roots. The grass dies out and the moss, which doesn’t mind poorly drained soils, takes over. The soil core aeration will also help improve soil drainage. In small areas the soil can be dug up to a depth of 12 inches or more and liberal amounts of organic matter added to improve the drainage.
Even if your lawn has plenty of sun and the soil is in good condition you can still encourage a poor grass root system by practicing “social” watering. Watering lightly each evening encourages a shallow grass root system and grass failure during the hot, dry summer weather. Better is to only water once a week or less, but put on enough water to moisten the soil to a depth of at least 4 inches. Remember that moss doesn’t kill grass and take over a yard. The grass fails first and the moss simply moves in and takes over the vacant spaces. Therefore, just killing the moss will usually not solve the problem. The answer lies in figuring out why the grass failed *in the first place and taking care of that problem(s). Once you’ve got the grass growing well you’ll not see another patch of moss.