Mosses are a group of primitive plant species that can form a creeping green mat on home lawns. When conditions are favorable for its growth, moss can grow to a sizable thickness prohibiting turfgrass from growing in these areas. Mosses reproduce by spores that are scattered in the wind or rain.
Mosses are not a very competitive group of plants, therefore, they most commonly occur on neglected lawns or in sites where growing conditions are unfavorable for good turfgrass growth. Moss does not kill lawns but rather occupies those sites with unfavorable growing conditions. Some of the most common unfavorable growing conditions include low soil fertility, low soil pH (acid soil), poor drainage (wet soil conditions), compacted soil, insufficient sunlight due to shade from trees and buildings, surface tree roots that compete with turfgrass and turfgrass types that are not adapted to the particular site. While mosses are most likely to be found growing in moist, shaded areas, home gardeners will often find them growing in thin covered turfgrass areas and full sunlight. Closely mowed turfgrass of a non-aggressive species can be especially prone to moss.
The home gardener may spend many hours and a great deal of money in an attempt to control moss. The results are often few if particular attention is not paid to correcting the above mentioned causes which allow moss to become established in a lawn.
Soil with low fertility levels will not produce a vigorous stand of turfgrass. Weak, thin turf will provide niches for plants with lower nutrient demand (like moss) to become established.
Low soil pH (acid soil conditions) is often blamed for the presence of moss. Many gardeners will make rather heavy annual applications of limestone in an attempt to kill moss. Moss however can grow nicely on soil that does not have a low pH. Don’t guess, have the soil tested before applying limestone.
Soils that are poorly drained and remain wet for extended periods of time may have low levels of oxygen present. Adequate soil oxygen levels are necessary for good overall turfgrass growth. Improving soil drainage is not always easy. Improvements may be seen through the installation of drain tiles, French drains, or by elevating depressions. These improvements will move excess water to areas on the site where drainage problems do not exist. Also adjust irrigation practices to avoid excessive water in these sites.
Compacted soil can physically interfere with root growth in addition to contributing to low soil oxygen levels. This type of soil can sometimes be improved with core cultivation. This is accomplished by means of a hollow-tine core aerator. This machine will remove plugs of soil, which are approximately ½ inch in diameter and 4-6 inches in length, from the lawn. These cores are deposited on the surface of the lawn. The cores are then “dragged” to break them up and deposit them back into the holes made by the aerator. It is important to aerate the lawn when the soil is not too wet. To gain the full benefit of core aerating the sides of the holes must be able to collapse. This procedure should be done in the spring or fall when the turfgrass is not under any type of stress.
Another common cause of moss growth is insufficient sunlight to support good turfgrass growth. The lack of sunlight is often due to shading by nearby trees or shrubs. Leaves on trees absorb light rays necessary for turfgrass growth. It may be possible to increase sunlight by pruning and/or removing some of the trees. You may need to set a priority – trees or a lawn. One method of pruning is called “crown raising”. This includes the proper removal of the lower branches of trees, which are casting the shade. This will allow sunlight to make its way under the tree canopy and reach the turfgrass below. Pruning very large branches from a mature tree could result in wounds that may not close and this could result in the formation of a cavity. It is recommended that a certified arborist be consulted first to determine how to correctly prune such branches.
Another method of increasing sunlight to the turfgrass below is thinning. This is accomplished by selectively removing smaller interior branches which are determined to be less important to a trees overall growth. It is important to follow correct pruning practices to avoid damaging the trees. Pruning which increases the amount of morning sunlight is particularly helpful.
Some trees produce numerous “surface roots.” These roots make it almost impossible to keep turfgrass growing under the canopy of the tree. On the other hand moss will usually tolerate this situation rather well. Homeowners always want to know how many tree roots can be removed (pruned) without harming the tree. There are many variables that make it difficult to give a rule of thumb answer to this question. It is probably not a good idea to prune tree roots. Since roots will usually sprout new growth from just behind the point of pruning this would not be a long-term solution anyway. In addition removal of too many roots will effect growth of the tree as well as increase the risk of having the tree blow over due to the loss of structural roots.
It is also not wise to spread topsoil over the root system of a tree in the hope that grass seed or sod will survive. The increased soil depth can interfere with the gas exchange carried on by the tree roots, which results in root death. Also soil piled against the bark of the lower tree trunk which is supposed be exposed to air can start to rot.
Turfgrass decline in shade (which results in the growth of moss) is often a result of incorrect species selection. It is necessary to plant shade tolerant grass species and varieties. Some turfgrass species are more efficient in capturing and using energy from sunlight in comparison to others. Others resist diseases such as powdery mildew, which frequently are found in the shade and the cause of turfgrass decline in these sites. If the site is shady with dry soil plant a grass mixture that is predominantly fine fescue. These include creeping red, hard and Chewings fescue. If the shady site is consistently wet fine fescue will not tolerate the site and you should switch to rough bluegrass (Poa trivialis). Rough bluegrass does not hold up well to traffic though. Some of the Kentucky bluegrass varieties listed as being shade tolerant can be mixed with fine fescue. Although they tolerate shade better than other Kentucky bluegrass varieties they are not better than fine fescue in the shade.
Finally do not over water turfgrass in the shade. This is often one of the reasons for the decline of turfgrass and the increase of moss in a shady site. Avoid frequent irrigation (e.g. daily or several times each week) as this will prolong the period of leaf wetness and will likely increase disease severity. If fine fescue is the predominant grass in the shade (which is often the case) it will rarely if ever require irrigation and natural rainfall will supply sufficient moisture.
If you try to kill or eliminate moss without first identifying and correcting the cultural conditions that favor its growth it is likely that a resurgence of moss will eventually occur. If it is impossible to correct the conditions, consider keeping the moss as a permanent low maintenance groundcover or plant another groundcover such as pachysandra, English ivy or myrtle, which are shade tolerant.
After correcting the cultural conditions you will need to renovate the areas where moss is present to allow new grass to become established. Moss can be eradicated by mechanical methods. Raking with an iron rake or scraping will remove the moss. Moss control products can be purchased that may help ease the task of moss removal before you renovate and spread new grass seed. These products will not be a long-term solution to moss control on their own.
Remember that unless the cultural conditions responsible for the decline of the turfgrass are not corrected the moss will eventually reestablish in the site.
Resource Moss in Lawns: Causes and Corrective Measures, by: Edmond L. Marrotte, Consumer Horticulturist, Department of Plant Science, University of Connecticut, and Moss, by: Dr. Norman W. Hummell, Jr., Department of Floriculture and Ornamental Horticulture, Cornell University, 12/6/88.
Prepared by Thomas Kowalsick, Extension Educator, Cornell Cooperative Extension – Suffolk County, 11/99.