How to Prevent and Control Moss in Lawns

In this article:

1. What is moss?
2. Factors contributing to the presence of moss
3. Top tips for keeping moss under control

All turf managers will encounter problems with moss at some time or another. Its presence is usually indicative of an underlying problem causing a weakness in the turf, giving an opportunity for moss to ingress. Identifying and then dealing with the problem will usually be enough to strengthen growth and keep moss under control.

What is moss?

There are many different species of moss in the UK and worldwide, and they are adapted to grow in a wide variety of situations. Mosses are non-flowering plants, and their growth is usually restricted to 25 mm (1 in) in height. They are generally soft and fleshy in nature with a slender stem and minute brown or green leaves.

Of the many different species of moss, there are three main types that occur in turf situations:

1. Fern or feather type mosses usually have a trailing appearance. For example, Hypnum and Eurhynchium. These mosses are mostly found in less intensively managed turf, usually where excess moisture due to too much organic matter is a problem.
2. Tufted or mat-type mosses are mostly found on acidic soils, for example, Ceratodon and Bryum. These mosses tend to form dense accumulations creating a “mat” or “cushion” type structure. Ceratodon purpureus is more commonly known as “winter moss” as it tends to colonize turf during the fall when sward vigor declines, and dies back in the spring as growth picks up.
3. Upright mosses, for example Polytrichum, are mostly found in dry, infertile situations such as mounds, south-facing embankments, and sandy soils.

Factors contributing to the presence of moss

The factors responsible for the colonization of moss within a lawn include:

1. Acidity, as many mosses are better adapted to deal with acidic conditions than the majority of grasses. Where soil pH drops to 4 or below sward health will be weak and start to thin out, allowing space for moss and weed colonization.
2. Shade may be cast by surrounding vegetation or by physical structures such as buildings. This reduces the level of photosynthesis within the grass plants, affecting the ability of the turf to maintain healthy growth. Additionally, shade may be accompanied by a damp surface due to poor air movement or absence of heat from the sun, or result in a dry soil where large trees remove moisture from the soil. Either will further exacerbate the problem, giving greater opportunity for moss ingress.
3. A wet surface can be caused by a number of factors including poor drainage, excess organic matter, lack of aeration (compaction), overwatering (irrigating), and shade.
4. Low fertility may simply be due to a lack of fertilizer, leaving the sward weak and lacking in vigor. It is more likely to occur in sandy soil where nutrients are easily lost through leaching.
5. Mowing can be a factor associated with close cutting especially where ground conditions are uneven, leading to scalping. Letting clippings fly can lead to excess organic matter accumulation at the turf base leading to problems associated with excess moisture.

Top tips for keeping moss under control

General maintenance should focus on creating favorable conditions for healthy turf growth and minimizing damage to the sward.

1. Identify any underlying problems that may be causing a weakness within the sward and providing opportunity for moss to invade.
2. For wet surfaces increase routine aeration, ensure that organic matter levels are controlled, and improve drainage if required. Any vegetation restricting air movement and casting shade should be removed or pruned.
3. Ensure that a balanced fertilizer program is in place, appropriate to the sward composition and soil make-up. The aim should be to provide adequate nitrogen to maintain healthy growth and to enable the turf to cope with any general wear and tear. Nitrogen inputs should be restricted in fall to prevent the creation of soft growth that may encourage disease.
4. Shade should be minimized by removing vegetation or thinning and pruning where removal is not possible. Eastern and southern directions are the most important.
5. Apply lime (calcium carbonate) to increase soil pH to 5.0 and above to provide more favorable conditions for healthy grass growth.
6. Collect clippings if possible, to minimize the build-up of organic matter at the turf base.
7. Avoid close mowing for prolonged periods and always raise the cutting height during periods of hot dry weather to minimize stress and avoid any weakening of the grass plants.
8. Prevent damage and thinning to the turf by spreading wear and minimizing stress that may result in disease or a reduction in turf density due to dry patches and drought stress.
9. Where thinning does occur or bare patches appear, always carry out repair work as soon as possible to reduce the likelihood of moss colonization.
10. Well-timed fall renovation works, in particular overseeding, will help to reinvigorate the sward and restore sward density to reduce the likelihood of “winter moss” invasion.