Moss and its control in turf

Typical moss found in a general lawn, mown regularly at 40mm

Typical moss found in a general lawn, over the winter

Moss: A low growing plant found in many different situations. Moss is particularly prevalent in turf surfaces that are shaded and damp, which can be typical of many lawns, as well as those that are neglected or have not had the correct type of maintenance for the lawn conditions.

With there being some 600 plus species of moss in the UK, it can be found growing almost anywhere: wet or dry soil, acid or alkaline soils, compacted or more open sandy soils, shade or open situations, long grass or short grass, and soils of low or high fertility.

Surfaces that are put under stress through compaction, poor drainage, nutrient deficiency, close mowing or the impact of very acidic soils can all contribute to moss invasion.  The presence of moss is a symptom of poor grass growth and not the cause: Identifying and then rectifying the cause, or causes, as there will probably be several contributory factors to moss invasion, is essential rather than just applying a material which will temporarily provide some control of a mossy surface.

Moss plants do not have a root system but rely on the presence of moisture in sufficient amounts to enable the transfer of water and nutrients through its leaves. Water is essential for moss to spread and reproduce.

Moss reproduces by two main methods: Primarily by spores which are spread through the air, and secondly by the spreading of plant parts which have become detached from the main plant, such as would occur during scarification.

Types of moss

There are three main types of moss that invade turf:

  1. Cushion type

The ‘cushion’ types have small, upright unbranched stems packed closely together that form a cushion.  It is soft to the touch and likely to be dark green in colour. This type of moss is an indicator of dampness and also of too close mowing which causes scalping of the surface.

  1. Trailing type

The ‘trailing’ type of moss is delicate with a form of branching that gives it a fernlike appearance.  As the name indicates it trails and can cover large areas of ground.  This type of moss is an indicator of very poor drainage and shade conditions.

  1. Upright type

These are much larger than the other two types and have dark green stiff leaves.  The leaves can be dark green at the tips, but the lower ones may be brown.  This type of moss is an indicator of drier acid soils.

Combating moss

In a turfgrass situation, the most appropriate approach to combating moss encroachment and colonisation is to develop a healthy and competitive grass sward. This will be achieved by undertaking correct and effective maintenance and management practices, thus making the sward a challenging environment for any moss to invade. This also means ensuring that soil conditions are made suitable, so don’t just focus on maintenance work on the turf surface

Occasional use of approved chemicals may be considered to help bring under control any initial heavy infestation of moss, but subsequent physical and cultural practices should be applied to negate further chemical applications.  Personally though, I have found that carrying out regular and correct maintenance practices undertaken at the right time (this latter point is essentially the key to goof turf care). Moss spores can be present and inactive within a soil for many years, so attempting to control moss just by chemical means is often futile.

Moss sporophytes: Capsules which contain spores, at the end of an unbranched stalk

Moss sporophytes: Capsules which contain spores, at the end of an unbranched stalk

Major factors that encourage moss

In practice it is unusual for a single factor alone to be solely responsible for moss invasion, and it is usually a combination of several factors which result in a heavy infestation of moss.

  1. Low Fertility and low soil pH
  • Low fertility can often mean poor grass growth, which gives the moss a chance to establish itself.
  • A planned and appropriate feeding programme should be maintained throughout the growing season, although do not overfeed.
  • Soils that are too acid will also tend to encourage moss invasion. Extremely acid soils can be corrected by liming, but this should not be done until a soil analysis has been carried out and appropriate recommendations made.
  • Neutral to alkaline acting fertilisers, such as nitro-chalk, can also be applied where the soil is overly acidic.
  • Top-dressing with neutral materials can also help to raise the surface soil pH of a lawn and may be more suitable than the application of lime, which can bring its own problems.

2. Moisture and shade

  • Many mosses are found in moist conditions, especially compacted and poorly drained surfaces. Moss adapts well to compacted soils and this condition is the opposite of what is required for turfgrass surfaces.
  • Improve surface compaction and drainage potential by regular aeration, using a variety of aeration equipment for the condition encountered, and top-dress with a sand-very sandy soil where the soil is of a heavy nature and prone to retaining water. Do not use builders’ sand or fine sand as this will clog up the pore spaces in the soil and produce a poorly drained area.
  • Where a moss is present in a dry soil then it may be necessary to improve the organic matter content of the soil by using an appropriate top-dressing which is applied after the turfgrass surface has been hollow-tined.
  • Reduce shade by trimming back overhanging trees, hedges and shrubs to allow more light and airflow onto the grass surface. This will also reduce the time any surface moisture, such as dew, might stay on a surface during the day.
  • Brushing or switching a grass surface could potentially spread moss spores during their reproductive phase, however, the benefit of removing surface moisture / dew far outweighs any potential negative outcome from not carrying out this type of maintenance activity.

3. Mowing

  • Many turfgrass surfaces, especially fine ones, are cut too short and scalped, in the mistaken idea that it will reduce the frequency of cut required. Scalping causes bald spots and thinning of the grass cover and leaves an ideal spot for moss to invade because the competition is less.
  • The ideal cutting height for a turfgrass surface will vary, but the important point to consider is that of not cutting too low, which will be obvious if the grass looks yellowish after cutting or scalped patches are present.
  • The opposite of cutting too short is that of leaving the grass too long. This will encourage moisture to be retained on the surface, the grass will thin out and moss will be able to more easily colonise as well.
Picture showing moss in a bowling green

Picture showing moss in a bowling green

Treatment of moss

Only used approved materials, which might include:

  • a straight moss killer in liquid, powder or mini-granular form;
  • in combination with a fertiliser,
  • in combination with a selective moss killer and fertiliser,
  • or a traditional lawn sand.

Be aware that moss killer for hard surfaces, such as paths and tennis courts, is not suitable for turfgrass areas and will kill the grass.

Any material applied to control moss only provides a temporary respite and not a solution. This point can’t be emphasised enough, especially to home lawn owners and volunteers maintaining sports surfaces.

One last consideration and that is of whether to rake moss when it is active and reproducing: It can either reproduce by little pieces of the plant being broken off or by the spread of spores and this can help spread the moss. However, if the purpose is to improve the turf surface with a range of well-planned maintenance activities then some form of prior material application is unlikely to be a particular issue, but this will be influenced by the extent and density of the moss: the benefits of getting on with moss control / removal as part of a renovation programme will typically far away any potential disadvantage of potential spread.

Do make a significant effort to improve soil conditions with a range of maintenance and renovation activities as part of the moss control / removal programme, as typically moss encroachment in a lawn is down to soil conditions, shade and incorrect maintenance practices.

In some cases, it might be beneficial to kill off moss by chemical control (assuming any still approved for use), or lawn sand, first and then rake it off when the moss is dead, especially when there is a lot of moss as this loosens the hold the moss has with the soil (because it has been killed off) and can be more easily removed. Whether to undertake any form of material application control will depend upon each turf surface and the extent of moss colonisation.

Improving turf surfaces

Improve the grass growing conditions so that the grass can out compete the moss, by considering some of the following (these will depend upon the actual conditions found on site though):

  • aerating with a solid or hollow tine fork or machine;
  • apply a sandy top-dressing to help keep the surface free draining;
  • remove moisture retentive thatch;
  • does the area need draining, or major renovation works?
  • improve soil biology, by adding organic products such as seaweed extracts, biostimulants, or similar;
  • test the soil for pH and nutrients to ensure an approve feed is given: apply a fertiliser; it can be difficult to raise the soil pH in a substantial way, so be careful not to try and change the pH measures to any great extent;
  • cut back over-hanging branches, thin trees, shrubs and hedges to improve light contact on the grass sward and also to improve air circulation;
  • scarify to keep the surface open;
  • remove leaves (by raking or sweeping) as soon as practical from a grass surface in the autumn;
  • brush the surface during the autumn and winter to keep the grass blades more upright and the sward more open for air movement amongst the leaves;
  • keep an eye on your grass surface and monitor its condition throughout the year.

In conclusion: Look after your turf on a regular basis, tend to it with love and attention and don’t forget that your turf is a living organism. So don’t carry out activities that are likely to stress the turf and focus on helping it develop into a strong, relatively hard wearing and compact sward.

Chris Gray, 24th February 2019