What is the problem?
When a soil surface becomes compacted, waterlogged or sparse, growths such as algae (or algae-like growths), liverworts and moss may thrive. As they have no roots that can only grow in very moist soils. Rockeries, shrub borders and neglected flower borders are most prone, especially those in shade.
Algae, liverworts and moss may be found growing on the surface of compost in containers, including seed trays. Such growths make container plants unsightly and can inhibit seedlings from growing.
Although these growths are not directly harmful to plants, they can look unsightly and risk smothering small, delicate plants such as alpines. Without roots they cannot rob plants of water or nutrients.
Algae and algae-like growths: Algae found covering the surface of borders may be either a green, powdery deposit or a dark jelly-like growth, known as Nostoc, a form of cyanobacteria.
Liverworts: Liverworts on borders usually have a green, flattened, plate-like body and no leaves. A common example is Marchantia, which is often topped with umbrella-like sexual organs.
Moss: There are several types of moss that colonise borders. These mosses may form large, coarse, loose, green or yellowish-green tufts, densely matted tufts, or compact green cushions.
Rockeries and borders: A carpet of algae, liverworts or moss usually indicates a compacted soil surface. It may also suggest little plant competition, impoverished soil and, in the case of liverworts and some mosses, acid soil conditions.
Containers: Algae can be a problem if containers are not thoroughly cleaned and sterilised before use, or where home-made or recycled compost are used. Growth is worst where plants are potted too firmly or there is compression of peat-based composts, so that the surface tends to remain damp.
Borders and rockeries
For long term control of liverworts and mosses such as Polytrichum, aim for a soil pH that is close to neutral, i.e. pH 7.0. pH testing kits can be bought from most garden centres. See our advice on understanding soil pH and lime and liming for information on how to make acid soil neutral.
Where the soil surface compacts readily after rain, increase the organic content of the soil by digging in or mulching with well-rotted farm manure, garden compost, processed bark or leafmould.
In rockeries, weed out any moss and liverworts before loosening the soil surface and mulching with grit, gravel or slate.
Keep borders and other cultivated areas well stocked with plants. Bare soil can be kept free of unwanted growths by regular hoeing and raking in order to maintain a rough and well-drained surface.
Algae, liverworts and moss may also thrive in shrub borders where the soil has gradually become impoverished and the shrubs are sparse in growth or in decline. Hoeing off the growths will provide only temporary control in such conditions and may well damage shrub roots. Rejuvenate the shrubs by pruning out old and weaker growths in late winter or early spring, then feed them and mulch. The mulch will effectively smother existing growths and improve soil texture.
Algae, liverworts and moss are less likely to be a problem if containers are clean before use and a clean water supply is used. Where rainwater is used, ensure that gutters, fall pipes and water storage tanks are kept clean and keep stored rainwater in covered tanks to stop debris falling in.
With containers it is a simple task periodically to remove the liverwort from the surface of the compost and top up with fresh compost, or add a top dressing of coarse gravel. Where possible, replace with peat-free or loam-based potting compost.
Where plants are to remain in their containers for some time, a surface layer of grit will check unwanted growth of algae, liverworts and moss. This is particularly helpful with containers of slow-germinating tree and shrub seeds or with seeds of bulbous plants, such as lilies, which may remain in containers for one or two years before reaching a suitable size for pricking out.
Products based on natural fatty acids (Bayer Moss Killer) or pelargonic acid (Scotts Weedol Max) should be used with care around garden plants, including those in containers. However, they will kill green plant tissue so should be used with care around plants with a woody base and are not suitable for using around seedlings or plants without a woody base. These products are contact herbicides based on natural ingredients. They biodegrade on soil contact and have limited environmental impact.
To guarantee clean rain water for containers, add potassium permanganate crystals to fresh water, in a thoroughly cleaned rainwater tank, until the water is faintly coloured pink. Repeat from time to time as new rainwater dilutes the solution. Water butt cleaners based on naturally-occuring micro-organisms or plant extracts (Aquahydrotech Water Butt Fresh, Biotal Water Butt Treatment and Just Green Water Butt Cleaner) will help maintain clean stored water.
Algae, liverworts and moss will return unless soil compaction and impoverishment is relieved.
Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see section 2b and 2c)