When a soil surface becomes compacted,

Describes soil or potting compost that is saturated with water. The water displaces air from the spaces between soil particles and plant roots can literally drown, unless they are adapted to growing in waterlogged conditions. Waterlogging is common on poorly drained soil or when heavy soil is compacted. 

Algae, liverworts and moss may be found

Transferring young plants from one container to a slightly larger one, where they will have more room to grow and fresh potting compost. This encourages continuous, healthy growth.

Can refer to either home-made garden compost or seed/potting compost: • Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own.

compost

A seedling is a young plant grown from seed.

seedlings

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.

This page looks at options for gardeners when algae-like growths, lichen and liverworts are becoming a problem.

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. Growth is worst where plants are potted too firmly or there is compaction of potting composts, so that the surface tends to remain damp.

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

Cultural control

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 well-rotted farm manure, garden compost, processed bark or leafmould. Mulch with a coarse material such as bark or gravel.

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.

Containers

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 to periodically 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.

Weedkiller control

Products based on natural fatty acids (Job Done Garden Ultrafast Weedkiller) can be used with care around garden plants, including those in containers. However, they will kill green plant tissue so are not suitable for using around seedlings or plants without a woody base. Weedol Gun Fast Acting or Spot On Fast Acting Weedkiller based on acetic acid can be used around the bases of roses, ornamental shrubs and trees, and under hedges that are established with woody bark. These products are contact herbicides based on natural ingredients. 

To guarantee clean rain water for containers, thoroughly clean rainwater tanks and water butts. There may be a build-up of debris collected in the bottom of the butt. Emptying and cleaning it out, and checking if gutters need clearing is a better solution than using chemical additives as the chemical will be lost once the water is used but the cause of the problem will still be there

Algae, liverworts and moss will return unless soil compaction and impoverishment is relieved.

Inclusion of a weedkiller product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.