Algae, lichens, liverworts and moss on hard surfaces It is common to find growths such as algae, lichens, liverworts and moss growing on hard surfaces. Contrary to popular belief, they do not damage what they are growing on, but can cause patios, drives, paths and steps to become slippery.

Algae on paths can make them slippery. Image: John Trentholm/RHS Quick facts

Common name Algae, lichen, liverwort, moss Areas affected Paths, patios, drives, paving, stone features, garden furniture and fencing Main causes Damp, humid, shady conditions, clean air and/or poor drainage Timing More noticeable in winter or after a wet spell, but present year-round Jump to AppearanceCauseControl What is the problem? Slippery paths and steps covered in algae-like growths, liverworts, lichens and moss are hazardous. Winter is traditionally the time when algal, moss and liverwort growth is most significant, but build-up can occur during any wet period or in shady, humid areas.

On stone and timber features lichens and moss can be very attractive and give a mature look to the garden. Such growths do not harm the surfaces on which they grow, and are a natural part of the garden ecosystem.

Appearance Algae and algae-like growths: A green film or powdery deposit is typical of algae on paving, stonework and garden furniture. The dark green or blackish jelly-like growths that often appear in damper, cooler weather on paths and areas of tarmac are incorrectly known as blue-green or gelatinous algae, but are in fact a cyanobacteria called Nostoc.

Lichen: These are common on paving and timber structures such as garden benches. The colour of lichen varies with species, but most are silver-grey, grey-green, yellow or orange. They can be crust-like, leafy or scurfy in texture.

Liverwort: Liverworts that grow on hard surfaces usually have a green, flattened, plate-like body and no leaves. A common example is Marchantia, which is often topped with umbrella-like structures carrying sexual organs.

Moss: Mosses commonly found on hard surfaces are usually cushion-like.

Cause Algae, lichens, liverworts and moss are found in damp places as they need moisture for both growth and reproduction. Lichens are particularly common in areas with clean air. However, they grow only very slowly so, unlike moss and algae, are slow to spread.

Growth of algae, lichens, liverworts and moss on paths and garden furniture often appears in sheltered areas overhung by plants.

Poorly drained and/or shady conditions contribute to the growth of algae, moss, liverworts and lichen on paths and hard surfaces.

Control Where growths of algae, moss, lichens and liverworts present no hazard, such as on stone sculptures and features, gardeners are encouraged to allow them to flourish. Their appearance signifies a mature garden, blends in harsh stonework to the environment, and adds to the biodiversity of the garden. Indeed, in rural areas it is possible that gardens could harbour very rare species of lichen.

For areas where growths are a slip-hazard or are contributing to the weathering of wooden structures, the following controls are recommended.

Non-chemical control Dislodge moss from between paving by running a sharp knife along the cracks. Alternatively, use a block paving brush with a long handle, narrow head and wire bristles for effective cleaning without stooping A pressure washer will remove moss and algae effectively. However, use this method with care in areas where drainage is unsatisfactory as the extra water could exacerbate damp problems. Always wear goggles when using a pressure washer. This is the best method for removal of moss and algae from wooden garden features. After spraying, consider treating fences and sheds with wood preservatives and garden furniture with teak oil Brush hard surfaces with a stiff broom on a regular basis to help prevent growths from taking hold. Raking loose surfaces such as gravel helps to keep these areas free of both moss and weeds Prune overhanging plants to improve air flow – this will allow the drying effects of sun and wind to reach the site Ensure surfaces slope slightly to prevent standing water Improving drainage in the surrounding area will also help to deter growths. Dig out shallow channels along the edges of paths, patios and drives and fill with coarse gravel to absorb run off water Fork over beds close to damp surfaces to maximise drainage and water absorption Only pave areas essential for access. Choose permeable paving when constructing new hard surfaces and keep drains clear of leaves and debris Surface finishes that are raised to give grip in wet weather are ideal for shady spots. On wooden surfaces try tacking down some chicken wire as this too will make it less slippery. Spreading coarse sand over garden steps is another simple anti-slip solution Chemical control Algae, lichens and liverworts can be removed with most proprietary patio cleaners. Moss can be controlled with products approved for use on hard surfaces. Most products are non-persistent and repeat applications will be required.

Use products based on benzalkonium chloride (Brinton Patio Magic, Vitax Easy Clean or Doff Super Concentrate Path, Patio & Decking Cleaner), acetic acid (Algon Organic Path & Patio Cleaner), fatty acids (Bayer Garden Moss Killer), nitrilo triacetic acid/trisodium salt (Bayer Garden Path & Patio Cleaner) or other surfaces cleaners such as Jeyes Fluid Path & Patio Cleaner which claim to control algae on hard surfaces or natural paths. They may also give some control of lichens and liverworts For moss control on hard surfaces use acetic acid (Scotts Weedol Fast Acting Gun), fatty acids (Bayer Garden Moss Killer) or pelargonic acid (Westland Resolva Moss Killer) Just Patio and Concrete Cleaner is a natural surfactant (detergent) based on seaweed extracts and should be especially safe to use near planted areas. It claims to remove algae on hard surfaces Path and patio cleaners based on hydrochloric acid or bleach have some effect but are not recommended for use near plants. They can also discolour certain types of stone 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. to read more follow the link below https://www.rhs.org.uk/advice/profile?pid=418

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