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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
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.
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.
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.
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.
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.
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.
Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see sections 2b and 2c)
Algae, lichens, liverworts and mossAlgae, lichens and liverworts on lawnsAlgae, lichens and moss on trees and shrubsAlgae, liverworts and moss on borders and containersAlgae, liverworts and moss on greenhousesFurniture: maintaining garden seats and tablesWeeds on hard surfaces
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