There are more than 1,700 species of lichens in the British Isles. Some are very rare, restricted to specific sites but most gardens have at least a few.
Lichens are composite organisms: a fungus growing with an alga. They are not parasitic, so do not injure plants on which they grow, but can be unsightly. Infestations are most common on plants lacking vigour.
Lichen species are difficult to identify but for ease of recognition they can be divided into two types:
Foliaceous lichens are attached at the base, giving the impression of leafy or bush-like plants. The strands of lichen arising from the base are flattened, leaf-like expansions whose margins are divided into segments, either strap-like (left) or rounded (centre). In species of Cladonia these often end in cup-shaped structures.
Crustaceous lichens appear as thin, flat crusts, with or without distinct margins (right). They are common on paving and can be removed with proprietary patio cleaners.
In turf the most common lichen is Peltigera canina (dog lichen); it is brown or grey and formed of flat, lobed structures. Lawn moss treatments should be effective against it.
Tony Dickerson