Algae, lichens, liverworts and moss are often found growing in damp or shady places in the garden on plants, soil and hard surfaces. They do not cause any harm, and can usually be tolerated as they can give a mature look to a garden. But they can make paths and lawns slippery and make ponds and borders unsightly so control is sometimes necessary.
What are they?
Algae vary in appearance depending on where they are growing:
- In ponds, algae may be single-celled, turning the water green, or filamentous, forming hair-like blanket weed
- A green film or powdery deposit is typical of algae on paving, stonework, garden furniture, greenhouse glazing and the surface of containers
- Nostoc, the dark green or blackish jelly-like growths that appear in damper, cooler weather on paths, areas of tarmac and on lawns are incorrectly known as gelatinous or blue-green algae, but are in fact cyanobacteria
- On tree trunks and evergreens, algae can commonly be seen as a green powdery deposit. The alga Trentepohlia shows as a vivid orange powdery deposit on tree trunks and branches
Lichens are composite organisms: a fungus growing with an alga. There are more than 1,800 species of lichens in the British Isles. Some are very rare, restricted to specific sites but most gardens have at least a few. Lichen species are difficult to identify but for ease of recognition they can be divided into three types:
- Foliose, or leaf-like, lichens are flat and creeping, of various colours, attached over their whole base to the substrate on which the lichen is growing. Some, such as Cladonia, grow erect cup-shaped reproductive structures known as ‘pixie-cups’
- Fruticose lichens are attached to the substrate usually by a single point at their base. They grow in an erect or pendulous pattern and give the impression of bush-like plants
- Crustose lichens appear as thin, flat crusts, with or without distinct margins. They do not have a lower surface and grow directly on their substrate. They are common on paving and timber structures such as garden benches, producing interesting mosaics that, given a few years, will help to blend in the harsh appearance of new timber, brick or stone
The colour of lichen varies with species but most are silver-grey, grey-green, yellow or orange. Lichens growing on trees and shrubs are often grey to green in colour.
In turf the most common lichen is Peltigera spp. (dog lichen); it is brown or grey and formed of flat, lobed structures with conspicuous rootlets (rhizines) underneath.
Liverworts are small plants related to mosses. Nearly 300 species are native to the British Isles. Broadly liverworts can be divided into two types: thallose and leafy.
- Thallose liverworts have a flattened, plate-like body – the thallus – and no leaves. A common example is Marchantia, which is often topped with umbrella-like sexual organs
- Leafy liverworts have two ranks of flattened ‘leaves’ growing out from a stem. A third rank of smaller, forked ‘leaves’ lies on their underside
Marchantia polymorpha and Pellia spp. are common liverworts in gardens.
There are over 600 species of moss in the UK, some of which are endangered. Mosses may form large, coarse, loose, green or yellowish-green tufts, densely matted tufts, or compact green cushions.
Some lawn mosses include Bryum, Ceratodon, Eurhynchium, Hypnum and Polytrichum.
Where growing conditions are favourable, these algae, lichen, liverworts and moss usually appear in gardens of their own accord. These conditions include:
Algae: Algae will form on hard surfaces or plants, especially in shady, wet or poorly drained conditions. They proliferate in water such as ponds or water butts where light and nutrients are available.
Lichens: Lichens are favoured by humid, still conditions and clean air, so are often more common in rural areas and mature gardens. They are particularly adaptable as they are able to exist where nutrients, and sometimes water, are scarce. However, they grow only very slowly so, unlike moss and algae, are slow to colonise. They can be found most frequently on trunks and stems of trees and shrubs, on paths, patios, paving,and walls, and less commonly in lawns.
Lichens often grow more profusely on trees and shrubs which have been neglected - especially where the branches have become overcrowded or the plant is in poor health. However, they can also appear on vigorous new plants in humid areas and are fairly common in western districts, where they form a natural part of the garden ecology.
Liverworts: Research indicates that some liverworts, such as Marchantia polymorpha, multiply under conditions of high humidity, high soil moisture, low pH and little plant competition, especially where the soil is compacted.
Moss: Moss is favoured by wet, poorly drained, compacted or shady conditions. They occur naturally on the stem and trunk of trees and shrub, hard surfaces, borders and the top of compost in containers. On lawns their presence is encouraged by a lack of aeration, low fertility, over-acidity of the soil, very wet conditions, excessive shade and very close mowing. Rhytidiodelphus squarrosus is the species most usually encountered in turf or lawns, but Polytrichum juniperinum may also be found on acid heathland soils and on compact sandy soils. Polytrichum commune may be found in wetter conditions.
Algae, lichens and moss can add character to a garden, patio or containers, and you can encourage their growth on stone troughs or walls by painting the surface with organic materials including a weak yoghurt solution.
Algae, lichens, liverworts and mosses all require a moist environment to reproduce so keeping the area damp and shaded will encourage them to flourish.
When transplanting mosses and liverworts, keep them wet at all times. Never take mosses from the wild, or without the landowner’s permission. Lichens cannot generally be transplanted.
Algae, mosses, liverworts make a valuable contribution to the biodiversity of our gardens. Some rural gardens may have very rare species of lichen growing in them. See the links below for information on RHS research and advice on biodiversity in gardens.
Further information on moss gardens and lichens
Moss Grower’s Handbook by Michael Fletcher (pdf)
Lichens can be seen extensively at RHS Garden Rosemoor, Devon. Occasionally courses are run on these; see Rosemoor's current adult learning brochure.
Although algae, lichens, liverworts and moss can be tolerated or even encouraged in a garden, they can make paths and lawns slippery, cut out light to greenhouses and evergreen leaves, and make ponds, borders and the top of containers unsightly so control is sometimes necessary.
See the links below for more information on control;
Algae in ponds
Algae on leaves
Algae, lichens and moss on trees and shrubs
Algae, liverworts and moss on greenhouses
Algae, lichens, liverworts and moss on hard surfaces
Algae, lichens and liverworts on lawns
Algae, liverworts and moss on borders and containers
Moss on lawns