Algae 

Algae are simple, plant-like organisms. They can photosynthesise (harness the energy in sunlight) but don’t have true roots, stems or leaves.  

Branches and trunks host algae that appear as green, orange or rusty-red fine deposits; these are most likely to be seen on the northern side of a tree. Trentepohlia is a common type of algae on trees in the UK; the species T. umbrina is easily noticed, due to its conspicuous rusty-red colour. 

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Orangey-red growths of Trentepohlia are often seen on tree trunks 

Lichens 

A lichen is not a single organism; it is a symbiotic (mutually beneficial) association between a fungus and an alga, or a fungus and a cyanobacterium.  

Some lichens grow well on the trunks, stems and branches of trees and shrubs. They may form crusty patches, leafy mats, or upright branching or hanging growths on the bark. Many are green or grey in colour, such as the grey-green tufted growth of Ramalina farinacea (shaggy strap lichen). However, some display yellow and orange colouring, such as the commonly seen Xanthoria parietina, although it is often grey when growing in shade.
 

Mosses 

Mosses are small, low growing plants that reproduce by spores. There are over 700 species of mosses in the UK, ranging from dense mats of flat growth, to loose tufts or rounded cushions. They have stems and leaves, but instead of roots they have shallow rhizoids (small hairlike structures) that anchor them to surfaces.  

Some mosses that are common on trunks and branches include – Hypnum cupressiforme (cypress-leaved plait-moss), Lewinskya affinis (wood bristle-moss) and Brachythecium rutabulum (rough-stalked feather-moss). 

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Brachythecium rutabulum is commonly seen on living trees and shrubs, and on old stumps and logs 

Bark is the outermost layer of woody plants (trees, shrubs and woody climbers). It is several cells thick and provides protection against physical damage, disease and environmental stresses. Bark comes in a wide variety of colours and patterns, and these can help gardeners when identifying plants. The fissures and crevices of bark on older plants also creates valuable habitat for many garden creatures as well as lichens and small plants.

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There is no need to remove algae, lichens and mosses from woody plants as they are not parasitic and do not cause any harm – they are just using the trunks and branches as a surface to grow on.

They can, in fact, be beneficial and allowing them to grow:

• Boosts the biodiversity of your garden – a woody plant can support many native species of algae, lichens and mosses. Some species are rare, and many are threatened by habitat loss, climate change and pollution, so you can help conserve native species simply by allowing them to grow.  
• Provides food and habitats for garden wildlife – they offer shelter to small creatures, including beetles, spiders, springtails and woodlice. Mosses are used by garden birds as nesting material and are a food source for many moths. 
• Adds character – algae, lichens, and mosses add another level of interest to trees and shrubs, creating a sense of maturity and permanence. Growths can form tapestries of colour and texture on bark and wood, providing winter interest after leaves have fallen.

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Mosses and lichens add interest and character to these old oak trees 

Algae, lichens and mosses tend to be more common on plants lacking vigour, so their presence could indicate a need to improve the overall health of a plant. Algae and mosses, for example, tend to favour the damp, shady conditions created by congested growth on neglected trees and shrubs, while many lichens favour the bright conditions of a sparse canopy. If algae, lichens and mosses are seen on sickly or neglected plants, it is important to remember that their presence is a result of the plant’s condition, rather than the cause of it. 

Lichens and algae are sometimes mistaken for fungal growths; if you are an RHS member you can use the Gardening Advice service to identify growths you are unsure about.

Moss grows on the old wood of this willow, below the vigorous new stems that are pruned annually 

If you have algae, lichens and mosses growing where they are not wanted, they can be discouraged and controlled in two ways:

• Adjust the growing conditions – growths of algae, some mosses and a small proportion of lichen species will be discouraged by pruning out overcrowded branches and removing overhanging vegetation. This will improve air circulation and reduce shading.  
• Encourage the tree or shrub to produce new growth – vigorous growth is less likely to host algae, lichens and mosses. Try to stimulate new growth by mulching, watering during prolonged dry spells, and feeding if your soil is poor or there are signs of a nutrient deficiency. Once your plant regains vigour, the most affected twigs and branches can be pruned out. However, lichens and mosses can grow well on vigorous new growth in areas with high rainfall and humidity, particularly in western parts of the UK.  

Gardeners who don’t want algae, lichens or mosses on woody plants might be tempted to physically remove them. Brushing or jet washing the growths off risks damaging the bark and branches, which could affect the overall health of the tree or shrub. And, as physical removal is just a temporary solution, algae, lichens and mosses will return if the growing conditions remain unchanged.