Algae, lichens and moss often form green or grey, powdery or mossy, crusty growths on the stems, branches and trunks of trees and shrubs. While this can worry gardeners, these growths are harmless, although may occasionally indicate a lack of vigour in the affected plant.
What is the problem?
Algae, lichens and moss are non-parasitic plant-like organisms that colonise bark, rock and other hard surfaces. Lichens and algae are often mistaken for a fungal disease but, fortunately, they do not harm plants on which they grow. Furthermore, they can give a mature look to a garden, preferring damp areas with minimal air movement.
However, growths of algae, lichens and moss may be more common on plants lacking vigour, so their presence could indicate that attention is needed, particularly on old fruit trees and azaleas.
Algae: On tree trunks and leaves of evergreen trees and shrubs, algae can be seen as a green, powdery deposit. It is not unattractive on trunks but can make leaves dull and unsightly. The alga Trentepohlia is seen as a vivid orange powdery deposit on tree trunks and branches.
Lichens: Lichens growing on trees and shrubs are mainly grey to green in colour. They may form as crusty patches, leafy mats, or upright branching or hanging growths on the bark or wood.
Moss: Various mosses can grow on the trunks or branches of trees and shrubs. These mosses may form large, coarse, loose, green or yellowish-green tufts, densely matted tufts, or compact green cushions.
Algae, lichens and moss are found in damp places, as not only do they need moisture for growth but also for reproduction. Lichens 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. Lichens prefer areas with clean air, so are more common in rural districts.
Conditions that favour such growths on branches and twigs include:
- Trees or shrubs which are lacking in vigour, particularly those which are already beginning to die back. In these circumstances the growth of lichen in particular is often unjustly blamed for the poor condition of an affected plant
- Trees and shrubs which have been neglected; especially where the branches have become overcrowded. However, lichens and moss can also appear on vigorous new plants in humid areas and are fairly common in western parts of the UK
- The side of tree trunks facing the prevailing wind and rain may be colonised by moisture-loving mosses and lichens
- The shady side of tree trunks may be colonised by algae, often giving a grey or rusty-red appearance
If algae, lichens and moss are considered unsightly, they can be controlled to some extent by improving air circulation; prune out overcrowded branches and cut back overhanging vegetation. Following this, try to stimulate new growth by feeding, mulching, watering and applying a foliar feed. Once an affected plant regains vigour, badly affected shoots can be pruned out. Control is not necessary on tree trunks.
There are no chemical controls for algae, lichens and moss on plants.
Common name Algae, lichen, moss
Areas affected Trunks and stems of trees and shrubs
Main causes Humid, damp, still conditions and poor plant vigour
Timing Algae are more noticeable after wet weather; mosses and lichens are present year-round, but are more noticeable in winter