Hornbeam hedge: Lichens unfairly blamed for lack of vigour in plants and trees

Question: I am enclosing a piece of my hornbeam hedge which encloses my front north-facing garden. Can you identify the growth on the twigs and tell me how to treat it? (R Pettit, North Walsham)

Answer:

The problem on your tree is lichens and falls into a group of algae, lichens and moss that 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 completely harmless, although may occasionally be an indicator of a lack of vigour in the affected plant.

Algae, lichens and moss are non-parasitic plant-like organisms that colonise bark, rock and other hard surfaces.


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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 welcome mature look to a garden, and they prefer damp areas where there is minimal air movement.

However, growths of algae, lichens and moss may be more common on plants lacking vigour, so their presence could indicate that some remedial attention is needed, particularly so in they are growing on old fruit trees and azaleas.

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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.

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, and 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. This is especially the case where the branches have gradually become overcrowded.

However, just to complicate the issue you should be aware that 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.

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 but you could try a winter wash suitable for fruit trees.

•This article was first published on December 31, 2011.

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