Encouraging lichens to grow on new surfaces

A new concrete or rendered building can often be a glaring eyesore, and even a new, white garden trough may look out of place amongst the earthy tones of its surroundings. The Design Council suggests that a building will fit more harmoniously into the landscape when it appears darker than its surroundings. The question is then often asked as to how it might be more rapidly toned down and given an older, more muted appearance. If left alone a natural succession will take place, so that eventually it becomes covered with the mottled camouflage of lichens, algae and mosses. New concrete is too alkaline (about pH 11 or higher) for most living organisms to survive on it. It may take as long as five years for this alkalinity to drop down to a level (about pH 8) where colonisation can take place. It might take 12 years for 60% of the surface to be covered and as much as 20 years for this cover to become complete. In urban environments the higher levels of acid pollution may bring about this change more rapidly. At the same time, the number of species that can survive in this polluted situation is reduced and the colourful surfaces found in clean air districts may not be achievable. Conversely, the neutralising effect of the very alkaline concrete reduces the acidity and it has been seen that, within four years in some urban areas, 80% of the surface of new concrete coping stones was already covered with small lichens.

A roughish texture is best for colonisation as this enables air-borne dust and the small incipient lichens, algae and mosses that land on the surface to lodge there and not be immediately carried away again by the wind. On the damper northern side of a building the first plants to colonise the wall or roof are probably green algae. These may form a layer which holds the moisture and dust particles enabling this algal layer to become rather thick and thus often preventing colonisation by lichens. If a soil, consisting of dust and broken down plant remains, builds up in the cracks this will enable mosses to grow. In time the soil in the cracks may well be sufficient for flowering plants. On the drier, better lit surfaces there will be fewer algae and it is the lichens that are more suited to this harsh regime of wetting and rapid drying out. Small, inconspicuous lichens are likely to be the first to appear, but by their great numbers they can quickly soften the colour of a harsh surface. These are normally followed by larger, grey and orange, lichens that will in time give a wall an attractive weathered appearance. If this surface is, by some means, nutrient enriched this will encourage the large orange, leafy lichens (Xanthoria species) and also a number of grey species (eg Diploicia canescens). These orange patches will often be seen under television aerials, chimneys and overhanging branches. where birds find good perching points. It could be useful therefore to provide perches to ensure a steady supply of nutrients to a surface where the growth of lichens is to be encouraged.

The levels of air-borne sulphurous pollution have been dropping and this has enabled many species to return to areas where they have been absent for over 100 years. Indeed the appearance on concrete paving stone in Middlesex of the grey-green lichen Lecanora muralis caused some consternation. It was not recognised as a lichen by many people and correspondence to the press suggested that it had come from outer space! As the pollution levels drop, it should become easier to encourage the growth of these larger, more sensitive lichens.

The microhabitat of the building will have a profound effect on which species will grow at a particular site. A low-pitched, north-facing roof will be colonised most readily by mosses. A steep-pitched, north-facing roof will frequently be covered in grey Physcia species as well as the mosses. A south-facing roof is better illuminated and also drier and would favour the yellow Xanthoria and other sun-loving species such as Lecanora muralis. Any shading from other buildings or trees. will also affect the range of species that colonise the site. Overhanging trees may increase the nutrient enrichment not only from bird droppings but also through pollen, dust and the "honey-dew" from aphids. Damp retained in cracks on roofs, or between tiles, will help the colonisation of mosses and lichens. If soft mortar is used in walls, this will retain the moisture for longer periods. Soft or cement mortars will, over a period, leach and make the surrounding brick-work more alkaline and therefore more suitable for the species that need an alkaline situation. Shaded damp sites will also encourage the growth of green algae.

Over the past few years many different substances have been painted onto buildings to encourage more rapid colonisation. These include yoghurt, beer, skimmed milk, thin porridge and, in Japan, rice water. To all these substances a small quantity of PVA (polyvinyl acetate) adhesive may be added. This acts as a binder, improves the adhesion of the nutrient and possibly allowing more gradual release over a longer period. On very alkaline materials, such as new concrete, a slightly acid substance will assist in neutralising the high alkalinity. Dilute cow slurry is frequently used, the urine present providing the acid content and the brown staining caused by the slurry giving an immediate toning down of the concrete. Little work so far has been done to determine the frequency of application or strength required. The evidence from those who have tried these methods seems to show that they work. Various timings have been suggested but it is probably worth trying about four applications at yearly intervals. Even a single application would probably assist, but due to the very alkaline nature of new concrete it would be more effective to give at least a second coat after about two years. On more acid stones, such as granite and sandstone, it is suggested that, especially in polluted areas, powdered chalk is added to the mixture to neutralise this acidity to some extent. To aid colonisation, coarsely ground up pieces of lichen can be added to the mixture before it is painted onto the surface. If this is done, care should be taken to use only lichens that are growing abundantly in the local area, and which are found in a similar microhabitat to that on which they are placed. In this way, the chances of success are greatly increased, as many of the lichens require fairly precise conditions in which to grow.

Removing lichens from surfaces

The grey and orange patches formed by lichens on gravestones give a distinctive character to a churchyard. These attractive "time-stains" not only enhance the appearance of the churchyard but are often of some rarity for which, like many other organisms, the churchyard is awildlife sanctuary. Many lichens require a particular type of stone on which to live and, in many lowland districts, the churchyard may be the only undisturbed location in the area for many of these types of stones.

There are differing views as to whether lichens damage the stone on which they are growing or whether they protect it. There is evidence that the acid substances produced by lichens can attack the stone, but this effect is limited to a very thin layer immediately under the lichen. Any small cracks present or caused by this process will probably be infiltrated by the fine root-like hairs (fungal hyphae) of the lichen and this may cause more damage. It has, however, been argued that any damage caused by these processes is less than would be brought about by the weather if the lichen was not present. The tough, rather thick, lichen can protect the underlying stone from the weathering effects of wind, rain and frost. On some soft stones in exposed sites the lichens may eventually cover raised areas where the surrounding stone has been eroded away by natural weathering.

In some circumstances it may be necessary to remove lichens and various methods have been used with success. If the purpose is to enable an inscription to be read, other ways of doing this should be tried first before the removal of the lichens. These methods, to increase the clarity of an inscription, include wetting or looking at it in the twilight with a torch shone along the inscription on a gravestone at a low angle. This will enable many worn inscriptions to be read. If it is deemed that cleaning is essential, only the minimum area necessary should be treated. This may be done by physically rubbing the lichens from the surface. Where this is done on a smooth stone the result may be unsightly as it is almost impossible to remove many crusty lichens from the lettering of the inscription. The lichens remaining in the lettering and cracks will probably regrow but rare lichens may have been lost from the surface. Another physical method that has been used is to cover the area to be cleaned with black polythene. It may take some months for the lichens to die but they may then be removed with a stiff brush. A more practical method may be to use fungicides such as benomyl, cheshunt compound, thiophanate-methyl etc. Domestic bleach has also been used with effect.

Whatever method is used care should be taken to treat as small an area as possible and not allow the chemicals to drip onto adjacent parts of the stone or statue. Before commencing try to get an experienced lichenologist to check that there are no rare lichens present. Remember, before you kill them, that these lichens may have been growing on the stone for several hundred years and so, if possible, please allow the colour to remain on the cupid's cheeks.

© British Lichen Society 1996

This [article] (with line illustrations) is obtainable, free of charge, from the British Lichen Society.