Algae, Moss and Lichen


A thatched roof is made up entirely from biological material; the surface can provide a habitat and a food source to support the growth of algae, moss and lichens. The unusually wet seasonal weather, in the past five years or so, has created conditions favourable for the growth and colonisation of many surfaces, not just thatch, by specialist plants of wet areas.
Algae forming a biofilm on the north face of a roof in Devon. The algae form a mass of cells embedded in a mucilaginous gel. This holds moisture and inhibits the drying of the roof and when it does dry out, it forms a crust, which becomes brittle and cracks appear in the thatch.

Algae forming a biofilm on the north face of a roof in Devon. The algae form a mass of cells embedded in a mucilaginous gel. This holds moisture and inhibits the drying of the roof and when it does dry out, it forms a crust, which becomes brittle and cracks appear in the thatch.

Algae are a large and diverse group of simple, plants, ranging from single celled to multicellular forms. Algae will colonise thatch in wet conditions, they do no harm but demonstrate that the area is wet. In windy conditions and spells of hot dry weather will destroy colonies of algae Algae forming a biofilm on the north face of a roof in Devon. The algae form a mass of cells embedded in a mucilaginous gel. This holds moisture and inhibits the drying of the roof and when it does dry out, it forms a crust, which becomes brittle and cracks appear in the thatch by drying them out, as this dry material cracks it may flake from the surface, sometimes taking with it fragments of thatch to which it has adhered.
Lichens and moss growing on water reed. Lichens are slow growing and indicate that this roof is not wearing quickly at the surface.

Lichens and moss growing on water reed. Lichens are slow growing and indicate that this roof is not wearing quickly at the surface.

Identifying mosses and lichens can often be confusing; in fact, the two organisms are radically different. Lichens are now considered to be fungi especially adapted to obtain food from algae living within their tissues. Lichens form a fascinating example of cooperative relationships in nature, with the fungus using the algae or bacteria to produce energy, while the algae or bacteria enjoys the protection the fungus provides. Lichens most likely to be found on thatch are leafy or stringy (foliose) these can also be found growing on the ground or around trees.
Moss on north facing roof surface. There is also bird damage on this roof.

Moss on north facing roof surface. There is also bird damage on this roof.

Lichens can be extremely difficult to properly identify, often requiring the use of magnification and specialized staining techniques to discover the mingled identities coming together to make a lichen colony. Moss, on the other hand, is a plant; which reproduces by sending out spores, like lichen, moss can reproduce from broken off parts of the parent plant. In general, mosses grow in moist dark areas and have small leaf-like structures, in addition to stems. Lichens often appear grey or pale white in appearance, while moss is usually green.
Prolonged periods of rain encourage the colonisation of the straw thatch surface with algae and mosses. These hold moisture and prevent the thatch from drying out.

Prolonged periods of rain encourage the colonisation of the straw thatch surface with algae and mosses. These hold moisture and prevent the thatch from drying out.

At present there are no tried and tested chemical treatments for the safe removal of algae or moss from thatch; although there are manufactures with a growing interest in a thatching application for their products. It is illegal to use any commercial chemicals for applications not specified in their license.