Thatch moss  (Leptodontium gemmascens)

Thatch moss, as its name implies, is sometimes found on old thatched roofs. In the semi-natural state, it is found on the decaying bases of rush and grass tussocks, occasionally on rabbit droppings, and very rarely, on trees. It forms dull-green patches. The tiny leaves are spear-shaped with reproductive structures called gemmae at the tips. These disperse and aid the spread of the moss, and no other British moss has these gemmae on the tip of its leaves.

This moss is distributed widely over Europe but is known to be rare everywhere. In Britain, it used to be found on thatched roofs across much of southern England. Today, it is restricted to three sites on thatch in Wiltshire, Dorset and West Sussex, with a recent discovery in Somerset. It is also found in four semi-natural locations on grass tussocks in Norfolk, Suffolk, Hertfordshire and Greater London.

The moss is found in association with decaying vegetation, usually in acid soil areas.

Mosses are among the lower order of plants, which includes liverworts and hornworts. They are very different from the higher plants in that they do not have true leaves, that is, leaves furnished with ribs and pores. Neither do they possess roots, but mosses do have structures called rhizoids which anchor the moss to the surface on which it grows. Mosses usually reproduce by spores, but also spread by budding off small clusters of cells and by fragmentation.

Thatch moss has suffered through changes in thatching methods. Thatched roofs are much less common than in the past and thatch is replaced more frequently as owners are more affluent and can afford to do so. This does not allow the moss to form colonies, and the re-roofing process destroys those that do get established. The zinc-coated wire, used to prevent bird damage, may also be toxic to the moss.In its semi-natural habitats, thatch moss suffers from drought conditions, and the encroachment of scrub.

Thatch moss is listed in the UK Biodiversity Action Plans (UKBAPs), and included in English Nature's Species Recovery Programme (SRP). Its status is still precarious, and it is important to preserve the existing populations of the moss.

Several of the semi-natural sites are managed as nature reserves, and scrub control is being carried out to maintain the right conditions for the moss. Surveys are being conducted to find new populations. Thatch moss was found recently on the thatched roofs of the National Trust's estate at Honeychurch in Somerset, and they are taking steps to conserve it. It may prove possible to alter modern thatching methods so they are more thatch moss friendly. Advice is being provided to owners of thatched properties on which the moss grows to ensure it is preserved when any re-roofing takes place.