This little moss is seldom seen and readily mistaken for young plants of Atrichum undulatum, whose leaves are often not undulate. A good field character for A. tenellum can be seen by examining the upper surface of the nerve with a hand lens, where several dark parallel bands will be easily visible. The bands are the lamellae, which in this species are few in number, relatively high and well spaced. A. undulatum usually has more lamellae, which are squished together on the nerve and so are less obvious through a hand-lens.
Atrichum undulatum sometimes behaves as a pioneer on damp, clay-rich disturbed ground and in places such as reservoir margins, which may have only been exposed for a few months, small plants without undulate leaves may be encountered. To be absolutely sure about the identification, it’s critical to cut leaf sections (use a brand new, sharp razor blade) so you can see the lamellae clearly. Images on this page show more details.Read the Field Guide account