Hoary rock-moss (Racomitrium lanuginosum) belongs to a group of mosses that collectively are known as rock-mosses. Almost all of them are found on very dry sites usually on rock outcrops and boulders or on talus slopes. Originally, they were a large group of mosses belonging to the genus Racomitrium which has now been split into four genera that include Racomitrium, Niphotrichum, Codriophous, and Bucklandiella. Hoary rock-moss is only one of three species left in the genus Racomitrium and the only Racomitrium species that occurs in North America.

Many of the species in these genera often form turfs with branching stems and leaves with white hair points and many species can be very difficult to tell apart even with careful microscope examination. Hoary rock-moss is rare in the Kamloops area but it can be very common at high elevations in dry alpine areas often on rocky substrates.

It can easily be confused with species in the other three genera. However, the distinctive feature of hoary rock-moss that distinguishes it from everything else is the margins of the hair-points have distinctive scalloped edges which can be seen with a small 10X hand lens in the field.

When viewed under a microscope, the leaf cells are covered in distinctive bumps called papillae. You can also see how the edge of the white leaf tip is regularly “scalloped” along the edge giving this moss a its key field identification feature.

Like many rock-mosses, the leaves as shown below in cross-section are mostly one leaf-cell thick and are deeply concave with leaf margins that curve downwards and inwards (“recurved leaf margins” is the technical term to describe these leaf margins).

Another distinctive feature of rock-mosses are that the leaf cells are often sinuous and contorted particularly near the base of the leaf.