Pityrogramma (pityro, scurf or loose scales, and gramma, line, in reference to the lines of sporangia under the powdery scales on the frond's lower surface) is a genus of 16 to 20 species (as currently revised) and is primarily distributed in the Americas.
Pityrogramma calomelanos (calo, beautiful, and melano, black or dark) is probably the greatest of those louche and contemptible, noble and indispensable species, the weed ferns of the world. There are only a few of these worldwide, and these few are mostly national or even more local in habitat.
A cloak of Pityrogramma calomelanos covers a dry stone wall. The fern fabric is made up of dozens of self-sown sporelings. Companion plants include Alternanthera ficoidea (at bottom), a red-leafed Neoregelia (lower right), Iresine herbstii (upper right), and white-flowered Buddleia davidii (center). Photo by George Schenk.
(The Pacific Northwest, for example, is the home of a big, pushy regional fern, Athyrium filix-femina subsp. cyclosorum, which gives fits to meticulous gardeners but is also of major value in casual shade gardening.) Quite the opposite of the regionalist weed ferns, P. calomelanos is a world-beater. From its original range in the New World (the West Indies to Florida to Central and South America) it has ridden the wind and settled into most of the world's tropics and subtropics. It adores the fresh dirt of road embankments and agricultural edges, and delights equally in stone walls, rock garden crevices, and sidewalk margins where feet do not go. It sneaks into flower pots and planters already planted with something the gardener treasures, and there it elbows out the rightful occupant as callously as any cuckoo chick. Yet there are places where its invasion is a betterment, places such as the stone wall which it dresses with fine, rich greenery at no cost to the gardener other than grooming the fern about once a year. This species is a tuft-maker not a stroller on stolons. A form of this variable species known as silver fern wears a coat of silvery white powder on its frond undersides; other forms are powdered yellow, orange, or even pink, or are not powdered at all. (Description by George Schenk.)
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