pillwort

Pillwort is not a carpeting plant, whatever its appearance suggests. The thin upright shoots look like hairgrass, but they arise sparsely from a slowly creeping rhizome and stay scattered rather than knitting into turf. Expect a loose stand of green filaments, and it becomes a useful plant rather than a disappointing one.

It is a fern, related to the four-leaf-clover Marsilea species though it looks nothing like them. Its shoots are coiled fronds that unroll as they extend, in the way ferns do. The common name comes from the small spherical sporocarps produced at the base of the shoots, which are the plant's spore capsules and are unlike anything else growing on a European pond margin.

It grows wild on the muddy edges of ponds, lakes and slow streams across temperate Europe, and it has declined enough across much of that range to be legally protected in several countries, so cultivated stock is the only sensible source. In the tank it is a cool-water plant: 18 to 24 °C and pH 5.5 to 7 in soft water suit it, and sustained warmth past the mid twenties weakens it until it fades out. CO2 is optional. Growth is slow. Propagate by dividing the rhizome with shoots attached; spores are produced but rarely germinate in aquarium conditions.