needle spike-rush

Most people buy dwarf hairgrass expecting a lawn and get a scattering of thin green spikes instead. The carpet is real, but it is earned: Eleocharis acicularis fills in by sending runners sideways through the substrate, and that takes months rather than weeks unless conditions are pushed.

Light is the first lever. Under strong light the culms stay short and multiply densely; under weak light they reach upward for it and the patch thins into something closer to grass than turf. CO2 is the second, and it is optional in the literal sense: the plant grows without injection but covers ground far more slowly. Trimming the tops with scissors is the third, since cutting encourages lateral runners rather than height.

Chemistry is undemanding. It runs at 20 to 25 °C, accepts pH 5 to 7.5, and handles soft through moderately hard water. It is a genuinely global species, found on every continent except Antarctica, and its wild tolerance extends down to about 4 °C, which is why it also works in unheated and coldwater tanks where most carpeting plants fail.

The technique that works is planting in small clumps of a few culms each, spaced a couple of centimetres apart, rather than pressing in one dense mat. The gaps close from below as runners spread, and the result is even. Established clumps can be lifted, split, and replanted to seed new areas.