Most of the mosses people keep underwater came from warm tropical streams and reached the hobby through the aquarium trade. Willow moss did neither. It is a true native of cold rivers across Europe and North America, a coarse dark plant that streams out in the current like green hair, and for anyone with a wildlife pond or a coldwater tank it is the natural moss to grow.
Recognising it
Fontinalis antipyretica grows as long, branching strands, a hand's length or a good deal more, anchored at one end to stone or wood and trailing free for the rest. The leaves give it away. Set in three ranks along the stem, each is folded sharply down its midline into a keel, so a shoot held up to the light looks distinctly three-sided rather than round or flat. The colour runs from deep olive to a near-blackish green, darker than any tropical moss, and lifted out of water the whole plant collapses into limp, slippery ropes. Returned to the water it recovers at once, fanning out wherever the flow carries it.
Where it grows
This is a plant of moving, well-oxygenated, cool water. Look for it in clean streams and rivers, draped over submerged boulders and fallen branches, below the spill of weirs and along stony lake shores where the waves keep the water stirred. It puts up with a fair spread of conditions but draws the line at warmth and stagnation, so a sun-baked, sluggish pond in high summer is the one place it sulks. Since it fixes itself by tough anchoring threads rather than true roots and feeds straight from the water around it, the quality of that water counts for more than whatever it happens to sit on.
A name that means against fire
The species name puzzles everyone who meets it. Antipyretica is built from the Greek for "against fire", and the reason traces back to Linnaeus, who recorded that people in northern Sweden packed the dried moss into the gap between a chimney and the timber wall of a house. Rammed in thick, it was thought to lessen the chance of the wall taking light, a humble bit of fireproofing from an age before mineral wool. Whether it answered as well as hoped is hard to judge now, yet the name has held for two and a half centuries, a small fossil of old country practice carried along in the Latin.
In ponds and coldwater tanks
Willow moss suits the very setting that defeats the warm-water mosses. In a wildlife pond it oxygenates the water, shelters tadpoles and the larvae of dragonflies and caddis, and gives newts a place to fold their eggs; few submerged plants do as much for so little fuss. Under glass it belongs in coldwater and native setups, among white cloud minnows, sticklebacks or a temperate shrimp, rather than the heated tropical tanks where Java moss reigns. Wedge or tie a strand against rock or bogwood and it takes hold of its own accord in time. Tropical keepers who try it usually watch it melt away in the heat, which is partly why the warm-tank species are gathered separately in the aquarium mosses compared.
What a healthy stand tells you
Because willow moss absorbs everything across its surface, it takes up dissolved metals and pollutants and locks them away in its tissue. Freshwater ecologists turn this to use, lowering bags of the moss into rivers and later measuring what has gathered inside, a cheap living gauge of how clean a watercourse runs. For the pond keeper the reading goes the other way: a thriving stand is fair evidence that the water is cool, clean and well aired. That same knack for harbouring small creatures puts it among the more rewarding mosses for a wildlife garden, a thread picked up in moss and wildlife.
Keeping it
Cool water, a little movement and modest light are about all it asks. A pump, a filter return or the trickle off a small waterfall supplies the flow it prefers; still water it accepts only while the temperature stays down. Growth is unhurried, so put aside any hope of the quick sprawl a tropical moss gives, and thin it by pulling surplus strands once it crowds its neighbours. Should you lift a piece from a wild river, take a small fragment from where it grows thickly and leave the rest to close over, the same restraint laid out in collecting moss responsibly. Settled and content, it wants next to nothing and quietly does a great deal.