Wabi-kusa is a quietly beautiful corner of the aquascaping world: a mound or ball of substrate planted with aquatic plants and moss, grown emersed, up out of the water, as a living arrangement that can sit in a shallow dish or be dropped into a tank later. Moss is one of its natural ingredients.
What it is
The name nods to the wabi-sabi aesthetic of rustic, imperfect simplicity. In practice a wabi-kusa is a free-standing ball or low mound of soil, often bound with a little mesh or simply shaped by hand, planted with a mix of aquarium plants and moss and kept in humid air rather than underwater. The plants grow in their emersed forms, flowering and spreading in ways they never do submerged, and the whole thing reads as a miniature meadow on a dish.
Where moss comes in
Moss clothes the wabi-kusa as it does any hardscape, softening the base, knitting the planting together and holding moisture at the surface. The same aquarium mosses used elsewhere, Java moss and its relatives, take to emersed life readily and spread over the mound. Because the arrangement lives in damp air under a cover, moss thrives on it far more easily than it would on a dry surface in an open room.
Why moss takes to emersed life
It helps to know that these aquarium "mosses" are true mosses, the same broad group as the moss on a garden wall, simply species that will also grow underwater. Out of the water they behave like any land moss: they take up water across their whole surface, have no roots to speak of, and only need the air around them kept humid to stay lush. Under a cover the moisture they lose is trapped and recycled, so they never dry out. That is why a wabi-kusa in a lidded dish is such easy moss country, and why the same moss struggles on an open ornament in a heated room. More on that difference is in keeping moss as a houseplant.
Choosing moss and plants
Java moss is the reliable starting moss, undemanding and quick to spread over the mound, with Christmas and weeping moss giving a finer, more deliberate texture once you have the knack. For the plants, pick low, spreading aquarium species that grow willingly out of the water, which most stem plants and carpeting plants do. A restrained mix reads better than a crowd; a couple of contrasting leaf shapes with moss knitting between them is plenty, and it leaves room for the arrangement to fill in as it settles.
Making and growing one
Shape a ball of aquatic soil, sometimes around a core to hold its form, press in your chosen plants and a thin scatter of moss fragments, and stand it in a shallow dish with a little water, under a cover or in a humid spot with good light. Mist it, crack the cover daily for a little air to keep mould down, and let it grow in over several weeks; the technique is the same emersed growing described in the dry start method. A thin scatter of moss beats a thick pad, which tends to rot in the middle before it takes hold.
Display it, or flood it
Kept as a display, a wabi-kusa is an arrangement in its own right, changing through the season as its plants come and go. Dropped into a flooded tank it becomes an instant planted scape, and the moss and plants gradually convert to their submerged forms over the following weeks, which can mean some melt-back and regrowth as they adjust. Either way the moss earns its place, holding the mound together above the water and carrying on below it. It is a good first project for anyone curious about growing moss out of the tank, and it sits among the wider family of moss balls and mounds in things to make with moss.