To make a carpet, you can just tie riccia with fishing line onto stainless steel meshes, riccia side facing up. Then just lay the meshes side by side; the riccia will carpet the whole mesh really, really fast.
Another way would be to lay the riccia on a flat surface, such as a rock, then secure the riccia with coarse netting (mosquito net, plant net, etc). The riccia will grow out of the mesh and create a lush carpet.
A tip when growing riccia submerged: riccia grows best in lower temperatures, so try to keep the water cool with a fan.
Hope that helps!
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