Planting stems individually will ensure they get the best light from top to bottom. If you don't have yourself a set of these
yet, go get one. Very helpful when it comes scaping time.

That's what I had to do with my Rotala rotundifolia. No problems once they root, but before that, they won't stay in the gravel.If you landscape with rocks, you can also consider sticking the bottom parts of stems into the soil, per Speedie's and Caton's advice, then holding them down with rocks, to keep the stems from floating away when your back is turned... Of course, most of the stem would be above the substrate, and using a small rock or rocks would be okay, just large enough to show who's boss![]()