Thanks for all the ideas.
I gave it some thought. In addition to surviving in very wet soil, I need to find a plant that can grow in poor soil (due to constant exposure to the hard water in the waterfall). And I also need something whose roots are shallow and noninvasive (won't burrow into or through lava rock).
I don't know much about the root system of ferns, can they survive with shallow roots and will they burrow?
For that matter, will semp roots burrow into lava rock?
