I've never grown HC before and am not sure how it will work with my plan. Basically, I am going to have a pile of pool filter sand, with medium to large lava rocks sticking out of it. What I'd like to do is grow the HC in the sandy area between the rocks. If I planted a small amount of it, would it naturally only grow/spread in the sand areas, or will it grow over the rocks (which I dont want it to do)? The only other allternative would be to buy enough of it to fill in the sand areas myself, and then trim it alot (which would suck).