Do I just trim it and stick the top back in the gravel? Will the bottom portion grow another top? I love the look of really bushy cabomba! I only have around five stalks right now at about 12" in length.
Yes, just trim the tops and replant. The old one won't continue to grow from the part that you cut but will send off side shoots giving it a fuller appearance.
Not really, I usually try to cut just above the last leaves so there isn't a bunch of stalk sticking out whilst it re-grows. Don't worry, it is near impossible to screw it up. lol