Anne, since this is a bathtub with a drain in the end, you could attach a piece of pipe to the the drain, outside the tub. Then add a shutoff valve. This would allow you to fill the tub and flood the plant, then open the valve to drain just enough water so that the plants are wet but not submerged. Periodically you can open the valve and add fresh water to the top, allowing it to replace the old water that has been standing in the tub. Then shut it off and allow the tub to fill again. You would simulate a natural bog, which floods during rainstorms then slowly drain, but never becoming dry.
