We have two answers for this one!
Q. I have a water lily plant that is hanging over the pot in the water and I have never transplanted one of these. Do I cut it off somewhere? Repot it in small pea-rock like I did the others (to eliminate the dirty water)?
A. Water lilies should be planted in poor, clay soil. Keep it in a shallow pot unless you have created a plant pocket in your pond liner. You may want to transplant your lily into a larger pot.
To keep the soil from floating away into the pond, cover with one inch layer of cat litter plus one or two inch layer of gravel.
Dan Eskelson, Clearwater Landscapes, Inc., Idaho
A. You generally want water lilies to hang over the pot, but it sounds like yours is getting over-grown. I assume this is a hardy type lily you’ve let overwinter in your pond. Remove it from the pot and with a sharp knife divide the roots in half. I like to use calcined clay (Oil Dry) as my rooting media for aquatics and I prefer clay pots containers. Just like planting a terrestrial perennial put the roots in the pot and backfill with the calcined to about 3/4 from the top. Then top off the pot with gravel to give it weight and to keep the clay in place. You may also use a fertilizer tab in the clay, but mine do fine without. Submerge the pot in the pond slowly to get the air out. You might want to wedge the pot between some rocks to keep it in place.
Drew Effron, Ball Horticultural Company