A koi pond makes a beautiful addition to any landscape, and it's educational as well. Taking care of a pond is a wonderful way to teach your children about habitats and ecosystems. Plants compose an essential part of any koi pond, and many pond owners include exceptionally attractive water lilies (Nymphaea spp.). Water lilies can be loosely divided into two groups: Hardy water lilies are perennials in U.S. Department of Agriculture plant hardiness zones 3 through 9, while tropical lilies survive year-round in USDA zones 10 and warmer. Tropical water lilies can also be grown as annuals in cooler climates.
Choose a container at least 15 inches wide and 10 inches deep for hardy water lilies. For tropical water lilies, choose one that is twice as wide as it is deep. Containers that are 20 inches wide and 10 inches deep work well for tropical lilies, according to the University of Illinois.
Fill the container about two-thirds of the way full of heavy clay loam soil. Lighter mixes, such as potting soils, do not work well for water lilies. Then dampen the soil so it is moist but not waterlogged.
Place a lily rhizome on top of the soil. Angle hardy lily rhizomes at about a 45-degree angle so their growing points are pointing downward and into the center of the pot. The growing point will be covered with "eyes." Each stem will sprout from an eye. The other end of the rhizome should almost be touching the edge of the pot. Do the same with tropical lily rhizomes, only center them exactly in the middle of the pot.
Cover the rhizome with soil until the growing point is just barely peeking out above soil level. This means the other end will be protruding a bit more above the soil level than the growing point.
Insert a fertilizer tablet into the soil. Many are marketed specifically for water lilies, but any tablet or granular fertilizer with a ratio of 20-10-5, 10-6-4, 5-10-5 or 12-8-8 will work, according to the University of Illinois Extension. The recommended dose is about 4 ounces for every cubic foot of soil. Give the lilies another tablet each month until the end of the growing season in August.
Add about 1/2 inch or so of pea gravel on top of the soil to help stabilize it and prevent fish from digging into the soil to nibble on the rhizome.
Water the soil, and then submerge the container into the water. Hardy lily pots should be placed at a depth of between 12 and 18 inches as measured from the top of the pot once it is underwater. Tropical lilies should be placed about 12 inches under the water. Use bricks or flat stones, if needed, to raise a container so it sits at the appropriate level for water lily growth. Both types of lilies should be located in a spot that receives at least six hours of sunlight per day.
Cut off any yellow lily pads by clipping the stems under the water, near the rhizome, after they die.