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Hardy Water Lilies are the mainstay of the water garden. Most species grow rapidly and need repotting every couple of years- at least! They are called hardy because they can survive through freezing winters in the pond. (Tropical lilies won't withstand freezing and have different requirements.)
You know your hardy lily is ready to repot when the roots are jumping out of the pot and running around the pond. Other signs are that the plant is utterly root bound and is not blooming well, or the pot is beginning to split. Some of the larger varieties will split a plastic pot in a couple of years.
The rough, knotted root is called a rhizome. When you remove the plant from its pot, rinse the root with water from the garden hose. You will see along the rhizome "crowns" where groups of new lily pads emerge. Using a sharp knife, slice between these crowns and each section can become a new plant. Compost any sections that are mushy or look rotton. You may end up composting good sections too if your pond is overloaded with lilies and you can't find anyone to share with!
When you repot your hardy lily, provide it with the largest diameter pot you can get. Depth is not of great concern since it has a shallow root system. New oil change pans make great water lily pots! Hardy lilies bloom best if they are not divided often so you want to provide it with as large a container as you can manage. Remember that you will eventually have to haul that sucker out of the pond, so don't go overboard!
The most common potting medium is heavy clay or topsoil, but these cause the water to become cloudy. Hardy lilies will happily grow in plain sand or generic clay cat litter (no additives, please!) Gravel will work, too, but is not highly recommended because it makes dividing more difficult when the roots have woven the rocks into a large mass. If you opt for clay or topsoil, be sure to put a layer of gravel over the top to prevent it from escaping into the water. The amount that does cloud the water will settle soon.
Hardy water lilies that are grown in ponds with no fish may need occassional fertilizing to produce great blooms. Be careful about over fertilizing, though, because the fertilizer also feeds the algae! Hardy lilies grown in ponds with fish will be fertilized by fish waste.
Infrequent division of the rhizome will produce the best blooms. Try to divide your hardy lilies no more than once in three years. Sometimes they will grow too much, though, and it will not harm the plant to divide it more often.
Most of them require at least six hours of direct sun to bloom well. A few varieties bloom on as few as three hours of light. These guidelines may vary depending on how long and hot summers are where you live.
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