The best time to divide water lilies is in early spring once the pond has thawed and the lilies are beginning to sport new growth. Some pre-emptive gardeners choose to divide their water lilies every year. However, water lilies only need to be divided when their production of leaves and pads drops considerably. This is a sign that their roots have become too large to effectively absorb nutrients from their container.
Remove the water lily from its container.
Use a hose to wash off all the soil from its roots.
Select the rhizomes that have the most new root growth (located near the top of the rhizome) and cut them away from the larger mass with a sharp, sterilized knife.
Trim away all of the old and new root growth, one-third of the rhizomes' length from the bottom, and any damaged foliage.
Fill the container three-quarters full with aquatic planting soil and 10 grams of 10-20-10 fertilizer for every gallon of soil.
Plant each divided tuber into its own plant so that it is growing at roughly the same height that it was in its original container.
Cover the top of the soil with a 1-inch layer of pea gravel.
Submerge newly divided water lilies in roughly 1 foot of water until you see new leaf growth. Once they have sprouted a few leaves, they can be moved to a deeper, more permanent home.