Here’s a curious contradiction: hardy water lilies (Nymphaea spp.), even those growing naturally way up north in the snow-covered boreal forest, are incapable of tolerating freezing! That’s because, even in the Far North, the water of a lake rarely freezes very deeply. So even aquatic plants adapted to very cold climates have never had to adapt to freezing. Therefore if you’re growing hardy water lilies in a garden pond, you have to take that into consideration.
In regions where ice comes and goes in the winter (usually zones 8 and above, as well as in certain zone 7 areas), the water in a pond will never freeze to any great depth and you can put your hardy water lilies just about anywhere as long as the crown is covered with water at all times, but in any climate where solid ice does form, you have to assume that the water will freeze to a depth of 12 to 16 inches (30-40 cm). Since water lilies are usually grown in pots placed on the bottom of the pond, that elevates their crown to about 8 inches (20 cm) above the bottom. In other words, if you add the two together, you’re essentially looking at needing a pond depth of 2 feet (60 cm) or more to ensure that the crown is not in danger of freezing.
If your pond is over 2 feet (60 cm) deep, just move your water lily pots to the deepest part for the winter. Or, if you or a neighbor has a swimming pool, place them at the bottom of the pool for the winter. Another possibility is to dig a trench in the garden and to bury the pots below the frost level.
The easiest solution, though, is to find cultivars capable of growing at a good depth (60 cm or more), such as ‘Sultan’, ‘Comanche’, ‘Attraction’ or ‘Moorei’, and to simply place them at that depth permanently. Yes, they’ll be slower off the mark in the spring (the deeper a water lily grows, the longer it takes for its leaves to reach the water’s surface), but at least you’ve saved yourself the effort of sloshing knee-deep through the cold water of a pond each spring and fall as you carry your water lily pots back and forth.