From the perspective of the hardy water lily (Nymphaea spp.), the approach of winter is not a threat but a signal to enter a state of dormancy. As water temperatures drop and daylight hours shorten, the plant's metabolic processes slow down significantly. The plant ceases producing new leaves and flowers, and its energy focus shifts entirely downward. The goal is no longer growth and reproduction but pure survival. The lily's vital life force retreats into its thick, buried rhizome, which acts as a stored energy bank. The leaves and stems above will naturally die back, a process that is both expected and necessary for the plant's health cycle.
Allowing the plant's foliage to yellow and brown naturally is a critical first step. This process allows the lily to reabsorb valuable nutrients from its leaves back into the rhizome for storage over winter. Once the foliage is clearly dead and softened, it should be trimmed away. Using sharp, clean shears, cut the leaves and stems back to within a few inches of the base of the rhizome. This is crucial for the plant's health, as decaying organic matter left in the pond will sink and decompose, releasing toxic gases like hydrogen sulfide that can harm the dormant rhizome below the ice. Removing this material creates a cleaner environment for the plant's rest period.
The single most important factor for a hardy water lily's winter survival is the depth of water above its planting container. The rhizome must remain in unfrozen water throughout the winter. If a pond is deep enough—typically 18 to 24 inches (45-60 cm) at its deepest point—the rhizome will be safely below the ice line. In ponds of sufficient depth, the lilies can remain in place. The constant temperature of the water just above freezing at the bottom provides a stable environment, preventing the rhizome from undergoing a freeze-thaw cycle that would be fatal. The plant relies on this consistent, cold, and liquid environment to maintain its dormant state without suffering cell damage.
In shallower ponds that may freeze completely, the plant cannot survive in situ. In this case, the gardener must act to protect the rhizome. The pot should be removed from the pond after the first hard frost. The remaining foliage is trimmed back, and the entire container can be moved to a cool, dark, and frost-free place, such as an unheated garage or basement. The critical need for the plant is that the rhizome does not dry out completely. The pot should be placed in a plastic bag or covered with damp burlap or newspaper to retain moisture, and the soil should be checked periodically to ensure it remains slightly damp but not waterlogged. This simulated cold, damp environment mimics its natural pond bottom habitat until it can be returned to the pond in spring.
