Pond plants are hardy or non-hardy, depending on their ability to survive the winter cold, and in particular freezing temperatures. The USDA hardiness zone map provides garden climate information for 11 separate zones in North America, but these are just overall guidelines, and other factors like snow cover, soil drainage and elevation also come into play.
The Japanese iris (Iris ensata) is among the pond plants that require special attention during the winter, according to the Water Garden. Remove this plant from the pond and plant it in the garden until spring, after which it can resume life in the pond, exhibiting blue, lavender, violet-red, pink and white flowers. The same applies to the cardinal flower (Lobelia cardinalis). Add a few inches of mulch, too.
Lovely as they are, water hyacinths (Eichhornia crassipes) and water lettuce (Pistia stratiotes) are not the easiest pond plants to care for during the winter. According to the Water Garden, they require about 10 hours of strong light and temperatures of at least 70 degrees F in order to survive. The easier alternative is to simply replace them in the spring. The University of Florida's Center for Aquatic and Invasive Plants classifies the water hyacinth and the water lettuce as weeds. The water hyacinth's striking lavender flowers and rounded, leathery leaves grow with ease in all types of fresh water. As its name suggests, the water lettuce looks like an open head of very light green lettuce with ridges and the addition of feathery roots.
The Resilient Marsh Marigold
The marsh marigold (Caltha palustris) is a trooper of the winter pond because it requires no special winter care. It will do just fine hibernating at the bottom of the pond at the coldest time of the year, and freezing does not affect it. It is among the plants that stir early in the spring and greet us in April with cheerful, bright yellow flowers. Its rounded green leaves form a mound about 18 inches across. Keep in mind you should bring the marsh marigold up early in the spring so the flowers appear without delay.
Shallow or Deep
Depending on the depth of your pond, hardy water lilies and lotus flowers should survive the freeze if you sink them to the bottom of the pond for the winter. If your pond is shallow and freezes solid, they need to be stored indoors, where they will remain dormant until spring.
Outdoors or Indoors Seamlessly
Some pond plants are also ideal house plants. Just move them indoors when the weather gets cold, with no special requirements except regular watering and sunshine. The umbrella palm (Cyperus involucratus) is an example.