Most hardy plants naturally fall into a state of dormancy as winter approaches, but before then, there are some maintenance steps you can take to prepare them for winter. To winterize hardy pond plants, wait until the first hard frost, then use your pruning tools to trim off foliage even with the top of the pot. This will divert the plants' energy to their already existing, healthy growth and will prevent decaying plant matter from dropping into the pond. If your plants are in pots, move the pot to the bottom of the deepest part of the pond. In wintertime, the warmest water remains at the bottom, so this is the location best able to protect roots from freezing temperatures. Tender plants like tropical water lilies, floating plants, or other temperature sensitive varieties will need to be brought indoors. Most annual aquatic plants can easily be replaced in the spring and are usually discarded.
It's typically a less expensive option to buy perennial plants that are able to survive through the winter, rather than repurchasing annuals each and every spring. If your plants are happier in warmer temperatures, you may be able to move them inside temporary tanks or tubs where they can be protected from freezing temperatures. Basements and garages can be good options, but don't forget that plants still need sunlight if they haven't gone completely dormant.
