Other aquatic plants such as water celery and water pennywort, which over-winter well, root easily, and spread by underground stems, can become weed problems in the landscape if added to the compost pile and not completely composted. These plants should be discarded with the trash. Never discard water garden plants by putting them into lakes, ponds, ditches, streams, or other natural waterways. (That is how so many of the plants we have enjoyed through the years have ended up on the noxious weed list, and we are no longer able to sell or purchase them.)
To store plants successfully, it is important to understand which plants should rest during the winter and which plants are tropical and must continue to grow all year long. Plants that are heavy sleepers actually need to rest during the winter. Without this rest period they may not bloom and could be less vigorous the following year.
The hardiest heavy sleepers are our native plants. Marsh Marigold, Yellow & Blue flag iris, native rush, water sedge, hard-stem bulrush, water plantain and bog bean are all hardy in our bogs and ponds and can be wintered right where they are. To prevent excess sludge in the pond, they can be trimmed back to 2” above the soil after the first freeze.
Most of us have cattails potted or planted in our ponds and they can be left on the ledge to provide vertical interest for much of the winter. Cutting them back before winter creates open wounds that allow bacteria to enter the root system, which can cause them to rot and die. In ponds where they have been allowed to become to vigorous or aggressive, Autumn would be a good time to cut sections back to thin out the excessive growth.
Some arrowhead, variegated sweet flag, floating heart, water iris and the double yellow flag iris can be cut back and dropped to the bottom of the pond for the winter.
Hardy plants that are not hardy to our zone (Zone 5) but hardy to zones 6 and 7 can be wintered in cold storage. Most water plants do not like to dry out entirely even when they are resting. Some water gardeners store them in plastic bags; others remove the gravel from the top of the container and cover the soil with wet peat moss. You can set the pots in a tray and pour a little water over the soil about once a month during the winter. This will also provide humidity in the cold room and is absorbed through the drainage holes of the pots. It is not wise to leave dormant plants sitting in water all winter as this could encourage root rot.
If you do not have a cold storage room that will reliably stay above freezing and below (50°F) you can remove the plants from their pots, wash off the dirt, put them in damp peat moss in plastic bags and store them in the refrigerator. (An old refrigerator, that is not frost-free, is best to store plants as it does not dehydrate them.)
Cardinal flower (Lobelia Cardinalis) should not be wintered submerged. Even where hardy, the plant should be removed form the pond before hard freeze and placed directly in the ground.
Calla Lilies can be set in a corner of the basement and allowed to dry out and go dormant for the winter. Since they are native to southern Africa, they rest during dry conditions, but cold is not required. Since they are vigorous growers they usually need transplanting each year. Young calla lily plants can be kept growing and make beautiful house plants. To prevent them from drying out completely, they should be placed in water and fertilized in about February. Placing them in a bright easterly window will encourage blooms.
Plants that do not rest during the winter months such as Canna, umbrella palm, dwarf umbrella palm and dwarf papyrus that are tropical plants, make beautiful houseplants in a south facing window. They do not rest during the winter, because their natural habitat is a warm region where they continue to grow all year. Since they do not rest, they must be kept under artificial light or in a location where they receive the full benefit of the low winter sun and are not subject to cold winter drafts. To prevent them from drying out, you can set the container in a pail of water and camouflage it in a large decorative container. If stored where there is inadequate light they will stretch tall and become leggy. This is not necessarily a problem, as they can be cut back in early spring, leaving just the new growth. Tropicals that have been divided and cut back acclimate and adjust to the move outside more easily.
I have also wintered Canna, by placing them in crates with moist peat moss and storing them in the crawl space or a root cellar, where the temperature remains above freezing and below (50°F) The tubers of some tender bulbs such as Cannas, can be rather large. Although it is preferable to cut these into smaller pieces before planting in the Spring, it may be necessary to do so prior to storing. Wait until after the first frost when the leaves start to dry and change color. Use a clean knife to severe the tubers at convenient joints. Rinse the knife in weak bleach solution before doing this to prevent contamination from other tubers. When you take the bulbs out of their storage area next spring, you should be surprised at how well they have done. You might even find that you have some bulbs that have sprouted. Remember though that the bulbs need to be removed from the storage material gently and not rushed into the ground next spring. Wait until the ground is well thawed and warm before planting them. By wintering over your frost tender bulbs you will have a new crop of perennials to enjoy for next year.
Taro is another tropical aquatic plant with specific wintering requirements. Since the new leaves emerge from the stem of the previous leaf, the plant should be kept growing in adequate light and moisture throughout the winter.
Other tropicals that should be wintered as houseplants with adequate light & humidity include creeping primrose, parrot's feather (Which we are no longer allowed to sell in the state of Washington), Blue Bell Compacta, Blue Bell Pink, ruby creeper, water hyssop and water zinnia.
Hardy water lilies are heavy sleepers and prefer to rest in a cool place for the winter. To encourage water lilies to prepare for winter they should not be fed after August 15. When the pond is being prepared for winter, many water gardeners lift their water lilies from the bottom of the pond and set them on the plant ledge for the last few weeks. Covered with at least two inches of water they can be left in the cold water until there is a thin layer of ice. Placing them on the ledge makes it easier to reach them when it is time to bring them in or in our climate, prepare them for over wintering in our ponds. The leaves will freeze, but this is not a problem as all leaves are cut off the rhizome before it is stored. It is very important that the rhizome not be exposed to frost or it will rot. cut off, do not pull out the leaves as it may damage the rhizome leaving a wound where rot can begin. After removing all growth you can lower the lily back to the bottom of your pond. If you do not have at least 18” of depth, you can Place the container in a plastic bag in cold storage and monitor it periodically during the winter to ensure it is never dry and there is no mold.
Tropical water lilies are entirely different. In their natural habitat they may continue to grow all year, but they can adapt if stressed by drought. When the lakes dry up, it will force tropical lilies into dormancy and when the rainy season returns the storage tubers begin to grow again. It is best to dry out the plant and store the tuber at about room temperature in a plastic bag, unless you have facilities to winter them indoors. They do not like to be placed in cold storage and the plant certainly does not enjoy cold pond water in the early spring and late fall. They are probably best considered an annual in our climate.
A little understanding of how your plants grow in their natural habitat can provide valuable insight that will allow you to artificially create the most favorable conditions for, and improve your success in, over-wintering your aquatic plants during our cold winters.