If you live somewhere that freezes in winter, you don't necessarily need to replace all of your pond plants every spring. Many hardy aquatic plants can stay outside all winter—as long as their roots, crowns or rhizomes are protected from freezing.
The important part is knowing what kind of plants you have. Hardy plants, tropical plants, water lilies, marginals and floating plants don't all need the same winter care.
Start With Hardy vs. Tropical
Before moving anything, identify whether the plant is hardy in your area.
A hardy aquatic plant can tolerate winter conditions within its recommended growing zones. Many hardy water lilies, cattails, rushes and iris fall into this category.
Tropical aquatic plants are different. Water lettuce, water hyacinth, tropical water lilies and many tropical marginals won't survive freezing conditions.
Don't rely on labels like "marginal," "bog plant" or "oxygenator" alone. Those terms tell you how or where a plant grows—not necessarily how cold-hardy it is.
When you're unsure, look up the specific plant or variety and compare its hardiness rating with your USDA growing zone.
How Do I Winterize Hardy Marginal Plants?
Start by removing dead, yellowing or damaged foliage. This keeps unnecessary organic material from breaking down in the pond over winter.
For hardy marginals such as many cattails, rushes and iris, trim dead foliage back as the plant goes dormant.
Where the plant spends the winter depends on the species and your climate. Some can remain at their normal planting depth. Others should be lowered deeper into the pond so the crown or rhizome remains below the portion of the pond that freezes.
That's the important part: protect the living portion of the plant from freezing solid.
If your pond is too shallow to provide that protection, move the plant to a cool, protected location that stays above freezing. Keep dormant plants damp so the roots and rhizomes don't dry out.
What Should I Do With Hardy Water Lilies?
Hardy water lilies naturally go dormant as temperatures fall.
Remove dead or yellowing leaves and spent flowers. Small, healthy growth near the crown can remain until it dies back naturally.
Before the pond freezes, lower the container into deeper water where the rhizome will remain below the freeze line.
The exact depth isn't the same for every pond. A pond in a mild climate may barely develop ice, while a shallow pond in a northern climate can freeze much deeper. What matters is keeping the rhizome from freezing.
If your pond is too shallow or is an above-ground container that may freeze solid, move the lily to a protected, above-freezing location for winter.
What About Submerged or Oxygenating Plants?
Some submerged aquatic plants can overwinter right in the pond, while others can't tolerate freezing temperatures.
For hardy species, remove excessive or dying growth and keep the remaining plant below the freeze line. Before assuming an oxygenating plant can stay outside, check the hardiness of the specific species you have.
Also remember that plant activity slows considerably as water temperatures fall. Your pond's winter oxygen and gas-exchange needs shouldn't rely on aquatic plants alone.
If fish are overwintering in the pond, maintaining proper winter aeration and an opening in the ice is a separate part of winter pond care.
Can I Keep Water Lettuce and Water Hyacinth Over Winter?
Technically, yes. Practically? Most northern pond owners are better off treating them as annuals.
Water lettuce and water hyacinth are tropical plants and will not survive freezing temperatures. Keeping them alive indoors requires warm water, strong light and enough space to maintain them for several months.
For many pond owners, the equipment and effort required aren't worth it compared with replacing the plants in spring.
Either bring them indoors before frost or remove them from the pond before they die and begin decomposing in the water.
How Do I Overwinter Tropical Marginal Plants?
If a marginal plant isn't hardy in your climate, bring it indoors before freezing temperatures damage it.
Depending on the species, some tropical marginals can be kept in containers of water while others can be grown more like houseplants through winter.
Trim dead or damaged foliage before bringing plants inside and check them for insects. Keep them in conditions appropriate for that specific plant rather than assuming every tropical marginal needs the same amount of water, light or warmth.
Growth will usually slow during winter, so fertilizer needs are also reduced.
How Do I Overwinter Tropical Water Lilies?
Tropical water lilies will not survive a northern winter outdoors if their tubers freeze.
If you want to keep one from year to year, it needs to be brought inside and overwintered using a method appropriate for that variety. Depending on the lily, that may mean keeping the plant growing in warm water under strong light or allowing it to go dormant and storing the tuber in protected conditions.
If you don't want to maintain a tropical lily indoors, there's nothing wrong with treating it as an annual and replacing it in spring.
Don't Forget the Rest of the Pond
Plant care is only one part of preparing a pond for winter.
Remove fallen leaves and dying vegetation before they have a chance to accumulate on the bottom. Organic debris that sits in the pond continues to break down, contributing to muck and nutrient buildup.
If fish will remain in the pond, winter aeration can also help maintain gas exchange when ice forms. The goal isn't to aggressively circulate the entire pond. It's to maintain appropriate winter conditions while keeping an opening in the ice.
A little cleanup in fall can save you quite a bit of work when the pond wakes back up in spring.
A Little Fall Prep Goes a Long Way
Winterizing pond plants isn't about doing the same thing to every plant in the pond.
Figure out what's hardy in your climate, protect the parts of hardy plants that need to stay below freezing conditions, remove dying vegetation and bring tropical plants indoors if you want to keep them for next year.
Do that, and you'll give your pond plants a much better start when warmer weather returns.