Using containers in your water garden for plants will stabilize the plants as well as make maintenance easier for you. The best plants for containers in a water garden are aquatic plants that will oxygenate the water and aid in reducing the growth of algae. Should you include fish in your water garden, the fish will benefit as well. The container plants will provide shade and hiding places--two things fish enjoy.
Water lilies are a natural choice for your water garden. Lilies float on the surface of the water, but their roots are held in place by a container. They have long stalks that allow them to appear as if they are floating, and not fixed in one spot. You'll want enough water lilies to cover up to 1/3 of the surface of your pond.
There are two types of water lilies, tropical and hardy. Tropical water lilies do not tolerate cooler temperatures and should be treated as annuals with a limited growing season. Hardy lilies are more temperature tolerant, but cannot tolerate freezing temperatures.
Submersible plants live within the water, under the surface of your pond. These plants provide oxygen to the water, and are sometimes referred to as oxygenators. These plants can be aggressive growers, and if planted in an earthen pond, may literally take over. Planting submersibles in containers is a practical way to control them.
Examples of submersibles for water gardens are water fringe, water snowflake, parrot feather and moneywort. Your water garden should be deep enough that the plant tops will remain at least three to six inches beneath the water's surface.
Marginals, sometimes referred to as edge plants, are placed near the edge of your water garden. These plants provide a landscaping element for your garden, as well as aid in reducing algae as they compete for nutrients from the water.
When using containers for your marginal plants, consider using attractive planters. Though the container will be in the water, they will be visible, as you want these plants near the edge. Think of these as partially aquatic plants; they just want to get their feet wet.
Papyrus is an excellent marginal plant, as is the chameleon plant, the Japanese iris and the Chinese water chestnut plant.
Several aquatic plants are prolific in their growth, and may be restricted in some areas. Even if they are potted in containers, and so restrained in your pond, they still may go to seed and once settled in any body of water, will multiply. Always check with your local extension agency to determine which aquatic plants are restricted in your area.