While pond plants are set in containers of all sizes to contain rapid plant growth, many are placed in small containers to allow freedom in water garden design. Large containers, on the other hand, can be used not only to stabilize plants but to provide adequate space for dense plant growth. When using large containers in the pond, consider their depth tolerance.
Because fish such as koi forage the pond floor for food, small plant containers are often knocked over in the process. Tall plants in small containers are even more unstable, easily falling to their sides with the slightest gust of wind or nudge of a fish. Large containers are an easy way to stabilize pond plants from both the elements and koi. Plants such as elephant ears (Colocasia ssp.), cattails (Typha ssp.), canna lilies (Canna ssp.), and umbrella palm (Cyperus alternifolius) can grow over 5 feet tall, making them very top-heavy when planted in small containers. Tall plants greatly benefit from being planted in large containers to help anchor them, especially plants with broad leaves like elephant ears (Colocasia ssp.) and canna lilies (Canna ssp.).
Most ponds will experience an algae problem at some point in their existence. Since sunlight acts as a catalyst for algae reproduction, one way to keep algae growth at bay is to shade the water with pond plants. Floating plants are a great way to provide shade for the pond, but they are tropical in most regions and may become invasive. Water lilies (Nymphaea spp.) and Lotus (Nelumbo ssp.), however, are potted plants that grow prolific leaves to shade the pond and come back every year when cared for properly. Lotus leaves can grow to be 2 to 3 feet in diameter, while water lilies have leaves of all sizes; most, however, do not get larger than 12 inches across. By potting water lilies and lotus in large containers, the plants are allowed to expand, creating more leaves and thereby more shade for the pond.
Plants for Aesthetic Effects
Marginal pond plants come in varying shapes, sizes and textures. Some plants, like canna lilies (Canna ssp.) are spectacular focal points of the pond standing tall and isolated. Other plants tend to have a better effect when densely packed together in a large container. Parrot's feather (Myriophyllum aquaticum), aquatic mint (Mentha aquatica) and water celery (Oenanthe japonica) are all trailing plants that look very nice cascading over rocks when densely packed together. These trailing plants may also be packed in the base of a large container holding a tall plant like elephant ears (Colocasia ssp.) to hide the pot if it slightly sticks out of the water. Other plants, such as cattails (Typha spp), yellow-flag iris (Iris pseudacorus), sweet flag (Acorus calamus), ribbon grass (Phalaris arundinacea) and zebra rush (Scirpus lacustris zebrinus) are grassy upright marginals with different colors and textures that have a stunning effect when packed together in a large container.
Fish such as koi are again a problem when using oxygenating plants, as these plants are easily knocked over, uprooted and even eaten if planted in small containers. According to Craig Mauney at the North Carolina Cooperative Extension, one to two bunches of oxygenating plants should be added per square yard of pond surface area to help "ensure a thriving, self-sustaining system." Anacharis (Egeria densa), Hornwort (Ceratophyllum demursum) and Cabomba (Cabomba caroliniana) are all plants you can submerge that oxygenate the water as well. Placing these plants in a large container anchored in pea gravel will keep them safe from rooting koi and easily maintainable should they need to be replanted or trimmed back.