For some low-growing specimens, James plants a water mint, a true mint that thrives in water and can spread just as fast as the terrestrial variety (figure M). "A number of aquatic plants do have a tendency to spread, often to the point of being invasive," he says. If you want to add a plant like this to your water garden, be prepared to keep its growth in check or consider using a floater as an alternative (figure N). Just place the container-grown plants in the pre-cut holes of the floating island, pots and all. In this case, James plants four variegated pennywort and some water celery (figure O). He launches the island into the pond, and in time the plants will spread to cover the edge of the exposed foam but not much beyond the edge of the island, especially if you have koi--fish that love to nibble on foliage and roots.
Among water plants that have a tendency to spread, the water hyacinth is perhaps the most notorious (figure P). If allowed to escape into a nearby body of water, it can quickly take over. As a result, its sale is outlawed in many states. "However, it's not a real threat here, so I'm going to complete my planting scheme by adding a few clumps," says James.
In addition to simply looking good, aquatic plants help keep the water clean because they're great natural or biological filters. "And they complement the nearby terrestrial plants that surround the outer edges of the pond and stream," James says.