Persicaria amphibia is a vigorous, flowering member of the knotweed family. This hardy, rhizomatous perennial can take a number of forms and is quite variable in morphology: it may be an aquatic plant, growing submerged or floating in ponds and lakes, it may grow in margins and wet or muddy areas which are periodically inundated with water, and it may grow in moist spots such as meadows and bogs. These various submerged and emersed forms are sometimes considered different named varieties of the species by some experts. The mid-green leaves are up to 30cm (12") long, lance-shaped and hairless when grown submerged; terrestrial leaves being downier and more rounded at the base. During the summer and autumn, the plant sends up thick erect stems, which bear dense spikes of pretty, five-petalled, pink flowers, followed by small, glossy, nut-like brown fruit. This plant has a very rhizomatous root system and does need space to spread, typically increasing surface coverage of a pond to around 2m (6.6ft) within 5 years. For this reason this fast-grower is best suited to larger sized ponds. To control the growth, plant up into pond baskets filled with aquatic soil and a top layer of pea gravel, ensuring that the crown of the plant sits just under the very surface of the planting medium. Position the basket in a sunny or partially shaded site, covered by 45cm (18") of water initially, moving it gradually to its maximum recommended depth as it becomes established. Alternatively, place directly into the boggy soil at the pond margins if you wish to grow the plant in its emersed form. This plant is usually low-maintenance and disease-free, but be sure to deadhead any faded flower spikes as and when required. Propagation is from seed or division, the latter being the easiest option during late spring. Plant the divided clumps into small baskets and submerge in shallow water until established enough to be moved into larger baskets, and to their desired position.
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