Water smartweed (Persicaria amphibia) is an interesting plant because of its variable morphology and the fact that it is amphibious (adapted to both land and water). Water smartweed can be found floating in the water, creeping along the ground or growing erect. Habitats include shallow water (lakes and ponds), slightly wet areas which are periodically inundated with water (irrigation ditches and flooded fields) and moist spots on land (meadows and seeps). Personally I always think of it as an aquatic and am surprised to find it growing in a non-floating form.
To further confuse the situation, water smartweed was only recently given the scientific name Persicaria amphibia. Previously this perennial in the buckwheat family was known as Polygonum amphibium. And to confuse matters even more, for a while it was grouped into the species Polygonum coccineum. Of course, because of its many different morphological forms there continue to be several subspecies of water smartweed. Bottom line: in the literature waters smartweed is found under several different scientific names. Arg!!
The flowers of water smartweed are a bright pink with five petal-like segments and eight stamens. Numerous flowers are arranged in a terminal, upright spike. The alternate leaves are an oblong, lance shape, have smooth margins and a somewhat pointed tip and are stalked (have a petiole). In water the leaves float and take on a reddish hue as they age. The stem is jointed with prominent cylindrical, sheathing stipules and arises from a fibrous rhizome. When nodes or joints come in contact with a substrate they will root. A raft of water smartweed leaves floating on the water’s surface with pink flower spikes rising above the leaves like sentinels is a beautiful sight.
Found throughout the world, except in Australia and Antarctica, water smartweed is also known as swamp knotweed or water knotweed – knotweed referring to the nodes or joints on the stems. In Europe, plants of this genus were formerly used medically on human hindquarters to treat bleeding or skin rashes and would cause irritation or the “ars” to “smart” – “arsmart”. Why one would want to treat a rash with something that causes irritation has no explanation. Anyhow, this early medicinal use is apparently the derivation of the name smartweed.
Smartweed is not toxic and is used as a potherb or in salads by some people. It can be bitter or peppery though. Although I have tasted water smartweed, I do not find it worth the effort to collect and clean. It contains oxalic acid so should not be consumed in large quantities over a long period of time. In addition, there are references in the literature that suggest a photosensitive reaction in persons eating liberal amounts of smartweed. Perhaps it is best to leave the plants and their seeds to waterfowl and other small animals.
Water smartweed is usually considered an aquatic or a wildflower. In some cases, such as when it grows thickly in rice fields or in irrigation ditches, it becomes an agricultural nusiance and is then classified as a noxious weed.
These pictures were taken on Lake Manzanita in Lassen Volcanic National Park.