The Temperate Database is in the process of being updated, with new records being added and old ones being checked and brought up to date where necessary. This record has not yet been checked and updated.
Common Name: Knotweed
Polygonum douglasii is an annual plant that can grow up to 0.30 metres tall.
It is harvested from the wild for local use as a food.
Although no specific mention has been made for this species, there have been reports that some members of this genus can cause photosensitivity in susceptible people.
Many species also contain oxalic acid (the distinctive lemony flavour of sorrel) - whilst not toxic this substance can bind up other minerals making them unavailable to the body and leading to mineral deficiency. Having said that, a number of common foods such as sorrel and rhubarb contain oxalic acid and the leaves of most members of this genus are nutritious and beneficial to eat in moderate quantities. Cooking the leaves will reduce their content of oxalic acid. People with a tendency to rheumatism, arthritis, gout, kidney stones or hyperacidity should take especial caution if including this plant in their diet since it can aggravate their condition[
N. America - Northwest Territory to British Columbia, south to Vermont, New Mexico and Oklahoma.
Rocky slopes and dry soils all over N. America[
We have very little information on this species and do not know if it is hardy in Britain, though judging by its native range it should succeed outdoors in most parts of the country. The following notes are based on the general needs of the genus.
Succeeds in an ordinary garden soil[
] but prefers a moisture retentive not too fertile soil in sun or part shade[
]. Repays generous treatment[
Plants seem to be immune to the predations of rabbits[
Leaves - raw or cooked.
Seed - dried then ground into a powder and mixed with cornmeal or other cereals for making bread etc[
]. The seed is rather small and fiddly to utilize, it is enclosed in a dry papery hull[
Seed - sow spring in a cold frame. Germination is usually free and easy. When they are large enough to handle, prick the seedlings out into individual pots and plant them out in the summer if they have reached sufficient size. If not, overwinter them in a cold frame and plant them out the following spring after the last expected frosts.
Division in spring or autumn. Very easy, larger divisions can be planted out direct into their permanent positions. We have found that it is better to pot up the smaller divisions and grow them on in light shade in a cold frame until they are well established before planting them out in late spring or early summer.