Named after David Douglas (1798 – 1834), a Scot who collected botanical specimens in North America, Douglas’ knotweed (Polygonum douglasii) is a species of variable morphology having many subspecies, most of which are difficult to distinguish. Mountain knotweed is another common name for P douglasii.
Found across Canada, throughout much of the West and in many of the northern states, Douglas’ knotweed grows in dry rocky or sandy habitat, often disturbed, from 1,000 to 10,000 feet.
A native annual, Douglas’ knotweed has thin, angular stems that are freely branching and hairless. At higher elevations the plant often assumes a smaller and trailing form.
The alternate leaves of this Buckwheat Family member (Polygonaceae) are linear or lance-shaped and terminate in a small “spine”. There is a pair of stipules at the base and a lighter colored midvein. Toward the stem tips the leaves are reduced to scales. Douglas’ knotweed leaves sometimes fall away as the plant matures leaving the stems naked when flowering.
The inflorescence is a long, loose raceme with 1 to 4 flowers per node. Many of the pink to white flowers remain closed most of the time. The flowers are composed of five overlapping tepals (neither distinctly sepals or petals) fused at the base. The tepals often display a greenish or purple midvein. There are eight stamens.
Douglas’ knotweed seeds are elliptical, black, three-sided achenes.
These specimens were growing in July along an abandoned logging road in Widow Valley near Lookout CA (Modoc County).