Polygonum ramosissimum is a North American species of herbaceous annual plants in the buckwheat family, widespread across much of Canada and the United States, where it is commonly called bushy knotweed.
Polygonum ramosissimum has erect stems growing 30 to 100 cm (12 to 39.5 in) (sometimes to 200 cm or 80 inches) tall, with yellowish-green to blue-green foliage. The stems are freely branched with closed flowers produced in groups of (1) 2 to 3(5) flowers in the upper ocreae of racemes that are up to 15 cm (6 in) long, the inflorescences are spike-like. The greenish-yellow, rarely pink or white marked flowers, are on pedicels that are longer than the calyx. The calyx is around 3 mm (0.12 in) long and 5-parted with the outer three sepals longer than the inner sepals. The seeds are produced in fruits called achenes, which are egg-shaped, dark brown and around 3 mm (0.12 in) long. The achenes also have a smooth shiny surface. The late season achenes are larger, from 4 to 15 mm long.
There are a number of forms and two subspecies, that vary in flower and foliage coloration. Polygonum ramosissimum has great morphological variability, which is notable on the same plant, between flowers and fruits produced early in the blooming season verses those produced late in the season, plants also show great variation over geographical areas. The following names are widely recognized.
- Polygonum ramosissimum subsp. prolificum (Small) Costea & Tardif
- Polygonum ramosissimum subsp. ramosissimum
- The Plant List, Polygonum ramosissimum Michx.
- Polygonum ramosissimum United States Department of Agriculture plants profile, retrieved 14 July 2008
- Biota of North America Program 2014 state-level distribution map
- Gleason, A., Henry (1963). The new Britton and Brown illustrated flora of the northeastern United States and adjacent Canada. New York Botanical Garden by Hafner Pub. Co. pp. 72–84. LCCN 63016478.
- Flora of North America, Polygonum ramosissimum Michaux, 1803. Bushy knotweed