Ontario Weeds: Japanese knotweed

Table of Contents

1. Name
2. Other Names
3. Family
4. General Description
   • Photos and Pictures
5. Stems and Roots
6. Leaves
7. Flowers and Fruit
8. Habitat
9. Similar Species
10. Related Links

Name: Japanese knotweed, Polygonum cuspidatum Sieb. & Zucc.,

Other Names: renouée japonaise, Mexican bamboo, bambou, Polygonum sieboldii de Vriese

Family: Buckwheat Or Smartweed Family (Polygonaceae)

General Description: Perennial, reproducing by widely spreading underground rhizomes and only rarely by seed.

Photos and Pictures

Japanese knotweed (A - plants; B - tips of young shoots in spring; C - flowering branch).

| Top of Page |

Stems & Roots: Stems erect, 75cm-3m (2½-10ft) high, branched or unbranched, round, smooth, often mottled reddish-purple, hollow except at the nodes, dying back to the ground each winter, arising in early spring and throughout the growing season from widely spreading, shallow or deeply penetrating, thick, vigorous, whitish underground rhizomes, the younger rhizomes whitish except for dark brown, papery sheaths, with distinct nodes and internodes, the older ones thick, brownish, firm and often somewhat woody.

Leaves: Leaves alternate (1 per node), long-stalked, broadly ovate, 7.5-15cm (3-6in.) long, 5-12.5cm (2-5in.) wide, square-cut or slightly angled at the base, abruptly pointed at the tip with the tip often stretched out; ocrea (membranous sheath) surrounding stem at and immediately above every node, colourless or light greenish and turning brownish, hairless, often somewhat inflated, but soon disintegrating.

Flowers & Fruit: Flowers individually small but numerous and forming showy, greenish-white branching panicles from the axils of upper leaves; flowers unisexual, the sexes on separate plants; seeds only rarely produced, these hanging from the branches, triangular, about 7mm (¼in.) long, shiny, enclosed in a papery, 3-winged, teardrop-shaped, dry calyx. Flowers from July to September.

Habitat: Japanese knotweed occurs in southern Ontario in gardens, around old buildings or former building sites, waste places and roadsides, having been introduced as a bushy, hardy perennial for use as a screen or foundation planting.

| Top of Page |

Similar Species: It is distinguished by its very vigorous growth beginning in early spring and continuing to late autumn, its thick, widely spreading. whitish or brownish rhizomes, its erect stems, often mottled, hollow except at the nodes, its broad leaves with abruptly narrowed tip and a prominent ocrea around the stem at the base of each leafstalk, and its showy bunches of small greenish-white flowers. Two or more similar but smaller and usually less vigorous relatives of Japanese knotweed, presently grown as foundation or ground cover, may occasionally also escape from cultivation.

Related Links

... on general Weed topics
... on weed identification, order OMAFRA Publication 505: Ontario Weeds
... on weed control, order OMAFRA Publication 75: Guide To Weed Control

| Back to the Ontario Weeds Gallery | Top of Page |