Comparing Knotweeds

In Minnesota there are 3 species of knotweeds:  Japanese knotweed ,  giant knotweed , and their hybrid,  Bohemian knotweed . All three are shrub-like herbaceous perennials native to eastern Asia. Knotweeds were introduced to North America in the late 1800's as ornamental plants. Since their introduction, knotweeds have escaped cultivation and are frequently found in natural areas. As of 2020, all three knotweed species are listed as Prohibited - Control species on  Minnesota's Noxious Weed List 

Shoots emerge in spring, and plants grow quickly reaching full height mid-summer. Plants flower in late summer and after hard frost plants loose leaves and shoots die.

Impacts: forms dense stands, out-competes native vegetation; alters waterways and facilitates erosion; can grow through cracks in pavement or building foundations

Means of Spread: knotweed fragments produce new plants through cutting/mowing/dumping, contaminated soil, water; occasionally reproduce via seeds

Shoots: hollow, segmented ('nodes'); purplish resembling asparagus after emerging in spring; grow quickly; green in summer, reddish brown in fall, remain standing in winter; alternate branching

Roots: extensive system of taproots and rhizomes; shoots emerge along rhizomes (underground stems); system up to 6 feet deep and 65 feet long

Flowers: clusters made up of many small flowers; bloom late summer, at leaf axil

Leaf size and shape are very useful characteristics in determining knotweed species.

Some Bohemian knotweed plants may be difficult to distinguish as they are a hybrid species and can take on varying characteristics from either parent species - Japanese or giant knotweed.

All knotweed leaves have a pointed tip.

• deeply heart-shaped base
• up to 16 inches long, 10 inches wide
• notably thin texture

• heart-shaped to square base
• up to 12 inches long, 8 inches wide

• square base
• up to 5 inches long, 3 inches wide
• thick with rough texture

• rounded shape
• up to 3 inches wide
• quite thick with sandpaper-y texture

can depend on the environment the plant is growing in and may be subtle. Therefore height and structure will not always be useful, and could also be misleading.

In general, height of plants and general shape of full grown plants vary between knotweed species.

of all knotweeds bloom around the same time in late summer. General structure of flower panicles (clusters) does vary from species to species though may not be the most obvious distinguishing characteristic.

All Japanese knotweed plants in North America are female and therefore flowers will have large pistils and no visible stamen - this is useful in identification.

• drooping flower clusters

• upright flower clusters large stamen no visible pistil if male (most common in Minnesota)

• horizontal flower clusters large pistil no visible stamen if female (most common in Minnesota)

• white or light to dark pink flowers small clusters