Ohio Noxious Weed Identification – Week 9 Japanese Knotweed

Japanese Knotweed

FamilySmartweed, Polygonaceae.

Habitat: The species occupies a wide variety of habitats in many soil types and a range of moisture conditions. It is most common along roadsides and on stream banks, but is also found in low-lying areas, utility rights-of-way, old home sites and along woodland edges and openings. The species requires a high light environment and grows poorly under full forest canopies. In Ohio this species is primarily found in the eastern part of the state.

Life cycle: Semi-woody perennial.

Growth habit: Semi-woody perennial that grows in large clumps reaching heights of 3-10 feet.

Leaves: Alternate and egg-shaped (4-6 inches long and 3-4 inches wide) narrowing to a point at the tip.

Flower: Tiny (1/8 inch) flowers are creamy white to greenish white and are borne in plume-like clusters in the upper leaf axils. The species is dioecious, producing male and female flowers on separate plants, however male plants are rare, flowers bloom in August – September.

Root: Fibrous, spreads primarily by its extensive rhizomes creating dense thickets.

Stem: Stout, hollow stems are reddish brown and the nodes are swollen giving them a bamboo-like appearance. Typical of the smartweed family, nodes are enclosed by a modified leaf-life structure. Stems die back in the winter and new ones are produced each spring.

Similar Plants: Resembles bamboo because of the robust hollow stems with distinct nodes and internodes; however, true bamboo is a grass.

The problem is…Japanese knotweed grows quickly and aggressively by extensive rhizomes and forms dense thickets that exclude native vegetation and reduce wildlife habitat. This species represents a significant threat to riparian areas where it can spread easily as small pieces of rhizome are washed downstream and deposited to create new colonies. Transfer of soil containing rhizome or seed may also cause the establishment of new colonies. Establishment can be prevented with careful monitoring and eradication of small patches when they first develop.








Red Stem and Swollen Nodes







Currently there are 21 weeds on the Ohio Prohibited Noxious Weed List:

  • Shattercane (Sorghum bicolor) – February 8
  • Russian thistle (Salsola Kali var. tenuifolia) – February 22
  • Johnsongrass (Sorghum halepense L. (Pers.))
  • Wild parsnip (Pastinaca sativa)
  • Wild carrot (Queen Anne’s lace) (Daucus carota L.)
  • Oxeye daisy (Chrysanthermum leucanthemum var. pinnatifidum)
  • Wild mustard (Brassica kaber var. pinnatifida)
  • Grapevines: when growing in groups of one hundred or more and not pruned, sprayed,cultivated, or otherwise maintained for two consecutive years. – February 15
  • Canada thistle (Cirsium arvense L. (Scop.)) – March 29
  • Poison hemlock (Conium maculatum)
  • Cressleaf groundsel (Senecio glabellus)
  • Musk thistle (Carduus nutans)
  • Purple loosestrife (Lythrum salicaria)
  • Mile-A-Minute Weed (Polygonum perfoliatum) – March 7
  • Giant Hogweed (Heracleum mantegazzianum) – March 14
  • Apple of Peru (Nicandra physalodes) – February 28
  • Marestail (Conyza canadensis)
  • Kochia (Bassia scoparia)
  • Palmer amaranth (Amaranthus palmeri)
  • Kudzu (Pueraria montana var. lobata)
  • Japanese Knotweed (Polygonum cuspidatum) – April 2

Each week, for the next 21 weeks, I will post information and pictures on how to identify these invasive and harmful plants.

Leave a Reply

Your email address will not be published. Required fields are marked *