WSU Extension

Hortsense

Weeds
 
Annual bluegrass 
Barnyardgrass 
Bentgrass, creeping 
Bermudagrass 
Bittercress (Shotweed, Hairy bittercress) 
Bittersweet nightshade (European bittersweet) 
Black medic 
Blackberry (Himalayan, Evergreen, Pacific) 
Blue mustard (Purple mustard, Tenella mustard) 
Brackenfern, western 
Buffalobur 
Bull thistle 
Buttercup, creeping 
Canada thistle 
Catchweed bedstraw (Cleavers) 
Catsear, common (False dandelion) 
Chickweed, common and mouseear 
Clover 
Comfrey 
Crabgrass 
Dandelion 
Dock (Curly, Broadleaf) 
Dodder 
Downy brome (Cheatgrass, Downy chess) 
Dwarf mistletoes 
English daisy (Lawn daisy) 
English ivy 
Field bindweed (Wild morningglory) 
Field pennycress (Fanweed) 
Flixweed 
Foxtail (Green, Yellow, Bristly) 
Garden loosestrife 
Giant hogweed 
Goldenrods 
Groundsel, common 
Hawkweeds 
Hedge bindweed 
Henbit 
Herb Robert (Robert geranium, stinky Bob) 
Horsetails (Scouringrush) 
Horseweed (Marestail) 
Knapweeds 
Knotweeds (Bohemian, Giant, Japanese, Himalayan) 
Kochia 
Lambsquarters, common 
Liverworts 
Mallow, common (Cheeseweed, Buttonweed) 
Nightshades 
Oxalis (Creeping woodsorrel) 
Parrotfeather and Eurasian watermilfoil 
Pineappleweed 
Plantain (Broadleaf, Buckhorn) 
Poison hemlock 
Poison ivy and Poison oak 
Prickly lettuce (China lettuce) 
Prostrate knotweed 
Puncturevine (Tackweed, Goathead) 
Purple deadnettle (Red deadnettle) 
Purple loosestrife (Purple lythrum) 
Purslane, common 
Quackgrass 
Red sorrel (Sheep sorrel) 
Redroot pigweed (Rough pigweed) 
Redstem filaree (Stork's bill, Crane's bill) 
Reed canarygrass 
Russian thistle (Tumbleweed) 
Ryegrass, annual (Italian ryegrass) 
Salsify (Goatsbeard) 
Scotch broom 
Shepherd's-purse 
Smartweeds 
Sowthistle, annual and perennial 
Speedwells 
Spurges (Prostrate spurges) 
St. Johnswort, common (Goatweed, Klamathweed) 
Stinging nettle 
Tansy ragwort 
Tumblemustard (Jim Hill mustard) 
Velvetgrass (Common velvetgrass) 
Velvetleaf 
Water primrose 
Waterhemlock, western 
Yellow nutsedge 



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Caption: Japanese knotweed leaves
Photo by: J.A. Kropf
  
Weeds : Knotweeds (Bohemian, Giant, Japanese, Himalayan) : Polygonum spp.
(revision date: 9/3/2015)

Family: Polygonaceae
Cycle: Perennial
Plant Type: Broadleaf

Biology
The giant knotweeds are aggressive plants that spread via long rhizomes and also by seeds. They are deciduous plants, with the woody, reddish-brown stems dying back in winter. Summer growth of Japanese knotweed or fleeceflower (Polygonum cuspidatum, also known as Fallopia japonica) can reach nine feet in height. The large leaves are truncate, somewhat heart-shaped to broadly oval, with pointed tips. The leaf blades range from two to six inches long and from two to four inches wide. The underside may have barely noticeable bump-like hairs, especially along the midvein. Leaves are borne on short petioles, alternating along the slightly zigzagging, jointed stems. Flowers are borne in cream-colored or greenish-white clusters in the leaf axils, appearing in late summer to fall. Giant knotweed or sachaline (Polygonum sachalinense, also known as Fallopia sachalinensis) is a larger (up to twelve feet in height) but otherwise similar species which is found west of the Cascades. Its leaves are about three times as large as those of Japanese knotweed, and have a more distinct heart-shaped appearance. Hairs on the leaf undersides are long and wavy. Bohemian knotweed (Polygonum x bohemicum) is a cross between Japanese knotweed and giant knotweed and has some characteristics of both parents. Leaves tend to be heart-shaped near the base of the stems and more spade-shaped on the upper portions of the stems. Leaf hairs are short and broad-based, intermediate between the parent species. Himalayan knotweed (P. polystachyum) is a smaller species (up to six feet in height) and has more oblong, lance-shaped leaves and white/pink flower clusters. SPECIAL INFORMATION: The giant knotweeds are very aggressive plants, spreading rapidly to become a problem weed in some areas. In WASHINGTON all four species are designated as Class 'B' noxious weeds. In OREGON, all except Bohemian knotweed are designated as Class 'B' noxious weeds. Management of these species may be required by law in your county. In addition, giant knotweeds are on the noxious weed quarantine list for both Washington and Oregon. Sale, purchase, and transport of plants, plant parts, and seeds is prohibited. Consult your local Noxious Weed Control Board for more information.
Habitat
The giant knotweeds are escaped ornamental species that may be found on waste places, roadsides, ditch banks, and other sites. Japanese and giant knotweed grow best in sunny, moist areas and can tolerate heavy soils. Himalayan knotweed, however, grows on most soil types and in both sunlight and partial shade.

Management Options

Non-Chemical Management
None recommended
Select non-chemical management options as your first choice!

Chemical Management

Apply according to label directions. Treatment is the same for all of the giant knotweeds. Spot treatments with certain post-emergent herbicides will control weedy grasses, but will also kill the turf. Glyphosate products should be applied as spot treatments only! NOTE: Some ingredients listed here are only available in combination. Read the label carefully on combination products to make sure the product is suitable for your specific situation.

Landscape areas
  • glyphosate
Turf areas
  • 2,4-D + MCPP + dicamba
Bare ground areas
  • glyphosate
Images

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Caption: Japanese knotweed leaves
Photo by: J.A. Kropf
Caption: Giant knotweed
Photo by: L.M. Baldwin
Caption: Knotweed leaves, left to right: Giant, Japanese, Himalayan
Photo by: L.M. Shiner
Caption: Bohemian knotweed adventitious rooting
Photo by: T. W. Miller
Caption: Bohemian knotweed flowers
Photo by: T. W. Miller
Caption: Bohemian knotweed stem
Photo by: T. W. Miller
Caption: Bohemian knotweed
Photo by: T. W. Miller
Caption: Himalayan knotweed
Photo by: T. W. Miller
Caption: Invasive knotweed early shoots
Photo by: T. W. Miller