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: Spotted knapweed flowerheads
Photo by: B.F. Roche
  
Weeds : Knapweeds : Centaurea spp.
(revision date: 9/3/2015)

Family: Asteraceae (Compositae)
Cycle: Biennial/perennial
Plant Type: Broadleaf

Biology
Knapweeds are short-lived perennials with stout taproots. They can have one or more stems, reaching one to three feet in height. Depending on species, leaves may be narrow and lance-shaped, lobed, or somewhat toothed to deeply divided. Spotted knapweed (Centaurea stoebe, C. biebersteinii) flower heads are solitary at the ends of branches, with stiff bracts tipped with a dark comb-like fringe. Ray flowers are pinkish-purple or rarely cream-colored. Fruits are about an 1/8 inch long, tipped with a tuft of persistent bristles. Diffuse knapweed (C. diffusa) flower heads are numerous and narrow. Ray flowers are white to rose or purplish. Bracts under the flower have yellow spines with teeth appearing as a comb along the spine margins. The leaves are pinnately divided. Meadow knapweed (C. jacea x nigra, C. debeauxii) flowers are borne in large pink to purplish-red heads at the ends of branches, with deeply lobed bracts below the flower. The lower leaves have long stalks and may be toothed. Upper leaves are smaller, lack stalks, and are smooth-margined. SPECIAL INFORMATION: In WASHINGTON and OREGON, several knapweed species are designated as Class 'A' or 'B' noxious weeds. Management or eradication may be REQUIRED by law in your county. In addition, several species are on the Washington and Oregon noxious weed quarantine lists, which prohibit sale, purchase, and transport of plants, seeds, and plant parts. Consult your local Noxious Weed Control Board for more information.
Habitat
Several species of knapweeds establish themselves on roadsides, waste areas, fields, and pastures both east and west of the Cascades. Their early spring growth makes them competitive for soil moisture and nutrients. Some evidence suggests that knapweeds release chemicals which inhibit surrounding vegetation.

Management Options

Non-Chemical Management
  • Cultivation (rototilling or hoeing) will effectively eliminate plants.
Select non-chemical management options as your first choice!

Chemical Management

Apply according to label instructions. Use glyphosate products 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
  • products containing 2,4-D
Bare ground areas
  • glyphosate
  • products containing 2,4-D
Images

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Caption: Spotted knapweed flowerheads
Photo by: B.F. Roche
Caption: Spotted knapweed flower
Photo by: T. W. Miller
Caption: Diffuse knapweed flowers
Photo by: D.G. Swan
Caption: Meadow knapweed flower
Photo by: Washington Noxious Weed Board slide collection
Caption: Diffuse knapweed
Photo by: D.G. Swan
Caption: Spotted knapweed
Photo by: D.G. Swan