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Pacific Northwest Research Station
Blue Mountains National Resources Institute

Forestry and Range Sciences Laboratory
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BMNRI Home > Publications > Weeds > St. Johnswort


Publications: Noxious Weeds

Explosion in Slow Motion: A talk by Jerry Asher about noxious weeds in the Blue Mountains


Click on a weed to learn about how to find it and kill it!


Common Crupina
Diffuse Knapweed
Russian Knapweed
Spotted Knapweed
Purple Loosestrife
Perennial Pepperweed
Puncturevine
Tansy Ragwort
Medusahead Rye
Rush Skeletonweed
Yellow Starthistle
Canada Thistle
Musk Thistle
Scotch Thistle
Dalmatian Toadflax
Mediterranean Sage
St. Johnswort
Leafy Spurge
Whitetop
Dyers Woad

Wanted: DEAD!

St. Johnswort

Alias: Klamathweed

  • Stems: Often reddish. Woody at base. Main stem erect, single or several from the base of the plant, usually accompanied by many slender weak sterile basal stems. 1 to 3 feet tall.
  • Leaves: Numerous, borne in pairs, if held to light appearing as if pinpricked and with tiny black dots on the margins.
  • Flowers: 3/4-inch in diameter with numerous stamens, bright yellow, petals with black dots on margins, borne in clusters. Flowering period is during June and July.
  • Fruits: Splitting at maturity into three segments. Gelatinous coating helps seeds stick to fur and clothing when wet. Seeds numerous, about 1/20-inch long, dark brown, microscopically pitted.

St. Johnswort is a perennial that propagates vegetatively by underground stems or runners. It is cumulatively poisonous to livestock. Light-pigmented animals develop blisters and scabs around the mouth, eyes, ears, nose, and feet, especially when exposed to sunlight. Dark-colored animals are less affected. It invades farm and pasture lands.


Control:

  • Biological: Several beetles have been released with good success in controlling this weed. Klamathweed beetle has been working since 1948 in some heavily infested areas in Oregon. A root-boring beetle (Agrilus hyperici) has had success in Idaho and Washington. Climate is a limit on beetle populations.
  • Mechanical: Repeated cultivation destroys the weed; it is not found in any cultivated crop. Mowing several times to prevent maturation also helps control the plant.
  • Chemical: Selective herbicides are available, although restrictions may apply. Consult your Weed Board for more information.

 

Report all sightings to your local Weed Board

US Forest Service - Pacific Northwest Research Station, Blue Mountains National Resources Institute
Last Modified: Monday, 16 December 2013 at 14:18:43 CST


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