BMNRI Home > Publications
> Weeds > St. Johnswort
Publications: Noxious Weeds
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!
- 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.
- 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