Common Name: Common St. Johnswort
Scientific Name: Hypericum perforatum
Code: HYPE
Group: Dicot
Family: Clusiaceae
Growth Habit: Forb/herb
Duration: Perennial

What does it look like?

Klamath weed grows 1 to 4 feet
tall and survives year-round due to its taproot. The two-ridged stems stand erect with many rust-colored branches. When this plant is in bloom it has numerous bright yellow, five-petaled flowers with purple-tipped stamens arranged in three groups. The petals are about 0.5 inches long with occasional black dots around the edges. The leaves grow opposite of each other, are oblong shaped and covered with tiny dots that can be seen when held up to light. The flowering period ranges from June through August (BLM 2003).

Where does it grow?

Native to Eurasia, Klamath weed now grows throughout the Northwest. It has been found on public lands administered by the BLM in Humboldt County, mostly in small numbers and along roadsides.

Klamath weed is common in pastures, abandoned fields, roadsides, and other disturbed areas. It can produce seed without fertilization, and is a highly aggressive invasive exotic that takes over areas and inhibits the growth of native species. Grazing often encourages the growth of this weed, which is a hazard to livestock since it is poisonous. Also known as St. John’s Wort, the toxin hypercin has been successful at fighting depression in humans (BLM 2003).

Is it in our watershed?

It is very noticeable along major roadsides in the Mattole watershed, including Mattole Road, Briceland/Honeydew, Ettersburg/Honeydew, Bull Creek, and Chemise Mountain Roads.

Why is it a problem?

Klamath weed is difficult to eradicate and often competes with and replaces native plants. It is also poisonous to livestock and causes sensitivity to sunlight, which leads to the development of blisters and scabs near the mouth, eyes, ears, nose, and feet.

How do you get rid of it?

Manual Removal: Pulling or cutting Klamath weed can be effective on small infestations or when minimal impact is necessary. Weed removal is best after seedhead production and before flowering, and it must occur multiple times during the growing season. This removes the vegetative structure, which disrupts the life cycle and stops seed production. Dispose of seeds, shoots and roots in a sealed garbage bag through the trash.

Mechanical Removal: Mowing Klamath weed several times can prevent the plant from maturing and helps to control spread. This removes the foliage and prevents the plant from producing viable seed. It also inhibits its ability to produce food, which weakens the root stocks. If timed properly, and done persistently, good results are possible within two to three growing seasons.

Biological Control: Klamath weed beetle, Chrysolina quadrigemina, is native to central and southern Europe and North Africa and now inhabits North and Central America, Africa, Europe, and Asia (WES, 2003).

Author:
Unity Peterson, Invasive Plant Program Coordinator, Mattole Restoration Council, Petrolia, CA.

Photographs:
CDFA, 2001

References:
BLM 2000. Noxious Weeds of the Arcata Field Office. Bureau of Land Management. http://www.ca.blm.gov//arcata/arcweeds.html

WES, 2003. US Army Corps of Engineers. Engineer Research and Development Center. Waterways Experiment Station, Vicksburg, MS. 2003. http://el.erdc.usace.army.mil/pmis/biocontrol/html/chrysol1.html