U.S. DEPARTMENT OF THE INTERIORBUREAU OF LAND MANAGEMENT
 
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Klamathweed (Hypericum perforatum)

St. Johnswort Family (Clusiaceae)
 

Hypericum perforatum (klamathweed).  Photo courtesy of John M. Randall, The Nature Conservancy, Bugwood.org
Photo: John M. Randall, The Nature Conservancy, Bugwood.org

         

Hypericum perforatum (klamathweed).  Photo courtesy of John M. Randall, The Nature Conservancy, Bugwood.org
Photo: John M. Randall, The Nature Conservancy, Bugwood.org

Description/Habitat:  Klamathweed (Hypericum perforatum), commonly known as St. Johnswort, is a perennial, rhizomatous herb that can reach over 1 meter in height. Plants have been used medicinally to treat mild depression, but have been shown to cause hyper photosensitivity when consumed. Klamathweed is native to Europe and may be poisonous to cattle in large doses. Plants inhabit rangelands, pastures, roadsides and forest clearings, but can become particularly aggressive in rangelands characterized by dry summers.
 
Roots/Reproduction:  The root system of Klamathweed extends to depths that enable it to obtain water during dry months. The roots are rhizomatous, which allows for the plant to sprout new shoots throughout its root network. This rhizomatous nature can allow for the plants to create a dense canopy up to 1 meter tall. The roots themselves are also characterized by an outer coating, which makes them extremely hearty.
 
Leaves:  The leaves are light green and attached directly to the stem. They are oblong with an entire margin that rolls backward. The veins are prominent underneath and small transparent dots cover the blade (pellucid glands). Leaves are 15 to 25 mm long and 2 to 4 mm wide. Short leafy branches are borne in the leaf axils. The first pair of true leaves is thin with fine particles covering the surface. Later leaves typically have prominent, clear dots. The undersides of the leaf blades display five elevated black glands along the leaf margin.
 
Flowers:  Flowering starts when bright yellow flowers develop at the tips of the stems. The flowers have five petals and many stamens. Petals typically have black glands along the margins.
 
Seeds:  Fruits are three-chambered capsules with three persistent styles. Each plant can produce between 15,000 and 33,000 seeds, which can remain viable in soil for up to 10 years.
 
Flowering Period:  June - September
 
Management:  Klamathweed is a hard plant to control. It has deep roots and unless the entire root structure, including rhizomes, is removed, remaining root segments can regrow into new plants. The only herbicide (glyphosate) BLM is using at Fort Ord National Monument is not always effective against Klamathweed. This species occurs mainly in meadows which contain many native grasses and annual native wildflowers and has recently been discovered growing near Contra Costa Goldfields (Lasthenia conjugens, a Federally endangered plant). Because of this we are continually working to refine our methods to find an effective management technique for this species.
 
Other treatment methods are being utilized, beyond Fort Ord National Monument, including the use of the Klamathweed beetle (Chrysolina quadrigenina) as a biological control.
 
                                            
Sources:
 
"Cal-IPC: Hypericum Perforatum." Cal-IPC: California Invasive Plant Council Home. California Invasive Plant Council. Web. 20 Dec. 2011. <http://www.cal-ipc.org/ip/management/plant_profiles/Hypericum_perforatum.php>.
 
"Common St. Johnswort, Hypericum Perforatum (Theales: Clusiaceae)." Invasive Species: Information, Images, Videos, Distribution Maps. Center for Invasive Species and Ecosystem Health. Web. 20 Dec. 2011. <http://www.invasive.org/browse/subthumb.cfm?sub=4411>.
 
"St. Johnswort - Bishop Field Office - Bureau of Land Management, CA." BLM - The Bureau of Land Management. Bureau of Land Management, 5 June 2009. Web. 07 Feb. 2012. <http://www.blm.gov/ca/st/en/fo/bishop/biological_resources/weeds/plants/st_johnswort.html>.

 
Last updated: 08-20-2012