Blecker, L., Creech, E., Dick, J., Gephart, S., Hefner, M., Kratsch, H., Moe, A., Schultz, B. 2020, Nevada Noxious Weed Field Guide – Common St. Johnswort, Extension, University of Nevada, Reno, Field Guide

Stem

  • Up to 4 ft. tall with a woody base; highly branched; lacks hair (glabrous) and often rust colored with 2 ridges that run the length of the stem
Photo of st. Johnswort plantCommon St. Johnswort seedling

Leaves

  • Opposite, oval to strap-like (linear), prominent veins, less than 1 in. long, lack hair and no leaf stems (petioles); edges (margins) are smooth with the lower surface lined with small black dots
  • Surface covered with tiny transparent dots that can be seen by holding the leaf up to the light
Photo of fingers holding st. johnswort leaf
Common St. Johnswort leaf

Flower

  • 0.75 in. diameter; 5 yellow petals that often have tiny black dots around the edges; many stamens; clustered at tips of branches
Photo of common St. Johnswort flower
Common St. Johnswort flower

Root

  • Stout taproot with spreading rhizomes
Photo of common St. Johnswort plant
Common St. Johnswort plant

Other

  • Grows best on coarse-textured, gravelly, well-drained soils; known to occur in Elko county 
  • Perennial; reproduces by seed and rhizomes 
  • Ingestion causes skin irritation and weight loss in white-haired animals; sometimes cultivated as a crop and used for medicinal purposes (as an antidepressant)
  • Also known as Klamath weed
photo of common st. johnswort plant
 

Control

  • Mowing, grazing and burning are NOT effective
  • A biological control agent is available
  • Apply 2,4-D or glyphosate to actively growing plants prior to bloom; aminopyralid, metsulfuron or picloram pre or post-emergence  

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Davison, J., Powell, P., Schultz, B., and Singletary, L. 2012, University of Nevada Cooperative Extension
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Blecker, L., Creech, E., Dick, J., Gephart, S., Hefner, M., Kratsch, H., Moe, A., Schultz, B. 2020, Extension, University of Nevada, Reno, Field Guide