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

Common 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

Common St. Johnswort flower

Root

Stout taproot with spreading rhizomes

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

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

Learn more about the author(s)

Heidi Kratsch

Also of Interest:

Nevada Noxious Weed Field Guide – Common St. Johnswort