Saint Johnswort Biology, Impact and Management


Saint (St.) Johnswort (Hypericum perforatum L.), also known as Klamath weed or goatweed, was introduced to the United States as an ornamental and for medicinal purposes. The weed readily escapes cultivation, and has spread to infest natural areas, pastures and rangeland throughout most of the United States (Figure 1). St. Johnswort is listed as a noxious weed in eight western states (CA, CO, MT, OR, NV, SD, WA and WY) (USDA, NRCS 2019) and is on the noxious weed list for the North American Weed Free Forage Program (NAISMA 2019).


St. Johnswort contains the pigment hypericin, which causes photosensitization when ingested by grazing animals. Livestock will consume the weed when more desirable forage is scarce. Weakly pigmented parts of the grazing animal’s body such as the mouth, nose, ears and udders become light sensitive. Sheep, cattle, horses and goats are susceptible, but goats are more resistant than other animals. Symptoms include blistering skin, hair loss, high body temperature, rapid pulse and respiration rates, salivation and diarrhea. Affected animals may die of dehydration or starvation because of swelling and soreness of the mouth following an episode of hypericism. St. Johnswort also forms monocultures, reducing native plant diversity and impacting wildlife habitat and livestock carrying capacity on rangeland and natural areas. 


St. Johnswort is a taprooted perennial weed that reproduces from seed and lateral roots. Plants grow from one to five feet tall with numerous stems that are woody at the base. In autumn, infestations are easily visible because of the upright, rust-colored stems. 

Leaves are opposite, sessile, entire, elliptic to oblong, and generally not more than one inch long. A diagnostic characteristic of St. Johnswort is the presence of tiny, transparent perforations on the leaves that are visible when the leaf is held up to a light source (Figure 2). A mature plant may produce up to 30 flowering stems annually. 

Flowers are clustered in terminal cymes and each flower has five sepals and five petals (Figure 3). Petals are typically twice as long as sepals with black glands along the margins. The seed capsule bursts at maturity (Figure 4). A single plant can produce about 30,000 seeds that are easily transported by animals, wind, humans, and water. Seeds can remain dormant in soil for ten years.



Early detection and treatment of newly invading plants, minimizing disturbance, and establishing desirable competitive vegetation will reduce the ability of St. Johnswort to establish and spread. 

On small, isolated infestations, hand pulling or digging young plants may be effective. Repeated pulling or digging is necessary because lateral roots of older plants can give rise to new plants. Extracted plants should be removed from the area and burned to prevent vegetative regrowth and/or seed dissemination.

Mowing is ineffective as a management tool but may reduce spread of the plant if done before seeds form. Mowing may also negatively impact desirable vegetation that can compete with St. Johnswort. Burning may increase the density and vigor of St. Johnswort infestations.



Figure 5: Saint Johnswort control with Milestone herbicide at 7 fl oz/A 27 months after application. note treatment buffers with non-treated saint johnswort (brown stems-fall photo). Photo by Vanelle Peterson

Figure 5: Saint Johnswort control with Milestone herbicide at 7 fl oz/A 27 months after application. note treatment buffers with non-treated saint johnswort (brown stems-fall photo). Photo by Vanelle Peterson

Field trials conducted in Washington and Montana show that Milestone® herbicide at 5 to 7 fluid ounces per acre (fl oz/A) provides excellent control (>95%) of St. Johnswort one year after treatment (Table 1). Applications should be made when the weed is actively growing in late spring, early summer, and in fall when basal regrowth occurs. Opensight® herbicide at 2.5 to 3.3 ounces of product per acre provides similar control as Milestone. In Washington, late fall (November) application of Milestone at either 5 or 7 fl oz/A provided greater than 95 percent control 27 months following treatment (Figure 5). Neither metsulfuron (Escort) at 1 ounce of product per acre, nor 2,4-D provide acceptable control of St. Johnswort. 

Control of large infestations should integrate herbicide application with biological control agents. Herbicides can be used on the perimeter of large infestations and on satellite patches, and biocontrol agents can be used in the core of the infestation. Efforts should be focused on management techniques that stop seed production and maintain a healthy plant community that reduces establishment of St. Johnswort.


The flea beetle Chrysolina quadrigemina was introduced into California in 1945 to control St. Johnswort. The insect effectively reduced St. Johnswort to about one percent of its former acreage in that state. The flea beetle and three additional agents are currently impacting St. Johnswort: Chrysolina hyperici, a foliage feeding beetle; Aplocera plagiata, a foliage and flower feeding moth; Agrilus hyperici, a root-boring beetle. Chrysolina hyperici is better suited for wet sites than C. quadrigemina. The success and population stability of biological control agents depends on the fluctuations of St. Johnswort populations and site conditions including cold temperature.

Medicinal Properties

St. Johnswort has been promoted as a natural anti-depression compound and is sometimes used to treat other conditions that accompany depression such as anxiety, tiredness, loss of appetite and trouble sleeping. In some areas of the country, the plant is cultivated and harvested for use in multiple health products. St. Johnswort extracts can cause serious sensitivity to sunlight in humans. Products containing the plant will describe warnings to stay out of direct sunlight, and extracts may also have negative interaction with other drugs.  


  • Dow AgroSciences Internal Field Data Reports. Accessed January 2016.

  • EDDMapS. 2019. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at Accessed April 20, 2019.

  • Krueger J and R Sheley. 2002. St. Johnswort (Hypericum perforatum). Montana State University Bulletin No. MT199810 AG.

  • North American Invasive Species Management Association (NAISMA), Weed Free Forage Standards. Avialable online at

  • USDA, ARS Poisonous Plant Research Lab. St. Johnswort (Hypericum perforatum). Online Accessed April 2019.

  • USDA, NRCS. 2019. The PLANTS Database (, 20 April 2019). National Plant Data Team, Greensboro, NC 27401-4901 USA.

  • WebMD. St. Johnswort - Uses and side effects. Available online Accessed March 2016.


Published: June 2016: revised and updated June 2019

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