St. Johnswort

by Jane Krueger for MSU News Service

St. Johnswort was brought into the United States in 1696 for its ornamental, medicinal and purported magical properties. It was first reported on western grazing land in 1893. Today it can be found in most U.S. states. Montana's Department of Agriculture reports that it covers about 500,000 acres here.

Also known as common goatweed or klamath weed, St. Johnswort was in Europe, North Africa, and Asia in early historic times.

St. Johnswort is a taprooted perennial that reproduces by seeds or short runners. Plants can grow from one to five feet tall with numerous rust-colored branches that are woody at the base. In autumn, infestations are easy to spot by the remaining rust-colored branches. Leaves are opposite, attached directly to the branch, elliptic to oblong and generally not more than an inch long. A diagnostic characteristic of St. Johnswort is the presence of tiny, transparent glands on the leaves, which are observable when held up to the light. Flowers are bright yellow with five petals. Black glands can occur along the margins in an open, flat-topped group.

Even with some biocontrol agents in place, St. Johnswort can infest significant amounts of rangeland. It likes well-drained gravelly or sandy soils and sunny exposures.

St. Johnswort on the National Bison Range is an example of the problem. A few years ago, it covered about 9.000 of the reserve's 18,500 acres.

"It's cyclic," says Bill West, the Bison Range manager. He says the range has had three biocontrol agents for St. Johnswort for many years, and recently added a moth as the fourth.

"They do a major clean up about every 10 years, and a minor one about every five years. The rest of the time we have to use conventional controls," says West.

St. Johnswort displaces native plants and is not palatable to wildlife and livestock. However, when desired forage is scare, livestock will consume the weed and then may suffer severe blistering and itching on light-haired or unpigmented skin areas, such as the mouth, nose and ears. Animals affected by this lose weight, are difficult to manage and may lose market value or even die of dehydration or starvation because of swelling and soreness of the mouth.

St. Johnswort has been promoted as a "natural" antidepressant drug. It is cultivated in some areas of the country for use in health products such as herbal tea and dietary supplements. St. Johnswort is given by herbalists for its astringent and diuretic properties in urinary complaints, coughs and colds. Historically, the plant was cultivated for treatment of mania, hysteria, hypochondriasis, depression, dysentery, jaundice and to protect against evil spirits.

Because St. Johnswort is difficult to control, preventing large infestations is the best management, or, barring that, early detection. It is important not to create areas devoid of more competitive plant species, so range management programs should focus on maintaining competitive, closed-canopy communities where St. Johnswort will find it difficult to establish.

On small infestations, repeated hand pulling or digging of young plants may be effective. Pulled or dug plants should be removed and burned to prevent regrowth and seed dissemination.

Small infestations of St. Johnswort also can be controlled by repeated applications of herbicides. Foliar applications of 2.4-D at two pounds acid equivalent per acre will kill the seedling and pre-flowering stages in uncropped areas. In pasture, rangeland and uncropped sites, spring applications of picloram (0.125 to 1.5 lb. active ingredient/acre) or glyphosate (0.187 to 0.375 lb. acid equivalent/acre) are recommended. However, glyphosate is nonselective and should not be used where non target vegetation may be affected. St. Johnswort can be controlled after emergence with metsulfuron (0.04 lb. active ingredient/acre). Repeated applications may be necessary. Herbicide use on large infestations may not be economical.

Four biological control agents are approved in the United States to control of St. Johnswort: Chrysolina hyperici, a foliage feeding beetle; Aplocera plagiata, a foliage and flower feeding moth; Agrilus hyperici, a root-boring beetle; and Zeuxidiplosis giardi, a midge which forms leaf bud galls.

If you would like more information, email MWCA at acamm@montana.edu with your questions. A weed control expert will be happy to answer your questions.