Common St. Johnswort

Description:

COMMON ST. JOHNSWORT - Hypericum perforaturm L.

Clusiaceae - (St. Johnswort family)

A perennial reproducing by seeds or short runners. Stems are 1 to 3 feet high, erect, with numerous branches, somewhat 2-ridged, rust-colored, woody at their base. Leaves are opposite, sessile, entire, elliptic to oblong, not over 1 inch long, covered with transparent dots. Flowers are 3/4 inch in diameter, bright yellow, numerous in flat-topped cymes, with 5 separate petals with occasional minute black dots around the edges. Petals are twice as long as the sepals. Stamens are numerous, arranged in 3 groups. Seed pods are 1/4 inch long, rust-brown, 3-celled capsules, each with numerous seeds.

St. Johnswort, originally from Europe, is frequently found in the Pacific Northwest, often on sandy or gravelly soils. The weed contains a toxic substance which affects white-haired animals. Affected animals rarely die, but will often lose weight and develop a skin irritation when exposed to strong sunlight. St. Johnswort is an abundant weed in Australia where research has identified three insect species native to Europe which selectively feed on the plant. Insects introduced to the western United States and Canada have provided partial control of the weed.

Clusiaceae synonym: Hypericaceae.

(Courtesy of Weeds of the West)

CLICK HERE FOR INTERACTIVE DISTRIBUTION MAP

CONTROL STRATEGIES:

As with any of our rangeland weeds, once St. Johnswort is widely established, the cost of control seldom produces economic benefits of equal value.  It is critical to catch this weed early so that we are only treating small patches that can be eradicated thus preventing a larger problem. Beside the biological control agents available, there are several herbicides that will provide excellent control. For small infestations like we have in Wyoming at the present time herbicide should be one of the first choices. Once infestations are wide spread the use of biological control will reduce the impact and spread, but never eliminate the weed.  Escort at 1 ounce per acre will provide excellent control as will 1 to 2 quarts of Tordon 22K. 2,4-D at 2 quarts per acre will provide burn down, but multiple treatments over several years will be needed to really reduce the stand in the same way a single application of the other herbicides will.

To learn about biological control agents for Common St. Johnswort visit this link.