Common St. Johnswort (Hypericum perforatum)
St. Johnswort Family (Hypericaceae)
Origin and Distribution:
Common St. Johnswort is a European native that colonists brought with them when they immigrated to North America. It is now naturalized throughout the eastern half and the west coast of the United States. In Ohio, it is found throughout most of the state. The species grows in open areas such as pastures, open woods, roadsides, parking lots, and waste areas. It also appears in such disturbed sites as tree plantations, orchards, and agronomic fields that are not regularly cultivated. Common St. Johnswort grows best in sandy soil but it is also found in heavy soils. It usually occurs in dry habitats.
Common St. Johnswort is a perennial weed that has the general appearance of a shrubby herb with many woody stems covered with narrow leaves. It can be distinguished from other herbaceous shrubs by clear dots on its leaves that look like pin holes if the leaf is held up to a light. To distinguish it from other closely related species, look for yellow flowers with petals that are about 1/2 inch long and have black dots along the edge. Reproduction is mainly by seeds, but vegetative spread is possible by rhizomes.
Roots are slender, branched, brown, and very tough. The root system generally consists of a long taproot with shallow roots extending several inches from the crown.
Seedlings and Shoots:
Seedlings consist of a slender, purple stem with 2 leaves, which have a powdery surface appearance and are unpleasant to taste.
Stems are smooth, upright, usually reddish, woody at the base, and grow up to 3 feet tall. Each plant is usually comprised of several stems with numerous branches.
Common St. Johnswort leaves are between 1 to 2 inches long, elliptic, and opposite (2 leaves per node). Leaves lack leaf stalks (petioles), so they attach directly to the stem. Leaves are covered with small dots, visible with a hand lens when the leaf is held up to a light, that look like tiny perforations or pin holes.
Flowers are less than 1 inch in diameter and have 5 yellow petals with black dots along the edge. About 25 to 100 flowers can be found in a broad clusters at the top of each stem.
Fruits and Seeds:
The fruit is a pod with 3 parts, each filled with numerous dark brown seeds. Seeds are 1/16 inch long, cylindrical, dark brown, shiny, and have a pitted surface.
Common St. Johnswort can be distinguished from the more than 15 other Hypericum species found in Ohio by its yellow petals that have black dots on their edges.
Flowers appear in June to September. A single common St. Johnswort plant produces as many as 100,000 seeds per year. Seeds are wind dispersed or, because they have a gelatinous coating that becomes sticky when wet, they may adhere to animals, machinery, and other objects that can disperse them. If buried in soil, seeds survive up to 10 years. Hand pulling, digging, mowing, and fire are generally ineffective control measures. Common St. Johnswort is tolerant of many herbicides. Regular tillage is an effective control measure but it is not suitable for use on rangelands often infested by this weed. In its native Europe, there are nearly 40 known insect predators that help control common St. Johnswort. Studies have shown that some of these insects are useful as biological agents for long term control of common St. Johnswort in North America. Common St. Johnswort was a troublesome weed of pastures on the West Coast until the implementation of such biological control measures.
Although people differ in their sensitivity to the toxin in this weed, contact can cause second degree burns in some individuals. Ingesting common St. Johnswort causes animals to develop sensitivity to sunlight resulting in dermatitis. Symptoms range from redness and blistering of the skin to loss of hair. Animals with light-colored skin are especially sensitive. If enough of the plant is ingested, death may result. Animals generally do not relish common St. Johnswort, but they will eat it if more palatable herbage is not available. Because common St. Joh