HPIPM:Common St. Johnswort

Taxonomy

Domain	Eukarya
Kingdom	Plantae
Phylum	Magnoliophyta
Class	Magnoliopsida
Superorder	Rosanae
Order	Malpighiales
Family	Hypericaceae
Genus	Hypericum

Scientific Name

Hypericum perforatum

Common Name

common St. Johnswort

Common St. Johnswort (Hypericum perforatum)

Author: Jim Jacobs, Plant Materials/Invasive Species Specialist, USDA-Montana NRCS.

Adapted from the USDA NRCS Montana Invasive Species Technical Note “Ecology and Management of Common St Johnswort (Hypericum perforatum L.)”

Identification and Life Cycle

Common St. Johnswort (Hypericum perforatum), also called klammathweed is a perennial in the St. Johnswort family (Clusiaceae). Common St. Johnswort is an erect herb that grows from 1 to 3 feet tall. Individual plants can produce many stems, and each stem can have many branches. Stems are two-sided with black glands along the ridges and hairless with distinct dark rings located at the lower nodes. The flowers are bright yellow and showy. The petals have black glandular dots along the margins. The fruit is a capsule that contains many small cylindrical, black seeds. The leaves are less than 1/2 inch long, hairless, oblong in shape, and bright green in color. They are simple, with entire or with weakly-wavy margins or rounded teeth. They are oppositely arranged on the stem, and they attach directly to the stem without a petiole. Glands occur throughout the leaves appearing as translucent dots when viewed as the leaf is held up to light. Common St. Johnswort can reproduce via its rhizomatous root system and by seed.

Habitats

Common St. Johnswort grows in open forests, dry rangeland, pastures, along streams and rivers, along roadsides and railroads, and in waste places. It is commonly found on well-drained gravelly or sandy soils and on sunny hillsides.

Impacts

Common St. Johnswort reduces available forage for wildlife and livestock. Its dense canopy and rhizomatous growth reduces the number of forage species and plant diversity, and the amount of forage mass produced. It is toxic at all stages of growth and although livestock usually avoid common St. Johnswort, animals may readily graze young plants when other forage is not available. Common St. Johnswort produces the photodynamic pigment hypericin in the dark glands on the stem, leaves, and flowers. Hypericin is activated by oxygen and visible light. Animals that consume enough hypericin show symptoms of blistering after exposure to sun light.

Biology and Ecology

Vegetative growth begins early in the spring and gives common St. Johnswort a competitive advantage over later emerging plants. Vegetative growth is completed by midsummer when soil moisture is limited. Common St. Johnswort populations form a dense canopy up to three feet tall. Flowering typically begins in late June and continues into August. Seeds mature by the end of August. Stems die and turn a rusty-red color in late summer or early fall when soil moisture becomes limited or after a hard frost. Fall re-growth can be significant when there is fall precipitation. Common St. Johnswort is a prolific seed producer, and individual plants can produce over 20,000 seeds. Seedlings are characterized by slow growth and are poor competitors with established native perennial grasses and other plants. Seedling flushes have been observed after rainfall, herbicide application, and fire. Seeds can survive in the soil for ten years.

Management Approaches

On small common St. Johnswort populations, aggressive herbicidal control should be combined with cultural practices that strengthen the competitiveness of the plant community. In areas with large scale infestations, herbicides should be used to eradicate small satellite populations and to reduce spread along the invasion front of the parent population. Biological control insects can be used to reduce the parent population and reduce invasiveness. On crop and hay land in rotation, tillage followed by herbicide treatment will be more effective than either treatment applied alone.

Biological Control

Five insects are available for biological control of Common St. Johnswort. The release of two foliar feeding beetles in 1945 and 1946, Chrysolina hyperici and C. quadrigemina, were the first attempt at biological control using insects in the United States. The St. Johnswort root borer, Agrilus hyperici, is a beetle that deposits eggs on the stems and the larvae mine the roots. The St. Johnswort moth, Aplocera plagiata, deposits eggs on the foliage in July and the inchworm-like larvae feed on the leaves and flowers through September. The Klamath weed midge, Zeuxidiplosis giardi, produces galls on the leaf and leaf buds.

Mechanical and Cultural Control

Hand pulling, grubbing, and hoeing may be effective on small populations of Common St. Johnswort if it is applied persistently. Mowing if applied before bloom will reduce flowering and seed production but will promote increased vegetative production and will not reduce populations. Grazing to control Common St. Johnswort is not recommended because of the potential for livestock poisoning. Fire favors seed germination and vegetative growth and by itself will not control Common St. Johnswort.

Chemical Control

Common St. Johnswort can be controlled with herbicides when applied before flowering.

Examples of herbicides that can be used to manage common St. Johnswort

Consult herbicide labels for additional rate, application, and safety information. Additional herbicide information can be found at http://www.greenbook.net.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.

References

For more information and images please visit IPM Bugwood. http://www.ipmimages.org/search/action.cfm?q=St.%20Johnswort