Roots: The roots of this plant are an abbreviated taproot system. The roots branch deeply into the earth, however some of them do not and are shallow, these are usually the ones that reproduce. (1)

Stems/Leaves: This plant has numerous erect stems, that appear rust colored. (1) The stems are also woody and grow any where from 30-120 cm tall, (2-5ft). The leaves of this plant are oblong and/or elliptic, they contain transparent glands, which produce an oil that makes animals photosensitive. (1,2) These leaves are attached directly to the stem and the edges of the leaves contain tiny black dots.

Flowers: These flowers are anywhere from 1 to 2 cm in diameter, they are yellow and develop in clusters.  Each flower consists of five petals, there are also transparent black dots around the edges of each petal. (1) The flowering period of this plant is May through September. This flower also contains numerous stamens. (1,2)

The seed capsules of this plant are rounded at the end, and all of them are about 5mm long. They are rusty in color and each capsule contains numerous seeds. These seeds are brown (almost black) in color and are any where from 0.6 to 0.7 mm long. Each seed contains a germination inhibitor, but the germination increases over time during rainy periods because the inhibitor is washed off. (1)

Methods of Reproduction/Spread: Reproduction is through seeds and roots.  Seeds can be dispersed by the wind, wild life, and of course by humans. (1) The seeds can be carried in animal's digestive tract. They are only carried short distances by wind, but can be carried far by water, animals, and machinery. (7)

Life Style/ Habits/ Life Duration: St. Johnswort grows in fields and can push out native plants. It is a perrenial, which means it comes back every year from the roots. (1) It commonly grows in grasslands, woodlands, open forest, pastures, forestry, roadsides, railway lines, and river banks. (7)

This weed has many impacts. It produces an oil from a gland that forms on the leaf, this oil can cause animals to blister, develop rashes, loose weight, be supersensitive to light, and if the animal eats to much it can even die. It also out competes the native North American vegitation decreasing the available forage for wildlife and ranching. (1)

Native Range:
The native range of this weed is Western Europe, Northern Africa, parts of Asia, India, China, and Japan. (2)

Chemical control: Chemical control for this plant is: selective herbicides. (4)

Cultural Control: Includes: 1) repeated cultivation, which destroys it, and 2) mowing it over and over again, so seeds do not mature. (4)

Biological Control: Includes: Agrilus hyperici, Aplocera plagiata, Chrysolina hyperici, Chrysolina quadrigemina and  Zeuxidiplosis giardi. (3)

Other: St. John's Wort is used for many medicinal purposes including: Depression,  Bacterial Infections, Seasonal Affective Disorder, Viral encephalitits, and ear infections. (6)


1. Rees, Nornan, et. al., Ed., Biological Control of Weeds in the West, Western Society of weed Science, in cooperation with USDA ARS, MT Dept. of Ag. and MT State Univ., Color World Printers, Bozeman, MT, Feb., 1995.

2. Bradfield, Patsy, Executive Secretary. St. Johnswort (Hypericum perforatum)., 9-30-99.

3. Breitenfeldt, Todd, Personal Interview, Biology Teacher, Whitehall Schools, Box 1109, Whitehall, MT 59759. (406)287-3862. 9-2-99.

4. Henry, Charlie, Executive Secretary for CWMA. St. Johnswort (Hypericum perforatum) [Online] Available:   9-30-99.

5. Created by: Jiasuey and Allaire, Wellesley College, Common St. Johnswort, Hypericum perforatum, Family Guttiferae [Online] Available: 9-30-99.

6. UMMC. April 2002. University of Maryland Medical Center. 20 Feb. 2007. <>

7. DPI. May 2003. Department of Primary Industries. 20 Feb. 2007 <>

By: Kristen Chadwick 11/16/99.   Updated by: CRH  2/07