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Plant Biology

Taxonomy

Common St. Johnswort is in the Clusiaceae family, formerly called the Hypericaceae. The name Hypericum perforatum was applied by Linnaeus in 1753 in his publication Species Plantarum. The genus name Hypericum is derived from the old Greek, Hyperikon, recorded by Dioscorides meaning “a plant from the heath.” Supposedly this is because it grew in heath habitats. The species name perforatum refers to the translucent glands on the leaves that appear as minute perforations. There are no varieties or sub-species recognized in the North American Pacific Northwest. However, two sub-species have been recognized by European taxonomists. Plants from populations of common St. Johnswort in western North America and Australia where it is a more serious problem are taller than in Britain and eastern Canada where they are not as weedy. Two types of common St. Johnswort have been indicated in eastern Canada, one growing on acidic soils and one growing on calcareous soils. There are three native Hypericum species found in Montana that can be distinguished from H. perforatum during flowering. Hybrids of H. perforatum with other Hypericum species have been named in Europe but no hybridization is known to occur in North America.

Identification

Common St. Johnswort is an erect herb that grows from one up to three feet (0.3 to 1.0 m) tall. Individual plants can produce many stems and the stems can have many branches. Stems are two-sided with black glands along the ridges, hairless (glabrous), and there are distinct dark rings located at the lower nodes. The stems dry to a rusty red color in the fall, can remain upright throughout the winter, and can accumulate over time (see Figure 2).

The leaves are small (less than one-half 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 (sessile, see Figure 3 and Figure 4). There are glands throughout the leaves that appear as translucent dots when viewed as the leaf is held up to light. There can also be black glands on the ventral leaf surface.

The flowers are bright yellow and showy (see Figure 1 and Figure 5). There are five green, linear-lanceolate sepals 5 by 1 mm, and five 14 by 8 mm irregularly-shaped yellow petals. The petals have black glandular dots along the margins. There are many stamens and one pistil with a three-parted style. The ovary is three celled. The flowers are heliotropic, tracking the sun from east to west as it crosses the sky. The flowers are arranged in short, broad and somewhat flat-topped cymes at the branch ends. The fruit is a capsule that contains many small (1.1 by 0.5 mm) cylindrical, black, seeds that are pitted in longitudinal rows. Seeds have a gelatinous seed coat that may aid in dispersal by animals.

Common St. Johnswort can be distinguished from the native Hypericum species by the flower characteristics. Hypericum majus found in wet places in the northwest part of Montana and H. anagalloides reported from Missoula and Ravalli Counties, both have shorter petals (2 to 7 mm long) than H. perforatum, the petals lack black glandular dots, and the ovary is one celled. Hypericum formosum found in moist areas in west and central Montana has sepals that are triangular in shape compared to the linear-lanceolate sepals of H. perforatum, and seeds that are more or less striated lengthwise.

Life History

Common St. Johnswort is a perennial. It has been listed as a hemicryptophyte, which means that the over-wintering buds are at the soil surface. The buds can form rosettes in the spring and also rhizomes that originate from the stem base in response to defoliation and mechanical disturbance. Its root system is capable of penetrating deeply into the soil enabling it to be invasive in rangeland with dry summers. Rhizomes develop under poor growing conditions.

Vegetative growth begins early in the spring and gives common St. Johnswort a competitive advantage over later emerging plants (see Figure 6). Vegetative growth is completed by mid-summer when soil moisture is limited. Common St. Johnswort populations form a dense canopy up to three feet (1 m) tall. Vegetative reproduction is stimulated by grazing, fire, and defoliation.

Flowering typically begins in late June and continues into August. Historically, bloom is associated with Mid-Summer Day, June 24. Flowers are pollinated by bees, self-pollinated, or embryos can develop without pollination (apomixis). 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. Capsules of plants collected in Nova Scotia averaged 73 seeds per capsule and as many as 500 seeds per capsule have been counted. Populations in Idaho averaged 23,350 seeds per plant. Some seeds are capable of germination upon maturity, but percent germination increases with time after maturity. Exposure of seeds to short periods of 212 degrees to 221 degrees F. (100 degrees to 105 degrees C.) increases germination which may facilitate seedling establishment after fire. Germination of seeds was not reduced after burial in the soil for three years, and seeds stored at room temperature for 15 years maintained a 50 percent germination rate.

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. Seedling mortality of 96 to 99 percent where moisture was lacking was reported in Idaho. Garden competition studies showed high seedling mortality of common St. Johnswort when growing with other plant species.

Habitat

Common St. Johnswort grows in open forests, dry rangeland, pastures, along streams and rivers, along roadsides and railroads, and in waste places (see Figure 2 and Figure 5). In Montana, it is commonly found on well-drained gravelly or sandy soils where annual precipitation is between 15 and 30 inches. It is commonly found on sunny hillsides. Disturbance favors its invasion and maintenance of competitive plant communities reduces invasion. In North America it can be found throughout Canada except the far north and in all but the most southern U.S. states. Calcium has been found to reduce seed germination and common St. Johnswort is not common on soils with moderate levels of calcium.

Spread

Historically, common St. Johnswort spreads by escaping garden cultivation. As early as 1696, European settlers brought common St. Johnswort seeds to North America for their gardens because it was valued for its medicinal, spiritual, and magical powers. It had escaped cultivation and spread along roadsides from Maine to Florida by the Revolutionary War. It was brought to California around 1900 and rapidly spread throughout the drier rangelands.

Common St. Johnswort is also believed to be spread by animals. The gelatinous seed coat enables seeds to adhere to fur, and initial infestations reflect animal movement. It is also spread along roadways, railroads, rivers and streams. Rhizome fragments can be spread by tillage.

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. Livestock poisoning has been reported from Australia, New Zealand, North Africa, and Europe. 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 (10-7M, or 1 percent to 4 percent of body weight) show symptoms of blistering after exposure to sun light. Areas of light hair or un-pigmented skin are most susceptible and animals with affected areas of the mouth may refuse to feed. Symptoms also include fever, rapid pulse, diarrhea, dermatitis, and excessive salivation. Hypericin remains chemically intact through ingestion, digestion, absorption into the blood, and passage through the liver. It is poisonous only through ingestion. Contaminated hay poses a poisoning threat because hypericin is stable during drying and resistant to heat destruction. Native Hypericum species do not produce hypericin.

Common St. Johnswort is a valued medicinal plant species. It was used as a treatment for wounds and was known in England as “balm of the warrior’s wound.” The oil from North American Hypericum species was used by Native Americans to heal wounds and to treat consumption. In Europe, common St. Johnswort was used as an astringent, diuretic and sedative. It was used to treat hysteria and many other affections. Recently, hypericin was found to inhibit human immunodeficiency virus.

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Last Modified: 07/27/2007