NRCS−Montana−Technical Note−Invasive Species−MT-14 1

United States Department of Agriculture

NATURAL RESOURCES CONSERVATION SERVICE

Invasive Species Technical Note No. MT-14 

July 2007

Ecology and Management of Common St. Johnswort (Hypericum perforatum L.) 

By

Jim Jacobs, NRCS Invasive Species Specialist, Bozeman, Montana 

Figure 1. Common St. Johnswort flowers bloom around mid-summer.

Abstract

Common St. Johnswort is a perennial weed accidentally and intentionally introduced to North 

America from Europe. It is a member of the Clusiaceae family and also bares the common 

names St. John’s wort, goatweed, and Klamath weed. Common St. Johnswort is perennial, 

relatively long-lived, and reproduces by short rhizomes that initiate from the stem base and by 

seed. Seeds can survive in the soil for ten years. 

Herbarium records indicate that populations of common St. Johnswort in Montana were first 

reported from Gallatin County in 1905 and as of 2007 have been reported from 26 of Montana’s 

56 counties infesting an estimated 68,065 acres. It is recognized as an economically important 

pest in temperate regions world wide and has a long and storied history as a medicinal plant. It 

invades foothill rangeland, pastures, and open forest sites where it reduces available forage to 

livestock and wildlife and poses a threat of poisoning. It also occurs in riparian areas, along 

roadsides, and along railroad rights-of-way. 
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Common St. Johnswort can be controlled using picloram (one to two quarts Tordon® per acre) 

or metsulfuron (one ounce Escort® or Cimarron® per acre) applied to actively growing plants 

before bloom. Grazing management of common St. Johnswort with all species of livestock is 

risky because it contains hypericin which causes blistering of skin of grazing animals that 

consume it and are exposed to the sun. In extreme cases, affected parts of the mouth may 

prevent animals from drinking and foraging. 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 and were very successful in reducing St. Johnswort populations in California. 

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 (0.3-1.0 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). 
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Figure 2. An infestation of common St. Johnswort on a south-facing slope as seen in early 

spring is indicated by the rust colored stems from the previous years’ growth.

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 Figures 3 and 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.

Figure 3. Stems, leaves and flowers of common St. Johnswort.
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The flowers are bright yellow and showy (see Figures 1 and 5). There are five green, linearlanceolate 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 flattopped 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.

 

 Figure 4. Leaves of common St. Johnswort.

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. 
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Figure 5. The population of common St. Johnswort pictured in Figure 2 blooming in mid-July.

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 midsummer 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. 
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 Figure 6. Vegetative growth of common St. Johnswort in late May 

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° to 221° F. (100° to 105° 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% 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 96-99% 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 Figures 2 and 5). In Montana, it is 

commonly found on well-drained gravelly or sandy soils where annual precipitation is between 
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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% - 4% 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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MANAGEMENT

Biological Control 

Currently there are five insect species available for control of common St. Johnswort (see Table 

1). This weed has the distinction of being the first target weed species in 1945 and 1946 for 

control using biological control insects. The Klamath weed beetles, Chrysoloina hyperici and C. 

quadrigemina, were very successful in reducing populations of common St. Johnswort in 

Australia and California. Adult beetles lay eggs on the leaves in the fall or spring. The eggs 

hatch in six to seven days and the larvae feed on the leaves and flowers, and can completely 

defoliate plants. Heavy larval feeding in the fall reduces winter survival of common St. 

Johnswort. Both insects do well in mountainous, open, sunny and warm areas, but C. hyperici

prefers more moist conditions than C. quadrigemina. Neither insect does well in shaded barren 

or excessively rocky locations. 

The St. Johnswort root borer, Agrilus hyperici, is a beetle that deposits eggs on the stems from 

the soil level up to about eight inches (20 cm) in July and August. The larvae feed in the roots 

from July to May of the following year. They consume the root tissues resulting in stunted 

plants and reduced flower production. They establish best in dry, mountainous areas. On damp 

sites, larvae are susceptible to fungal attack. They will attack common St. Johnswort plants 

growing in the shade. 

The St. Johnswort moth, Aplocera plagiata (also known as Anaitis plagiata) deposits eggs on the 

foliage in July and the inchworm-like larvae feed on the leaves and flowers through September. 

Large populations of this insect can defoliate plants and inhibit flower and seed formation. They 

do well in dry, open areas with sandy, rocky soils, and soils with limestone parent material. 

They do not thrive in areas receiving high rainfall. 

Table 1. Biological control insects for management of common St. Johnswort, the site of attack on 

the plant, insect life stage for collection, and the collection method for re-distribution.

Insect Type Site of Attack Collection Collection Method

Agrilus hyperici beetle larvae feed in 

roots 

adult sweep net in July and 

August

Aplocera plagiata 

(plagiata)

moth leaves and 

flowers

larval sweep net

Chrysolinia hyperici beetle leaves and 

flowers

adult sweep net or hand 

pick

Chrysolinia quadrigemina beetle leaves and 

flowers

adult Sweep net

Zeuxidiplosis giardi gall 

midge

leaf buds larvae galled plants
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Herbicidal Control 1/

On pastures and rangeland, common St. Johnswort can be temporarily suppressed by 

aminopyralid, metsulfuron, picloram, or 2,4-D (see Table 2). Metsulfuron should be applied to 

actively growing plants in the spring, summer or fall at a rate of 1 ounce product (Escort® or 

Cimarron®) per acre. A non-ionic surfactant is needed in the spray solution at 0.5% by volume 

for metsulfuron to be effective. Picloram should be applied at 1 to 2 quarts product (Tordon®) 

per acre to actively growing plants before bloom. The higher rate should be used to eradicate 

small populations. To reduce large populations, 1 quart picloram combined with 1 quart 2,4-D 

(4EC) per acre applied to actively growing plants before bloom is recommended. Pre-bloom 

foliar applications of 2,4-D at 2 quarts per acre with repeated applications to re-growth can also 

be used. The ester formulations of 2,4-D are more effective than amine formulations. 

Aminopyralid should be applied at a rate of 5 to 7 ounces (Milestone®) per acre to plants before 

bloom. Glyphosate applied at 1 to 2 quarts per acre will kill common St. Johnswort on cropland 

or where revegetation is planned. 

Table 2. Chemical and product name, recommended application rate, soil residual half life, and 

eco-toxicity of herbicides commonly used to control common St. Johnswort. The eco-toxicity is the 

lethal concentration of the herbicide when applied in a single dose kills 50 percent of the tested 

organism (the lower the number the more toxic the herbicide). Follow label guidelines for 

rangeland use and all other label requirements when applying herbicides to avoid damage to 

desirable plant species.

Chemical name Product name Rate

per Acre

Half life

(days)

Eco-toxicity (LC50/EC50)

2,4-D Many names 1 to 2 qts. 7 1-10 mg/L

Aminopyralid Milestone 5 to 7 oz. 30 >100 mg/L

Glyphosate many names 1 to 2 qts. 32 8.2 mg/L

Metsulfuron Escort/Cimarron 1 oz. 14-180 >150 mg/L

Picloram Tordon 1 qt. 90 10-100 mg/L

Hand Pulling

Hand pulling, grubbing, and hoeing may be effective on small populations of common St. 

Johnswort if it is applied persistently. Hand pulling that removes the short rhizomes will be most 

effective. Follow-up treatments will be necessary where a persistent seed bank exists.

Mowing

Mowing if applied before bloom will reduce flowering and seed production. Mowing will 

promote increased vegetative production and will not reduce populations. 

1/

Any mention of products in this publication does not constitute a recommendation by the NRCS. It is a 

 violation of Federal law to use herbicides in a manner inconsistent with their labeling. 
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Tilling

Common St. Johnswort is not normally a problem in cultivated crop fields because it is 

controlled by tillage procedures that clean crop fields of weeds. However, because it produces 

short rhizomes, it is possible to spread common St. Johnswort within a crop field and between 

fields. An application of glyphosate to common St. Johnswort plants that regenerate from 

rhizomes or seeds following tillage will reduce it on tilled fields. Cleaning tillage equipment of 

soil that may contain seeds or rhizomes is recommended after use on fields where common St. 

Johnswort has been growing and before use on weed-free fields. 

Prescribed Burning

Fire favors seed germination and vegetative growth and by itself will not control common St. 

Johnswort. Prescribed fire on land infested with common St. Johnswort should be combined 

with other control treatments such as herbicides or biological control. 

Grazing Control

Grazing to control common St. Johnswort is not recommended because of the potential for 

livestock poisoning. Prescribed grazing to maintain the health and competitiveness of pasture 

and rangeland plant communities is important in preventing and retarding invasion, and to 

increase the effectiveness of other control treatments. 

Cultural Control 

Common St. Johnswort seedlings are not strong establishers in healthy productive plant 

communities. Also, plant competition reduces the invasiveness of common St. Johnswort and 

increases the effectiveness of control applications. Therefore, practices that increase the 

competitiveness of desirable plant species such as conservation crop rotation and conservation 

cover will make the environment less hospitable for common St. Johnswort survival. 

Revegetation 

Species selected for revegetating disturbed sites and common St. Johnswort infestations should 

be appropriate for management objectives, adapted to site conditions, and competitive with the 

weed. Management objectives will determine if introduced or native species are seeded and the 

combination of species in the seed mix. The environmental conditions of the site including 

precipitation, soil texture and depth, slope and aspect, will affect species establishment. Refer to 

Montana Plant Materials Technical Note 46, ‘Seeding Rates and Recommended Cultivars,’ and 

Extension Bulletin EB19 ‘Dryland Pasture Species for Montana and Wyoming’ for seeding rate 

guidance and revegetation species selection. State and area resource specialists can help 

determine the most appropriate, site-specific species mix and timing of seeding. 

In most cases, herbicidal suppression of common St. Johnswort is needed for revegetation of 

infested lands. The herbicides listed in Table 2 will control common St. Johnswort and reduce 

competition during the establishment period with little or no injury to emerging grass seedlings. 

This is especially important for species that are slow to establish like many of the native grasses. 

However, where herbicides have been applied, chemical carryover should be assessed prior to 

planting permanent vegetation. 
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Integrated Pest Management 

Integrated pest management is the application of two or more management alternatives so they 

are complimentary in weed suppression, increase the longevity of control procedures, and 

improve crop production, or conservation of resources. 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. On disturbed 

sites, pastures, and rangeland where competitive plants have been lost, revegetation following 

control of common St. Johnswort will improve the longevity of the control application.

References

Coombs, E. M., Coombs, J. K. Clark, G. L. Piper, and A. F. Cofrancesco, Jr. (eds). Biological 

control of invasive plants in the United States. Oregon State University Press, Corvallis. 467 pp. 

Crompton, C.W., I.V. Hall, K.I.N. Jensen, and P.D. Hilderbrand. 1988. The biology of 

Canadian weeds. 83. Hypericum perforatum L. Canadian Journal of Plant Science. 68: 

149-162. 

Mitch, L.W. Intriguing world of weeds: Common St. Johnswort. 1994. Weed Technology. 8: 

658-661. 

Piper, G.L. 1999. St. Johnswort. In: Sheley, R.L. and J.K. Petroff (eds.). Biology and 

Management of Noxious Rangeland Weeds. Oregon State Press. Pages 350-361.