Common St Johnswort – Hypericum perfolatum
Stemonitis sp slime mold

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Common St Johnswort (Hypericum perfolatum) is native to Europe, Asia, and Africa. It is also known as Klamath weed, goatweed, and dotted St Johnswort (because the leaves are dotted). Introduced as a garden ornamental, it was discovered in 1793 in Pennsylvania. How many invasives can you name that were introduced in this manner? Seven US states consider is a noxious weed. As with many non-natives, Common St Johnswort has no substantiated value for wildlife as food or as cover. In fact, it is dangerous to livestock and birds do not feed on the fruiting parts either.

Common St. Johnswort contains hypericin

The danger to livestock is because the plant contains a chemical called hypericin. This chemical creates light sensitivity to lightly pigmented areas of livestock  – the mouth, nose, ears, and udders. Sheep, cattle, horses, and goats are susceptible; goats are more resistant than others. Their skin blisters, they lose hair, and generate a fever; they experience rapid pulse and respiration rates, increased salivation and diarrhea. Affected animals may die of dehydration or starvation because of swelling and soreness of the mouth.


Hypericum perfolatum
Common St Johnswort, Hypericum perfolatum
Common St Johnswort, Hypericum perfolatum

Photo on right: Common St Johnswort, Hypericum perfolatum, without blooms.

Photo above: Close up of the leaves of Common St Johnswort, Hypericum perfolatum. Notice the dots on the leaves.

Where it Prefers to Grow

Common St Johnswort likes full or partial sun and grows in a variety of soil types. It is found in grasslands and forested areas in North America. It does not like competition and establishes quickly where disturbance has created bare soil (e.g. animal scratches, burned areas, and new plantings). It also favors high soil moisture and wet seasons. Bosy and Reader (1995) found that a covering of grass litter significantly reduced germination and shoot extension.

Physiology of Common St. Johnswort

A single Common St Johnswort plant can have many aboveground vegetative parts strung together   with a system of vertical and lateral roots. This root system is extensive. It starts by putting down a taproot that extends 2 to 5 feet deep. The lateral root growth is shallow, usually 0.5 to 3 inches below the soil surface. In autumn and spring or following plant injury, these lateral roots produce buds from which new plants develop. These new plants can be as far as 3 feet from the parent plant with sprouting in between. Each seedling eventually matures and becomes independent, developing up to 30 stems per parent plant. This massive root system with its numerous young sprouts create an impenetrable mat near each parent plant. 

Common St Johnswort, Hypericum perfolatum
Common St Johnswort, Hypericum perfolatum

Notice the large taproot of this well-established plant.

This is a younger plant. Notice the offset of the taproot and all the little brown rootlets.

Common St Johnswort, Hypericum perfolatum

Lots of rootlets and seedlings arising from this taproot!

Common St Johnswort, Hypericum perfolatum

The lateral roots extend at relatively long distances and along these seedlings will arise. It’s easy to see how these can take up space in your prairie and outcompete other plants.

Common St Johnswort, Hypericum perfolatum

More root structure examples.

Common St Johnswort, Hypericum perfolatum

A young plant with a developing tap root.

Common St Johnswort, Hypericum perfolatum

A young adult plant without a fully formed tap root.

Seedlings of Common St. Johnswort are very small and grow slowly compared to seedlings of other plants. This slow growth makes seedlings susceptible to competition for light, nutrients, space, and moisture from not only the mature parent plant, but independent Common St. Johnswort plants and other species. Drought will severely reduce seedlings but when moisture levels are favorable an abundance of seedlings can be found.


Reproduction occurs via self-pollination as well as pollination by a variety of insects. Regeneration also occurs via vegetative propagation and is stimulated by disturbance (e.g. grazing, fire, mowing). Vegetative propagation can occur from the root fragments, the root crown, or sprouting from the lateral roots.

Common St Johnswort does not flower or produce seeds until the 2nd year. That is not much relief since estimates of seed production range from 15,000 to 34,000 seeds per plant with an average of 23,350 seeds (Tisdale et al. 1959). Variation in seed production is a function of site factors, seasonal growth conditions, and the level of disturbance and competition. That multiplies quickly when considering each plant can have 30 stems and each stem can contain 25-100 flowers! The seeds are produced in pods or capsules, which average to about 73 pods per plant (Compton et al. 1988).

Common St Johnswort, Hypericum perfolatum

The seed pods are forming.

We know from laboratory experiments and field studies that Hypericum seeds persist in the soil for many years. Some results suggest seeds of Hypericum species remain viable in the soil for more than 50 years (War et al. 1994). While germination is highest when the seeds are in the top half inch of the soil, they will germinate when they are 3 inches deep (Tisdale et al. 1959). Germination can occur in spring, fall, or winter and takes an average of 1 to 2 weeks. I have found young seedlings and plants in full bloom alongside plants setting seed.

Dispersal can be by wind, water, humans, and other animals. The pod or seed capsule has a sticky exudate which aids in two ways. It adheres the seed to something or someone and doesn’t allow the seed to germinate until this coat is removed by wear or water. Viability after storage is high; 94% germination after 5 years and 50% germination after 16 years.

How to Control Common St Johnswort

As with all non-natives and invasives, control must be sustained. Infested areas must be monitored each year. With this particular plant, keeping disturbance to a minimum is helpful until the infestation can be brought under control.

Cutting and mowing are ineffective as management methods for Common St Johnswort since sprouting may occur immediately after crown removal or defoliation. Mowing also negatively affects surrounding plants that could outcompete with it if allowed to grow and set seed. As with most other invasives I’ve been researching (i.e. Ox-eye daisy, spotted knapweedQueen Anne’s lace), burning could increase the density and vigor of these plants, thereby increasing the area of their infestations. If you use prescribed fire for other management reasons, be sure you schedule time for the requisite follow up.

I have found mechanical removal to be the most effective. It does leave soil disturbance but if done early enough in the spring, this provides space for the native plants. One can always overseed these areas in the fall, too. Because any root segments left on the soil surface could sprout, be sure to carry your pulled plants out.

Coupled with hand pulling, if I break off the taproot or leave a portion of a lateral root exposed, a spritz of 20% triclopyr in bark oil will ensure a new plant will not sprout. If the clump is large, to mitigate soil disturbance, I will lop it off at the base and spritz it with the tricolpyr mixture. Metsulfuron is another chemical that will kill the plants. It is usually applied foliarly and is successful with large clumps. The small leaves of first-year plants make foliar spraying difficult to do without a good deal of collateral damage; hand pulling is best for these. 

Persistence does pay off! We have been able to reduce the number of these plants at Driftless Prairies with annual monitoring and treatment.


Bosy, J. L.; Reader, R. J. 1995. Mechanisms underlying the suppression of forb seedling emergence by grass (Poa pratensis) litter. Functional Ecology. 9(4): 635-639.

Crompton, C. W.; Hall, I. V.; Jensen, K. I. N.; Hilderbrand, P. D. 1988. The biology of Canadian weeds. 83. Hypericum perfolatum L. Canadian Journal of Plant Science. 68(1): 149-162.

NRCS Plant Guide (Accessed 10 Jun 2016)

Rice, Barry Meyers; Randall, John. 2004. Weed report: Hypericum perforlatum–St. Johnswort, Klamath weed. In: Wildland weeds management and research: 1998-99 weed survey. Davis, CA: The Nature Conservancy, Wildland Invasive Species Program. 7 p. On file with: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT.

Tisdale, E. W.; Hironaka, M.; Pringle, W. L. 1959. Observations on the autecology of Hypericum perfolatum. Ecology. 40(1): 54-62.

Warr, Susan J.; Kent, Martin; Thompson, Ken. 1994. Seed bank composition and variability in five woodlands in south-west England. Journal of Biogeography. 21(2): 151-168.

Zouhar, Kris. 2004. Hypericum perfolatum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: [2016, June 10]. (Accessed 10 Jun 2016)

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