Common St Johnswort – Hypericum perfolatum
Stemonitis sp slime mold

Subscribe to Blog via Email

Enter your email address to subscribe to this blog and receive notifications of new posts by email.

Join 224 other subscribers
image_pdfPDFimage_printPRINT

Common St Johnswort (Hypericum perfolatum) is native to Europe, Asia, and Africa. It was introduced to the U.S. as a garden ornamental. As with many non-natives, Common St Johnswort has no substantiated value for wildlife as food or as a host plant for rearing young, but does provide nectar for pollinators.  Seven US states consider is a noxious weed. Wisconsin isn’t one of them. In fact, USDA-NRCS suggests it would be good erosion control (Sheahan 2012). Ugh! When our government conservation groups advocate using invasives for any reason, it’s dispiriting to landowners trying to do the right thing AND it lessens their credibility as an authority.

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, “bush” with profuse blooms.

Middle: Common St  Johnswort without blooms

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

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. This means every time you burn, you should expect to have plants needing removal.

Growth of Common St. Johnswort

Common St Johnswort is a perennial, relatively long-lived plant that reproduces via short rhizomes and prolific seeding. It has 2 growth periods: spring and fall. It starts by putting down a 2-5’ deep taproot with shallow lateral root growth that is ½ -3” inches below the soil surface. These lateral roots create dense colonies quickly and more so if the tops are injured. These new plants can be as far as 3 feet from the parent plant with sprouting in between, creating an extensive network vertically and horizontally.

Common St Johnswort does not flower or produce seeds until the 2nd year. But that is little relief when each plant can have 30 stems and each stem can contain 25-100 flowers resulting in 15,000 to 34,000 seeds per plant (Tisdale et al. 1959, Crompton et al. 1988). It can self-pollinate as well. Yikes!

Compounding its invasiveness is the seedbank. Seeds can remain viable in the soil for more than 50 years (War et al. 1994) and their viability is high: 94% germination after 5 years and 50% germination after 16 years. 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).

Dispersal can be by wind, water, humans, and other animals. The pod or seed capsule has a sticky exudate which adheres the seed to something or someone and doesn’t allow the seed to germinate until this coat is removed by wear or water.

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.

Managing Common St Johnswort

As with all non-natives and invasives, persistence is required. Infested areas must be monitored each year. There is no effective triage method for this plant so plan management activities with this in mind.

Herbicide

I have found herbiciding with Garlon 4 in bark oil to be the most effective. If done early enough in the spring, the death of Common St. Johnswort provides space for the native plants. Use the spritz method and you’ll reduce any collateral damage and won’t be putting out much herbicide.

Mowing

Cutting and mowing are ineffective 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.

Hand pulling

Pulling or using the parsnip predator isn’t terribly effective unless you carry the pulled plant out or bag it. Aside from it being very difficult to get all the roots, any root segments left on the soil surface could sprout. Be sure to bag and carry out any plants you pull!

Fire

Fire is not effective but rather it encourages these plants because the reproduce vegetatively. As with most other invasives, burning could increase the density and vigor of these plants, thereby increasing the size of their infestations. Fire also leaves open spaces, which will be occupied with whatever plant (native or non-native) that gets there first and it’s usually the invasives! Dang it!

Grazing

Grazing can be dangerous to livestock. Common St Johnswort 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 although 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.

Overseeding is necessary to fill in the gaps left by your removal efforts. Plan on overseeding for several years.

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

References

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 http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.370.857&rep=rep1&type=pdf (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.

Sheahan, C.M. 2012. Fact sheet for common St. Johnswort. USDA-NRCS. Cape May Plant Materials Center. Cape May, NJ.

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.

Discover more from Driftless Prairies: Native Ecosystems

Subscribe now to keep reading and get access to the full archive.

Continue reading