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An Assessment of Exotic Plant Species of Rocky Mountain National Park

Hypericum perforatum L.
Klamath weed, St. John's wort, goatweed (Hypericaceae)

Current level of impact
Known locations in RMNP: Found only at one site in RMNP, a recently disturbed road shoulder. However, Klamath weed is spreading from its original infestation point at the east base of the railroad tracks at Rocky Flats. Found in Boulder County at Walker Ranch and Boulder Canyon.
Assessment: Klamath weed has been found only at one recently disturbed area in RMNP. However, this species is capable of invading and modifying native plant communities and should be controlled immediately.

Origin: Introduced from Europe.
Geographic distribution: Found frequently in Pacific Northwest, throughout eastern half of U.S. Infestation in Colorado includes a large area in north-central portion of the state and scattered areas in western Colorado.
Ecological distribution: Old meadows, pastures, along roadsides and waste places, dry pastures, and rangelands.
Soils: Usually on dry, gravely, or sandy soils. Can tolerate pH ranges of 4.3-7.6, does best in calcareous soils.

Perennial, reproduces by seeds and short runners. Root system spreads horizontally on light soils, and are capable of forming new buds. Vegetative propagules quickly separate from parent plant as soon as the new plants become established. Flowers June to September.
Seed production: 15,000-30,000 seeds/plant produced in one season, contributing to its potential for rapid spread.
Seed longevity: Seeds remain viable in soil for 6 to 10 years.
Seed dispersal: Seeds may be dispersed by animals, wind, and water.
Germination: Seeds germinate well, if not more readily if submerged in water. Germination is retarded by even small amounts of calcium (calcium may alter the permeability of seed coat). Germination is generally inhibited during hot dry summers, or in high litter levels and when seeds are buried greater than 2 mm below the surface.

Grows early in spring when moisture is abundant. Aggressive invader of grazed lands, frequently the result of overgrazing, now spreading into mountains. Somewhat intolerant of severe competition, Klamath weed spreads rapidly in pastures on dry soils.
Level of impact: Noxious in many states.

Difficult weed to eradicate because of plant's extensive root system and long lived seeds.
Mechanical: Tillage can be used to control Klamath weed. However, vegetative reproduction may be stimulated by hand pulling, mowing, and fire.
Chemical: Chemicals which have been used to control Klamath weed include; 2,4-D ester formulations, 2,4-D ester plus Tordon(picloram), and Roundup (glyphosate). Wetting and uptake of herbicides may be inhibited because leaves are small and waxy.
Biological Control: Grazing animals generally avoid this plant unless forageis limited. Biological control has been relatively successful using several beetles (Chrysolina hyperici (Foster) and Chrysolina quadragemina (Suffrian)). These beetles feed on foliage as plants begin to flower in April and May. The following season, beetles emerge in spring and feed on plants as they begin growth. Halloway (1964) reports that three years of heavy feeding by beetles will destroy the weed. However, in Canada, these insects do not thrive in arid, lightly grazed Douglas fir forests, or along stream banks and other shady moist areas, or at high elevations near treeline.

Known to be poisonous to animals. Parts of the plant contain florescent pigments that, when exposed to light, cause severe dermatitis. However, most studdies show that Klamath weed poses a more serious threat to reducing range carrying capacity than as a poisonous weed.


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.
Halloway, J.K. 1964. Projectsin biological control of weeds. pp. 650-670.  
     In P. DeBach (ed.) Biologicalcontrol of insects pests and weeds.  
     Reinhold Publishing Corp. New York.
Huffaker, C.B. and C.E. Kennett, 1959.  A ten year study of vegetational
     changes associated with biological control of Klamath weed. Journal
     of Range Management 12:69-82.
Stubbendiek, J., C.H. Butterfield, and T.R. Flessner, 1992.  Hypericum 
     perforatum L. pp. 232-237.  In An Assessment of Exotic Plants of 
     the Midwest Region.  final Report.  Department of Agronomy, University
     of Nebraska, Lincoln.
Tisdale, E.W., M. Hironaka, and Pringle, 1959.  Observations on the autecology
     of Hypericum perforatum, Ecology 40:54-62.
Tisdale, E.W. 1976.  Vegetation Responses following biological control of 
     Hypericum perforatum in Idaho.  Northwest Science 50:61-75.

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