Not much mentioned in the literature, increased fire hazard form dried plant material is all that was found, but this is probably rare. in forested or wildland areas, dry flower stems can contribute to fire hazard risks

Weeds of California and Other Western States, J.M. DiTomaso and E.Healy, as yet published

Displacement of wildlife and livestock. Can greatly depreciate livestock and wildlife carrying capacities, and endanger the biological diversity of grazing lands;
Displaces desirable wildlife; toxic to livestock

Biology and Management of Noxious Rangland Weeds, Sheley and Petroff. (ST. Johnswort. Gary L. Piper)

Good deal of both types disturbance- lead to establishment, although, one source found citing establishment into a pristine rangeland. Seed are disseminated short distances by the wind; long distances by adherence to animals (facilitated by a gelatinous seed coat), animal ingestion and subsequent deposition in feces, water movement, and through the activities of humans.
*Can be established in either highly degraded or pristine rangelands

Krueger, J. and R. Sheley. Montana State University, Extension Service Montguide, MT199810 AG, St. Johnswort, July 2002; Sampson, A.W. and K.W. Parker, 1930. St. Johnswort on Range Lands of California. University of California College of Agriculture, Agricultural Experiment Station, Berkeley, California. Bulletin 503, December 1930, University of California Printing Office, Berkeley, California.

Seems to be increasing along roadsides at higher elevations in the NE part of the state, but populations eventually decline because of biological control agents.
Fluxuations occur naturally, cycling up and down, with fluxes in bioagent populations
Localized outbreaks of the plant sometimes occur after disturbances such as logging, fire or during low population cycles of the bioagents
Many St. Johnswort populations are still increasing in size, while others have remained static or decreased. Unfortunately, a reliable published estimate of the amount of land presently infested by St. Johnswort is not available. In the 1940s it occupied over 1 million acres and today it is only about 1% of that, or less. Varies widely across the state, especially once you factor in elevational differences (bioagents can't survive); At this time ( July 26, 2004) this weed seems to be increasing total area infested- personal observation and in speaking with other land managers; we could just be in a down-swing of the bioagent populations.

Weeds of California and Other Western States, J.M. DiTomaso and E.Healy, as yet published; Biology and Management of Noxious Rangland Weeds, Sheley and Petroff. (ST. Johnswort. Gary L. Piper)

Human caused disturbances and gardening largest methods of human-caused dispersal. Long distance dispersal by humans into wildlands is probably uncommon. Sometimes cultivated like a crop or grown in herb gardens; Herbal medicine, hypericin is the antidepressant ingredient in St. Johnswort remedies
Localized outbreaks of the plant sometimes occur after disturbances such as logging, fire or during low population cycles of the bioagents.

Weeds of California and Other Western States, J.M. DiTomaso and E.Healy, as yet published

Northwestern region, Cascade Range, northern and central Sierra Nevada, Sacramento Valley, San Francisco Bay region, Central Coast, Peninsular Ranges, to 1500m.
Most contiguous states except Arizona, New Mexico, Utah, Alabama, and Florida.
south-eastern Australia, Eastern Canada and British Columbia;
In California and E. Canada- populations of H. perforatum have been found to be more and weedy; In Britian and Eastern Canada, this weed is a minor problem
Widespread in: Europe, Asia, N. and S. Africa, Australia, and western and eastern N. America Invades elsewhere but only in ecological types that it has already invaded in the state.

Weeds of California and Other Western States, J.M. DiTomaso and E.Healy, as yet published

Rangeland areas and pastures, especially those that are poorly managed, fields, roadsides, and forest clearings or burned areas in temperate regions with cool, moist winters and dry summers. Grows best on open, disturbed sits on slighly acidic to neutral soils. Does not tolerate water saturated soils.
By 1940 more than 1 million acres of rangeland and was infested- several years later biological control agents were released - the bioagents tended to only survive below 1500 m- dramatically reducing infestation across the Pacific Northwest.
Seems to be increasing along roadsides at higher elevations in the NE part of the state; Small populations still exist in shady areas, boggy situations, north-facing slopes, and roadsides where the beetles are less active.
Fluctuations occur naturally, cycling up and down, with fluxes in bioagent populations
Localized outbreaks of the plant sometimes occur after disturbances such as logging, fire or during low population cycles of the bioagents.

Weeds of California and Other Western States, J.M. DiTomaso and E.Healy, as yet published; Poisonous Plants of California, Fuller, T.C. et al. pp179-180; Sampson, A.W. and K.W. Parker, 1930. St. Johnswort on Range Lands of California. University of California College of Agriculture, Agricultural Experiment Station, Berkeley, California. Bulletin 503, December 1930, University of California Printing Office, Berkeley, California.