More info for the terms: competition, cool-season, cover, density, fire management, forbs, formation, frequency, fresh, invasive species, natural, presence, succession
Impacts: Earlier literature (e.g. [41,154,178]) seems to indicate greater impact from St Johnswort than is currently indicated in many areas. This is perhaps due to suppression of many St Johnswort populations by introduced biological control insects (see Biological control), or to its ubiquity. In California, for example, where St Johnswort populations have been historically widespread, St Johnswort is listed as a "wildland plant of lesser invasiveness" by the California Invasive Plant Council (Cal-IPC), and as a weed that is "so widespread that the agency does not endorse state or county-funded eradication or containment efforts" by the California Department of Food and Agriculture [30]. St Johnswort occurs in 44 states and 7 Canadian provinces but is designated as noxious in only 7 states and 2 provinces [183].
In agricultural areas, St Johnswort is more of a problem in pastures than in cropland because it is controlled by regular cultivation [139]. The most commonly described impacts of St Johnswort are loss of forage production and carrying capacity on rangelands and pastures, and losses from livestock poisoning [45,143,154,178].
Hypericin, a chemical constituent of St Johnswort found at all growth stages in either fresh or dry foliage, causes photosensitization in animals that consume it. Symptoms such as blistered skin and edemas have been reported in cattle, horses, domestic sheep, and domestic goats, with goats being most resistant, and light-colored animals of all classes being the most sensitive. Horses are more susceptible to hypericin toxicity than cattle, cattle more than sheep, and sheep more than goats [19]. Livestock rarely die directly from St Johnswort ingestion; however, effects of poisoning such as blindness or swelling and soreness of the mouth may prevent affected animals from foraging and drinking, and thereby contribute to death by dehydration and/or starvation. Several authors provide more detailed descriptions of St Johnswort poisoning and conditions under which it is likely to occur [19,34,102,139,143,154,163]. Some authors (e.g. [102,139]) also suggest that St Johnswort may cause contact dermatitis in humans.
Control: Much of the information on control of St Johnswort presented in this review comes from literature reviews and literature that provided additional pertinent ecological information.
Control of St Johnswort infestations requires several strategies designed to impact different plant parts and different stages in the plants life history. Emphasis on particular strategies may differ according to site type and St Johnswort growth habit. For example on rich soils, large St Johnswort plants with less root development may be easier to control by killing aboveground St Johnswort plant parts and then planting desirable plants, compared to the more difficult-to-control St Johnswort plants with greater root biomass in harsher sites [22,41]. With all St Johnswort control efforts, it is important that control measures are begun early and sustained for a sufficient length of time, as populations of St Johnswort can build from just a few seeds up to dense infestations in a short period of time (about 10-20 years) [25].
St Johnswort shows considerable variation in growth form, extent of vegetative reproduction, response to stress, and flowering frequency (see General Botanical Characteristics). Much of this plasticity can be related to variation in site; therefore, management strategies must be site-specific. Genotypic differences and the effects that these differences may have on susceptibility to different biological control agents have also been noted in St Johnswort ([46], and references therein).
St Johnswort seedlings are highly susceptible to competition, and the presence of competitive plant species is important to reduce the impact of periodic peaks in St Johnswort germination, establishment, and spread. Maintenance of vigorous pasture and rangeland by effective grazing management can prevent such seedling recruitment and limit St Johnswort spread. Effective grazing management plans in Australia are discussed by several authors [28,46]. In areas where vegetation is too sparse to suppress germination and recruitment of St Johnswort seedlings, other control efforts may be emphasized [22]. Any management strategy must also consider the presence of a viable soil seed bank for at least 20 years following control of St Johnswort (see Seed banking).
Based on results using detailed data from a long-term (6-7 years) observational study of St Johnswort populations, and an individual-based modeling approach for St Johnswort, Buckley and others [25] predict that the most effective management strategies for both open and shaded sites would concentrate on reducing the size of vegetative parts of St Johnswort. This may be especially true for infestations on harsh sites that tend to support St Johnswort populations with more lateral roots and root sprouts [22,41]. Single stresses, even if severe, are usually insufficient as St Johnswort root reserves respond with increased rates of sprouting from damaged roots and root crowns [22]. For example, beetles introduced for biological control defoliate St Johnswort plants but are not effective alone, especially in shaded sites (see Biological control) [25]. Multiple stresses such as defoliation and plant competition, defoliation and drought, or defoliation and fire, may cause reductions in St Johnswort crown density, and management of these factors is important for control to be successful [22,46].
Exhausting St Johnswort root reserves may require repeated stresses over a long period of time. Continuous low level herbivory through managed stock grazing may achieve this, although St Johnswort toxicity may limit grazing and a rotation of different types of grazing animals is recommended [19,28]. In its native range in Europe, crashes in population density of St Johnswort are associated with the destruction or exhaustion of root reserves by natural arthropod enemies. This has led to the current biological control strategy of targeting this part of St Johnswort's life history ([22] and references therein).
More research is needed to determine which factors affect the early stages of St Johnswort growth and recruitment, both from seed and sprouts, under different conditions, including fire treatment [25]. Whatever combinations of management techniques are adopted, a clearer understanding of the population dynamics of St Johnswort in different habitats should enable them to be better meshed together and their effects evaluated [22].
Prevention: The most effective method for managing invasive species is to prevent their establishment and spread. Some methods of prevention include limiting seed dispersal, containing local infestations, minimizing soil disturbances, detecting and eradicating weed introductions early, and establishing and encouraging desirable competitive plants [157].
Even though St Johnswort has a widespread distribution in North America, there are many areas where it has not yet established and other areas where it remains at low densities. Preventing further introduction and spread to uninfested areas is much easier, more environmentally desirable, and more cost-effective than is the subsequent management of large-scale infestations. Some aspects of prevention include curtailment of weed development along transportation and utility corridors, inspections and cleaning of machinery and vehicles prior to their movement from weed-infested to uninfested sites, and revegetation of disturbed soils with site-appropriate plant species to inhibit St Johnswort entry. Upon discovery, pioneer St Johnswort plants should be eliminated [143]. A similar approach to St Johnswort management in National Parks in Australia is described by Knutson [105], but also includes a comprehensive biological control program for large infestations, utilizing as many available organisms as possible.
Weed inventory and monitoring are important for early detection of new infestations and prevention of spread [97]. St Johnswort colonies with as low as 30% ground cover are distinguishable from other rangeland vegetation in multispectral digital images with 0.5, 1, 2, and 4 m spatial resolution taken from airplanes. This procedure provides a method to establish baseline plant community composition and a way to monitor species population changes and dispersal over time. See Lass and others [110] for details.
Because St Johnswort infestations may be serious on overgrazed areas, adoption of grazing systems that increase or maintain cover of desirable plants and/or reduce the amount of St Johnswort seed produced is a worthwhile preventative tactic [139,154]. St Johnswort occurs along roadsides and fencerows but rarely invades "properly managed" pastures (Lane 1979, as cited by [45]).
Integrated management: Controlling St Johnswort infestations requires more than defoliation or removal of aboveground plant parts. St Johnswort's ability to spread vegetatively and its apparent stimulation by defoliation (e.g. [41,178]) necessitate an intensive, integrated approach to control. An effective suppression program for St Johnswort requires planning, use of appropriate management methods, monitoring/evaluation, and persistence [143]. Integrated management includes considerations of not only killing the target weed, but also of establishing desirable species and maintaining weed-free systems over the long-term.
It is important to determine whether St Johnswort is a new or established invader in a particular infestation, because management approaches to each type will differ. With new infestations, emphasis may be placed upon early treatment and prevention, followed by monitoring and removal of newly established plants. Revegetation may also be useful to deter further St Johnswort invasion of the site. When dealing with an established population, the management approach may be more involved and take longer to implement. An established St Johnswort population is comprised of outlier, perimeter, and core plants. Management activities are prioritized based on plant location within the infestation. The 1st priority is management of the outlier populations to restrict continued spread of the infestation, followed by plant suppression within the perimeter and core infestation zones, respectively. Physical extraction and herbicides can be used against outlier plants [143]. A mixture of physical, cultural, chemical, and biological methods, chosen to cause multiple stresses to target plants, may be used to contain and eventually reduce perimeter and core population plants [22,143].
It is not possible or realistic to detail procedures to be followed in every potential management situation. The decision to use a combination of management methods must be based upon an assessment of plant developmental status, a characterization of sites infested or susceptible to infestation, and constraints of particular methods. Emphasis should be placed on the use of methods that interrupt St Johnswort seed dispersal and longevity, and that minimize habitat perturbations [143]. Site-specific integrated management programs can be developed for St Johnswort or other weeds ([143], and references therein).
Physical/mechanical: Several physical methods have been used for St Johnswort management including hand-pulling or digging, cutting or mowing, tillage, mulching, and flooding. Effectiveness of various physical methods depend on age, size, and location of St Johnswort infestations. For example, St Johnswort can be effectively managed by repeated tillage in intensively cropped situations [45]; however, tillage is rarely appropriate for natural areas.
Hand-pulling or digging of young, isolated plants is often effective, but is not considered practical for large populations of established, deeply rooted plants. Sampson and Parker [154] suggest "the mere digging, grubbing, and hand pulling of St Johnswort have proven costly and ineffective as a control measure." They observed sprouting from St Johnswort roots wherever segments were left in the surface soil, and numerous St Johnswort seedlings where adult St Johnswort plants had been dug [154]. Conversely, on prairie and dune sites on conservation preserves in Michigan, persistent annual pulling of mature St Johnswort plants to prevent seed production has been an effective control method. At Kitty Todd, Ohio, pulling is very effective, especially for early eradication of new St Johnswort populations [149]. It is important to remove as much of the root as possible, while minimizing soil disturbance, and removing all St Johnswort plant parts from the area to prevent possible vegetative growth or seed dispersal [143].
Cutting and mowing are typically considered ineffective as management methods for St Johnswort since sprouting may occur immediately after crown removal or defoliation [34,45,149,154]. These methods may be useful for preventing seed formation in St Johnswort [143,154], by cutting 2 or more times during the growing season [143].
Repeated mowing or cutting may weaken and eventually "starve" St Johnswort plants, and thus reduce population density [41,154]. St Johnswort is particularly susceptible to defoliation in spring. In Australia, hand defoliation every 2 weeks from late April to mid-November or from mid-August to early December resulted in from 93%-98% to 87%-100% destruction of crowns in low and high density stands, respectively. Hand defoliation from mid-August to mid-November or from early October to early December killed high density stands, and reduced crown density of low density stands by 72%-92%. Hand defoliation of St Johnswort in a eucalyptus forest in Australia resulted in the death of only 45% of crowns [41]. Survival of St Johnswort plants after hand defoliation depends on the tendency of the plant to reproduce vegetatively and the life span of the individual crown [34,41]. Cutting and/or mowing may not be feasible on many sites because of inaccessible terrain and potential damage to desirable plants [143].
Cultivation/tillage: St Johnswort is usually controlled by tillage and is not usually a problem in cultivated crops. Tillage is more effective when combined with sowing of competitive pasture or crops and adding fertilizer ([34,45] and references therein). In western Washington, St Johnswort density was reduced nearly 70% within 2 years following disking and seeding to cool-season introduced grasses (Gates and Robocker 1960, as cited by [116]).
Mulching/solarization: Sampson and Parker [154] killed St Johnswort by cutting plants to 2 inches (5 cm) above the ground surface and covering them with heavy tar paper. They did not indicate how long it took.
Flooding: There is no literature available regarding flooding as a control method for St Johnswort, although it may be effective since St Johnswort does not thrive in waterlogged soils [154].
Fire: See the Fire Management Considerations section of this summary.
Biological: Biological control of invasive species has a long history, and there are many important considerations before the implementing a biological control program. Tu and others [179] provide information and considerations for biological control of invasive species in general in their Weed Control Methods Handbook. Additionally, Cornell University, Texas A & M University, and NAPIS websites offer information on biological control.
There is a great deal of literature on biological control of St Johnswort in North America and Australia. The following discussion is based on literature reviews (e.g. [45,48,82,116,143]) and primary literature that discusses impacts [177], and/or population dynamics of St Johnswort and biocontrol insects in California [93], northern Idaho [31], British Columbia [84], and Australia [20,21].
Biological control of St Johnswort was initiated in Australia. A total of 12 insect species was released over 70 years, 6 of which established ([21] and references therein). Although early results were promising and led to biological control programs in other countries, including parts of North America, biological control has not yet managed to reduce St Johnswort infestations to levels that do not cause unacceptable economic or environmental damage in many areas. The Chrysomelid beetles (Chrysolina quadregemina and C. hyperici), for example, impact St Johnswort populations in certain situations, but are unable to prevent its continued spread [21].
From 1945 to 1946, shipments of both Chrysolina spp. were obtained from Australia and released in California. Within 2 years, both species were well established. This was the 1st attempt at control of a weed species by the intentional introduction of insects into North America [143]. Similar to initial results in Australia, Chrysolina spp. released in North America had a substantial impact on St Johnswort populations in many areas. In northern Idaho, the overall abundance of St Johnswort fluctuates around 3% of what was present before insects were introduced [177]. On annual grasslands in northern California, St Johnswort was reduced to less than 1% of its former occurrence 10 years after establishment of biocontrol insects [93]. The beetles were so successful in California that a 6-foot bronze statue of C. quadregemina was erected in the town of Eureka [129].
In some areas, however, biocontrol impacts are not as severe, and beetle populations do not reach high densities, probably due to climatic differences [116,139], particularly rainfall patterns [22]. These beetles appear to be adapted to a climate with hot, dry summers and mild, rainy winters [116], but their shade tolerance varies [20,21,24,31]. Other reasons suggested for varying impacts include predation by birds, spiders and other arthropods, and the presence of competing plant species [22].
The following table provides a list of biological control insects introduced to North America and areas where they have established. See the references listed or other reviews for more information.
Biological control agent Locations where established References St Johnswort borer (Agrilus hyperici) CA, ID, MT, OR, WA; mostly in mountain areas; attacks plants growing in shade [31,142] St Johnswort inchworm (Aplocera plagiata) ID, MT, OR, WA, Canada; dry area such as rocky ground, open sandy places, and limestone regions are favored [142] Klamath weed beetle (Chrysolina hyperici) CA, ID, MT, OR, WA; prefers conditions more moist than C. quadrigemina, and avoids shaded or barren, rocky locations [31,84,142] Klamath weed beetle
(C. quadrigemina) CA, ID, MT, OR, WA; mountainous, sunny and warm areas; does not seem to do well in shaded, barren, and rocky locations [31,84,142] Klamath weed midge (Zeuxidiplosis giardi) CA, HI, OR; seems to prefer damp locations with moderate to high relative humidity and high elevations; does not seem to like dry summers, continuously windy areas, or areas heavily grazed by livestock; "ineffective" in CA, BC, and Australia [84,142]
While no systematic release of either Chrysolina beetle had been made in the eastern U.S. (as of 1993), C. quadrigemina has been collected from several widespread localities in New York, Pennsylvania, Maryland, West Virginia, and Ohio, since 1989. Its establishment and range expansion in the eastern U.S. apparently resulted from a natural dispersal of populations from eastern Ontario. Several other states in the East where St Johnswort occurs have not been surveyed for Chrysolinids [91].
It may be helpful to fire managers to be aware of which biological control insects may be established in their area, as this affects postfire succession and population dynamics (e.g. [20]). See Fire Management Considerations for more information.
Livestock grazing: Several authors suggest using domestic goats to graze St Johnswort-infested areas to keep St Johnswort at low densities [139,154]. Some authors also suggest that domestic sheep can be used to graze St Johnswort in a rotational grazing system [34,46,116]. In 1 example, a heavy stocking of sheep controlled St Johnswort after a fire. Sheep, however, are more sensitive to St Johnswort toxins, and heavy infestations require 2 week rotations (2 weeks on and 5 weeks off), black sheep, and special management [34]. Cattle may be more effective than sheep under some conditions as they are less sensitive to toxins [19,34,46]. Managing St Johnswort with grazing requires intensive management, as defoliated St Johnswort plants may be stimulated to sprout from roots and root crowns [178]. Also, heavy grazing may do more damage to desirable plants in some situations, thus encouraging spread of St Johnswort.
Chemical: Herbicides are effective in gaining initial control of a new invasion (of small size) or a severe infestation, but are rarely a complete or long-term solution to invasive species management [29]. Herbicides are more effective on large infestations when incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations. Control with herbicides is temporary, as it does not change conditions that allow infestations to occur. See the Weed Control Methods Handbook [179] for considerations on the use of herbicides in natural areas and detailed information on specific chemicals and adjuvants.
Based on an individual-based modeling approach for St Johnswort, Buckley and others [24] predict that herbicide control that causes a sustained reduction in survival of at least 90% would be an effective control strategy in both open and shaded sites. They, among others [34,46,143], suggest that other considerations must be taken into account when using herbicides in a control program for St Johnswort. Herbicides are often too costly to be of practical value as a management tool for extensive infestations of St Johnswort. Repeated applications are often required to achieve adequate management, small patches may be missed, the St Johnswort seed bank must be considered for up to 20 years, and potential damage to associated vegetation must also be considered, especially in natural areas. Herbicides are best used as part of a larger, integrated system, and with adequate follow-up (for example planting desirable species, and grazing management) [46,139,143]. Some authors suggest grazing prior to herbicide application to remove nontarget plant biomass that may intercept the spray [139]. Timing and rate of application are important. See specific references, product labels or extension agents for this type of information.
Several herbicides have been used in an attempt to control St Johnswort. Many reviews discuss early attempts at killing St Johnswort with chemicals, many of which are no longer in use. Herbicides found to be effective at suppressing St Johnswort include 2,4-D, glyphosate, picloram, triclopyr, metsulfuron, and fluoroxypyr [36,195].
Campbell and Nicol [36] tested several herbicides for controlling St Johnswort on a site in Orange, Australia. They compared rate and type of herbicide, timing of application, application at annual intervals, rate of water carrier, and spraying combined with sowing of improved pasture species. Herbicides tested were triclopyr + picloram, fluoroxypyr, glyphosate, glyphosate + metsulfuron, and 2,4-D amine + metsulfuron. Results indicate that the only treatment that completely killed St Johnswort was 2 applications at annual intervals in summer of fluoroxypyr. Other herbicides that "substantially" reduced St Johnswort ground cover were triclopyr + picloram, glyphosate, and glyphosate + metsulfuron. Fluoroxypyr had no effect on regeneration of annual legumes and did not damage grasses, whereas triclopyr + picloram killed legumes and other forbs, and glyphosate killed grasses. Spraying St Johnswort and sowing improved species appeared to be the best treatment for long-term control because desirable pasture species established and replaced St Johnswort. Authors conclude that St Johnswort is difficult to control with herbicides and that every effort should be made to promote successful biological control and/or effective grazing management procedures [36]. See [36] for more details.
On prairie and dune sites at nature preserves in Michigan, sponge wick application of glyphosate on cut tops of St Johnswort plants is 100% effective at controlling St Johnswort [149].
Cultural: St Johnswort "competes strongly with other plants" but is itself sensitive to competition in the young stages or after it has been suppressed by cultivation, chemical control, or insects. If St Johnswort is suppressed by 1 or more methods but its ecological niche remains unfilled, reinvasion by St Johnswort or invasion by other undesirable species will likely occur [31,34,143]. Long-term control of St Johnswort requires that competitive plant communities be established and maintained using site-specific range management recommendations [143]. This may be especially difficult in natural areas where native species are desired [46,80].
The choice of plant species to be seeded should reflect site conditions, management, and future use. The Natural Resources Conservation Service, or local Cooperative Extension Service can recommend appropriate plant species for revegetation purposes. Small-scale plantings, evaluated for several years, provide another means to determine which plant species are most competitive with St Johnswort under prevailing climatic and land use conditions [143].
In Australia, a program of seeding pasture species (subterranean clover (Trifolium subterraneum) and canarygrass (Phalaris spp.)) combined with cultivation and fertilization on arable land, and reseeding alone on non-arable land, may control St Johnswort populations in pastures and arable rangeland [34,72]. In New South Wales and Victoria, radical land use change from badly invaded pastures to pine plantations has controlled many areas of St Johnswort because it cannot survive in densely shaded areas. St Johnswort may continue to grow along roads in the forest or plantation and may return when trees are removed [34,72]. It is usually the last species to disappear in a new forest and the first to return when the forest is cleared [34,41,72].
It is important that grazing be carefully managed before, during, and after establishment of desirable species [34,143]. Campbell [33] provides a review of different approaches to grazing management with and without seeding of improved pasture species on St Johnswort populations in heavy and light infestations on Australian pastures.
