More info for the terms: competition, cover, density, fire management, forbs, invasive species, litter, marsh, natural, nonnative species, phenology, resistance, series, succession
Impacts: Perennial pepperweed is listed by the California Invasive Plant Council (Cal-IPC) on List A-1: a widespread, aggressive invader that displaces natives and disrupts natural habitats. These are the most invasive wildland pest plants in their classification [16]. Little research is available documenting or quantifying impacts of perennial pepperweed. However, several authors indicate observed impacts, especially in wetland and riparian settings. Observed and/or suggested impacts include altered species diversity, structure and function [10,67,83,103], displaced native species [74,83] including rare plant populations (Skinner and Pavlik 1994, as cited by [41]), decreased food and habitat for several wildlife species [41,46,83,102,104], changes in biogeochemical cycles [8,11,12] including emission of mercury from contaminated soils into the atmosphere [50], increased streamside soil erosion (personal communications with Susan Donaldson and Jim Young, as cited by [69]), and economic losses through reduced forage quantity and hay quality [4,28,41,46,102,103,104].
Observations of researchers and managers (e.g. [10,74,83,103,104]) suggest that perennial pepperweed has altered species diversity, structure, function, and succession in many wetland and riparian areas in the western U.S. Because perennial pepperweed is highly competitive, grows in dense patches that are near monocultures, and results in a buildup of heavy thatch and litter that may be rich in salts (depending on the site), seedling recruitment and productivity of important, native species may be adversely affected [10,103,104]. Few data are available to support these observations. Reports of perennial pepperweed replacing quackgrass (Elytrigia repens), another highly competitive, nonnative species, attest to the competitiveness of perennial pepperweed [11,104]. An inventory of rare and endangered plants in California indicates that perennial pepperweed is encroaching on several rare plant populations at Grizzly Island Wildlife Area in Suisun Marsh, including soft bird's-beak (Cordylanthus mollis ssp. mollis), Suisun thistle (Cirsium hydrophilum var. hydrophilum), and Suisun Marsh aster (Symphyotrichum lentum) (Skinner and Pavlik 1994 as cited by [41]).
Changes in vegetation structure caused by perennial pepperweed may interfere with management objectives and reduce habitat for various wildlife species. For example, observations along the Green River in Utah indicate that because of perennial pepperweed's increased canopy height and density as compared to native vegetation, it directly interferes with mosquito control efforts in the area (Steven V. Romney, personal communication as cited by [67]). Perennial pepperweed's tall stature, dense growth pattern, and accumulations of semiwoody stems (see General Botanical Characteristics) are also purported to negatively impact nesting habitat for wildlife [83,102,104]. Observations by Blank and Young [12] suggest that when perennial pepperweed populations reach a density of 50 stems per m2, no waterfowl nesting occurs. According to Howald [41], perennial pepperweed outcompetes grasses that provide food for waterfowl. Perennial pepperweed has invaded pickleweed-dominated marshes in some areas in California, and thus poses a threat to the habitat of the endangered salt marsh harvest mouse, California black rail, and California clapper rail [41,83]. No data are provided to support these observations. At the Malheur National Wildlife Refuge in Oregon, perennial pepperweed has displaced 5 and 10% of the meadow and grass/shrub uplands, respectively, that are critical habitats for nesting aquatic and neotropical birds (US Fish and Wildlife Service, unpublished data, as cited by [46]). Because perennial pepperweed makes hay from infested pastures unmarketable, perennial pepperweed jeopardizes the haying program on the Malheur National Wildlife Refuge, which provides short and medium grasses for sandhill cranes, shorebirds, and waterfowl [46].
Observations at the Honey Lake Wildlife Refuge in northeastern California, indicated "striking differences" in soil profiles in perennial pepperweed infested areas compared with soils in similar, noninvaded areas of native hayland [10]. These observations led to a series of research projects that indicated many differences in soil physical and chemical properties between perennial pepperweed infested sites and similar noninvaded sites [8,10,11,12] (see Successional Status).
Perennial pepperweed can take up mercury from contaminated soils and emit about 70% of that taken up during the growing season into the atmosphere (for every one molecule retained in perennial pepperweed foliage, 12 molecules were emitted) [50]. The most critical factors governing mercury flux from plants are mercury concentration in the soil, leaf area index, temperature, and irradiance [51]. See Leonard and others [50,51] for more details.
The combination of low root density and easily-fragmented perennial roots allows soil erosion to occur during flooding events or other high waterflow events along riverbanks infested with perennial pepperweed. The water will also carry root pieces (which float) downstream where they can establish new populations (personal communications with Susan Donaldson and Jim Young as cited by [69]).
Perennial pepperweed invasion also causes economic losses when it persists in hay meadows, pastures, and/or cropland. Where perennial pepperweed invades native hay meadows (e.g. in the Humboldt River Valley of Nevada and Lassen County, California), it reportedly lowers the quality of hay in terms of protein content and digestibility [103]. Rumors that perennial pepperweed may be poisonous are usually based on horses being fed hay containing perennial pepperweed under confined conditions, but no data are available to confirm its toxicity [104]. In infested pastures that are not mowed annually, the accumulation of perennial pepperweed stems inhibits grazing [4,104]. Fence rows and "waste areas" within fields may become dense, impermeable thickets of perennial pepperweed [102].
Control: Eradication of perennial pepperweed is no longer an option in western North America, and control and quarantine efforts for perennial pepperweed have been largely unsuccessful. Biological suppression may be a viable goal that is likely to require an integrated management approach, as no single technique is likely to control perennial pepperweed [103]. Perennial pepperweed has a deep, extensive root system with a high reproductive potential that allows it to sprout repeatedly following removal of aboveground growth. Perennial roots must be killed or removed to prevent reinfestation by perennial pepperweed. According to Renz [69], these roots may remain dormant in the soil for several years, resist desiccation, and have been found more than 9 feet (3 m) deep in the soil profile (personal communication from Jim Young as cited by [69]). Strategies to control perennial pepperweed must include removing aboveground growth and perennial roots, preventing seed production, monitoring for perennial pepperweed re-establishment for several years, locating and controlling potential sources of reinfestation (e.g. populations upstream, down the road, next door, etc.), and establishing desirable vegetation. Timing control efforts to coincide with vulnerable stages in perennial pepperweed phenology may increase the probability of success (see Seasonal development). It is also important to consider how different control techniques may affect perennial pepperweed phenology and distribution of energy stores in perennial pepperweed (see Physical/mechanical control and Chemical control) [27,69]. More research is needed in these areas, especially long-term research, as many studies report results for only 1 year after treatment.
If resources are available to control an entire infestation of perennial pepperweed, including large stands, efforts should be made to do so. If only part of an infestation can be treated, modeling and experience indicate that controlling outlying patches and the leading edge of infestations are most important [57]. For smaller, scattered populations, an early response strategy can lead to reduced long-term cost of control. If possible, early detection and eradication of small satellite populations is the least expensive and most effective way to control perennial pepperweed [69,74]. Management of perennial pepperweed will likely be more intensive and costly as infestations age. Without management, infestations are expected to increase in density, store energy in belowground tissues, and close the canopy structure. All of these factors increase the difficulty of managing perennial pepperweed [71]. Therefore, intense monitoring, early detection, and rapid removal of perennial pepperweed increase the probability of successful control.
Diligent monitoring in areas where perennial pepperweed is being managed is important since roots are difficult to kill. Areas should be monitored in early spring and late summer whenever possible. In many places perennial pepperweed is one of the first plants to emerge in the spring and can be identified early in the growing season. Later in the season, as other plants senesce, perennial pepperweed will be one of the last remaining plants alive and green. Rosettes can be difficult to detect, but they may form the leading edge of an infestation and so are important to detect and control [57]. The best time to detect new rosettes is late summer. Monitoring can also be done in fall/winter by looking for senesced stems [69]. Nearby populations should also be located and controlled in an effort to limit off-site propagule sources [69,83].
With all control methods, it is important to restore desirable vegetation [99]. When perennial pepperweed is controlled, it may be necessary to also remove its litter in order to stimulate germination and growth of desirable plants. Previously infested land can recover, but costs incurred will vary depending upon location, density, and length of time infested. If soil salinities are dramatically increased by perennial pepperweed infestation, an intensive soil remediation program may be necessary before desirable native species can re-establish [69]. More research is needed to identify plants that can effectively compete with perennial pepperweed.
Prevention: The most efficient and effective method of managing invasive species is to prevent their invasion and spread [79]. Preventing the establishment of weeds in natural areas is achieved by avoiding management activities that encourage invasion, maintaining healthy natural communities, and conducting aggressive monitoring several times each year. Monitoring efforts are best concentrated on the most disturbed areas in a site, particularly along roadsides, parking lots, fencelines, and waterways. When a perennial pepperweed infestation is found, the location can be recorded and the surrounding area surveyed to determine the size and extent of the infestation, so these sites can be revisited on follow-up surveys [42]. New infestations should be controlled promptly to prevent further spread [7,104], followed by monitoring and some combination of control methods. Prevention of new invasions is much less costly than postinvasion control [52].
Sources of infestations must be controlled to prevent further spread. Equipment used in perennial pepperweed infested areas must be thoroughly cleaned before transport. Water sources, imported soil, and hay bales used for erosion control should be monitored to ensure they do not contain perennial pepperweed roots or seeds. Many infestations of perennial pepperweed have been initiated by one of these sources (CalEPPC 1999, as cited by [69]). Seed sources of revegetation species should be checked to ensure that there is no perennial pepperweed contamination.
Integrated management: A combination of complementary control methods may increase effectiveness of control efforts for perennial pepperweed. Integrated management includes not only killing the target plant, but establishing desirable species and discouraging nonnative, invasive species over the long term. Components of any integrated weed management program are sustained effort, constant monitoring and evaluation, and the adoption of improved strategies. An integrated management plan includes efforts to place continual stress on undesirable plants while promoting growth of desirable plants.
Integrated perennial pepperweed control strategies consisting of mowing, disking, or burning combined with herbicide applications before or after treatment, have been studied (see Physical/mechanical and Chemical control sections). Kilbride and others [46] examined the potential to restore native vegetation in infested meadows in the Malheur National Wildlife Refuge using integrated management techniques including herbicides, disking, fire, and combinations thereof. Study plots were predominantly perennial pepperweed interspersed with trace amounts of beardless wildrye, squirreltail, basin wildrye, saltgrass, cheatgrass, and forbs (e.g. flixweed tansymustard), as well as rushes and sedges in lower (wet) areas. Percent reduction of perennial pepperweed density 1 year after treatment was reported as follows [46]:
Site chlorsulfuron metsulfuron methyl disk chlorsulfuron-disk metsulfuron methyl-disk fire chlorsulfuron-fire metsulfuron methyl-fire Big Sage 100 90 46 100 99 not reported 100 97 Oliver Springs 100 100 -2 100 99 lack of fuels lack of fuels lack of fuels Skunk Farm 100 not reported 32 100 98 lack of fuels lack of fuels lack of fuels
Herbicide treatments alone or in combination with disking or fire resulted in 90% to 100% reduction in perennial pepperweed density 1 year after treatment, with chlorsulfuron providing slightly greater reductions than metsulfuron methyl. All herbicide treatments were more effective than disking alone. It is unclear what effectiveness fire alone had on perennial pepperweed density, as no data are given. For more information on the constraints and effects of fire treatments, see Fire Management Considerations. Disking in combination with herbicide treatments reduced cover of native forbs and grasses and resulted in the establishment of undesirable, nonnative species (cheatgrass and Canada thistle (Cirsium arvense)). Combining herbicide treatments with fire or disking did not increase effectiveness over herbicide treatment alone [46]. However, data from only 1 year may be insufficient to judge long-term effectiveness of control measures.
With all control methods, it is important to encourage growth of desirable vegetation. According to Young and others [99] when herbicidal weed control is used on near monocultures of perennial pepperweed in native hay meadows in the Intermountain Area, spontaneous regeneration of meadows is slow and reinvasion of perennial pepperweed likely. This makes seeding of desirable species necessary to maintain suppression of perennial pepperweed [99] (see Cultural control).
Discussion of other combinations of control methods is included in the following sections when these were encountered in the literature.
Physical/mechanical: Physical and mechanical control methods such as mowing and disking alone are unlikely to control perennial pepperweed because new plants quickly regenerate from both undisturbed and fragmented roots in the soil (see Regeneration Processes). Small infestations of perennial pepperweed can be controlled by repeated removal of above- and belowground plant material. Care must be taken to remove as much of the root as possible as small pieces can sprout. If repeated several times this process can be successful, but it is labor intensive [69]. For larger infestations, combining mowing or disking with other control strategies may improve success (e.g. [71]). Neither mowing nor disking is usually appropriate in natural areas, as they are likely to damage desirable plants, expose soil, and increase erosion potential.
While it is generally accepted that mowing will not control perennial pepperweed (e.g. [41,69,83]), Baker [4] notes that haying (i.e. repeated mowing) perennial pepperweed infested fields in Wyoming, seems to prevent it from developing into a monoculture. There are no examples in the literature where repeated mowing was tested as a control method for perennial pepperweed.
Timing manual defoliation or other disturbances of above ground tissues during periods when minimum pools of stored energy are present can deplete stores of energy for future growth and thus enhance long-term control. Research has shown that minimum amounts of stored energy are in belowground tissues of perennial pepperweed at the bolting stage [70] (see Seasonal Development), indicating this as the optimal time to mow stems. Unfortunately, perennial pepperweed quickly recovers from mowing and produces leaves from previously dormant buds near the soil surface [71]. Sprouting may require less than 14 days (unpublished data as cited by [68]), and total nonstructural carbohydrate (TNC) pools in the top 16 inches (40 cm) of roots in mowed plants were not different than unmowed plants 7 and 19 days after mowing at 2 study sites, respectively. The authors speculate that TNC from roots deeper than 16 inches (40 cm) may have been mobilized, or that reserves were replaced through photosynthesis by new leaves [70,71].
Mowing changes the architecture of a perennial pepperweed stand. Stem density is reduced (64 stems/m2 in mowed plots compared to 142 stems/m2 in plots not mowed), as well as stem height (19.4 inches (49.21 cm) in mowed plots compared to 38.0 inches (96.42 cm) in unmowed areas), and leaf area distribution is altered within the stand (see General Botanical Characteristics) [69]. Unmowed plants have the majority of leaf area in the top 3rd of the canopy, whereas in mowed areas, 84-86% of perennial pepperweed leaf area was found within the lower 3rd of the canopy. Sprouting stems also had 21-59% less total leaf area than plants not mowed at the flowerbud stage. According to Renz and DiTomaso [71], this change may increase effectiveness of herbicide sprays used after mowing by depositing more herbicide on basal leaves where it can preferentially be translocated to roots. Also, perennial pepperweed plants sprouting after mowing are more uniformly synchronized in growth stage, so herbicide application at a time of maximal below ground translocation is consistent throughout the stand [71]. According to Renz [69], a potential drawback of this approach is that perennial pepperweed sprouting is limited in dry sites and/or low precipitation years.
Renz and DiTomaso [71] tested the effects of mowing and herbicide treatments, alone and in combination, in 3 contrasting sites (high desert, roadside, and floodplain) in California. Dense, monospecific stands with >85% perennial pepperweed cover were mowed to a height of 1 to 2 inches (2-5 cm) when flowerbuds were present on the main shoot and shoots from axillary buds. Shoots quickly sprouted after mowing, resulting in a dense stand of rosette plants. The majority of these remained as rosettes throughout the season. Herbicide treatments (glyphosate, 2,4-D, and chlorsulfuron) were applied to mowed plants when bolting shoots reached the flowerbud stage. Perennial pepperweed biomass and density were measured 1 year after treatments. Mowing alone did not significantly (p<0.1) reduce perennial pepperweed biomass or density 1 year after treatment. Chlorsulfuron was equally effective (97-100% biomass reduction) with or without mowing on the floodplain site, and mowing improved effectiveness on the high desert and roadside sites. Glyphosate was equally effective with or without mowing (83.5-87.4% biomass reduction) on the high desert site, while mowing enhanced effectiveness on the roadside and floodplain sites. Effectiveness of 2,4-D was not significantly (p<0.10) enhanced by mowing, and was the least effective of the herbicides tested, with or without mowing.
A similar experiment compared the effects of combined mowing and herbicide treatments on dense and sparse perennial pepperweed infestations [71]. Mowing enhanced the effectiveness of herbicides in reducing perennial pepperweed biomass 1 year after treatments in the dense infestation, but not in the sparse infestation. Following control measures, response of resident plants was limited in the dense infestation, but extensive in the site with the sparse infestation. Nonnative annual grass cover increased at both sites. The authors conclude that dense perennial pepperweed populations may require an integrated approach, while less dense or establishing populations may be controllable with chemicals alone. Additionally, recovery of resident plant populations increases when management programs are initiated before perennial pepperweed infestations become dense, monospecific stands [71]. Renz [69] presents data on various herbicides used in this manner.
Disking alone is also not thought to be an effective control method for perennial pepperweed because new plants sprout from root fragments [95]. However, incorporating tillage with other management approaches may improve control [69]. For example, tillage after herbicide application may be an effective way of bringing the treated roots to the soil surface where they will desiccate [97].
Disking perennial pepperweed may increase the density of an infestation (Renz and DiTomaso unpublished data, as cited by [69]). Periodic disking during the growing season over a 2-year period resulted in no permanent reduction in perennial pepperweed cover in native hay meadows in Nevada [102]. Disking perennial pepperweed at Grizzly Island Wildlife Area resulted in a serious increase in its distribution (Feliz, personal communication as cited by [41]).
Control following spring herbicide applications at the flowerbud stage was slightly improved in disked areas relative to areas not disked. When previously disked areas are mowed the following spring at the flowerbud stage and herbicides are applied to sprouting plants in the flowerbud stage, greatly enhanced control by herbicides is observed. The incorporation of disking into a control strategy had been shown to enhance plant diversity the following year (Renz and DiTomaso, unpublished data), perhaps by stimulating seeds in the seed bank [69]. Renz [69] presents data on various herbicides used in this manner.
Inundation: Case studies described by Howald [41] and Renz [69] suggest that perennial pepperweed may be intolerant of prolonged inundation.
Research presented by Chen and others [17,18] suggests that perennial pepperweed may tolerate and survive saturated conditions, but does not grow well under these conditions. This may be an adaptation to arid or semiarid riparian habitats where spring flooding and summer drought are characteristic. After 7 days of flooding, total biomass (p<0.001) and root/shoot ratio (p=0.002) of flooded plants were significantly less than those of unflooded controls (maintained at -20 kPa soil matric water potential) [18]. Further study of anaerobic metabolism in roots of perennial pepperweed seedlings indicates that perennial pepperweed roots have metabolically adaptive strategies to anoxia, but there is evidence of oxidative stress under anoxia and of postanoxic injury from free radicals upon re-exposure to air. Results suggest that perennial pepperweed exhibits a mixture of characteristics typical of hydrophytic, facultative, and anoxia intolerant species [17].
Fire: See the Fire Management Considerations section of this summary.
Biological: Development of a biological control program for perennial pepperweed seems unlikely because of risks to many important crop plants that are members of the mustard family [7]. Additionally, several native pepperweed species from the western U.S. are either listed as endangered or are considered for listing [7,41]. Based on molecular phylogeny, perennial pepperweed is more closely related to Californian species than are other members of the genus [59]. Acknowledging these difficulties, Birdsall and others [7] point out the limitations of other control methods for perennial pepperweed, and suggest that both classical and augmentative biological control approaches warrant further examination, especially the potential for use in conjunction with other available techniques.
According to Young and others [97,99] perennial pepperweed can be suppressed by grazing, and there are examples in both Colorado and Nevada where grazing management suppressed perennial pepperweed. They provide no data or specific examples, nor do they mention which grazing animals are effective. According to Wood [94], in a preliminary test, 13 goats ate perennial pepperweed with no ill effects, and domestic cattle and sheep graze perennial pepperweed growing amid other plants, but they do not eat pure, dense stands of perennial pepperweed. According to Baker [4], domestic sheep readily eat perennial pepperweed in Wyoming, and even heavily infested pastures appear weed free. Once the sheep are removed, however, perennial pepperweed comes back (see Palatability/nutritional value). Grazing management is most effective in long term suppression of perennial pepperweed when initiated before all perennial grasses are lost from the community [97]. More research is needed on the use of livestock for perennial pepperweed suppression.
Chemical: Before using herbicides for control of invasive plants, managers must consider the effectiveness of the herbicide on the target plant, appropriate timing and rates of application, the potential impacts on nontarget organisms, and residual activity and toxicity of the herbicide. If chemical control is used it must be incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations [15]. Use of herbicides may be restricted in some areas. See the Weed Control Methods Handbook for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.
According to Howald [41], attempts have been made to control perennial pepperweed with chemical herbicides in California, Oregon, Wyoming, Idaho, and Utah. The shoot portion of perennial pepperweed is susceptible to several herbicides. However, even with 98% initial control, sprouting perennial pepperweed plants may result in total stand dominance by the end of the next growing season [103]. The most effective herbicides appear to be chlorsulfuron, metsulfuron methyl, and imazapyr [4,20,67,71,102]. Glyphosate is effective when applied after mowing to sprouting stems at the flowerbud stage [71]. At Malheur National Wildlife Refuge in Oregon, chlorsulfuron and metsulfuron methyl were tested alone and in combination with either fire or disking, with chlorsulfuron reducing perennial pepperweed densities by 100% in all 3 sites tested, and metsulfuron methyl resulting in density reductions of 90 to 100% [46] (also see Integrated management and Fire management considerations).
Chlorsulfuron delivers the most consistent long-term control of perennial pepperweed. Metsulfuron methyl appears to work well but is less studied. Imazapyr controls perennial pepperweed but is a fairly nonselective herbicide and is therefore more likely to damage desirable plants. Chlorsulfuron is not registered for use in many areas where perennial pepperweed occurs, particularly areas adjacent to water [69]. Other problems with chlorsulfuron include its adverse effects on valuable woody species [103], and difficulty in establishing perennial grass seedlings following control of perennial pepperweed [99] (see Cultural control).
Based on perennial pepperweed's seasonal carbon allocation pattern (see Seasonal Development), one might expect the optimal stage for herbicide application to be full-flowering to fruiting stages. However, control seems to be maximized when herbicide is applied at the flowerbud stage [102]. Application in late summer, after the haying operation has removed much of the top growth, may also be effective. "Excellent control" was also obtained with early spring or late fall applications in native hay meadows in Nevada [101]. However, fall applications of 9 herbicide treatments had minimal effects on perennial pepperweed in Utah [67].
Longer term studies are needed to better evaluate control potential of herbicides for perennial pepperweed [67]. The potential for perennial pepperweed to develop a resistance to particular herbicides/families also needs to be investigated [69]. For a more detailed synopsis of chemical control and more detail on particular herbicides, rates, timing, and other considerations see Renz [69].
Cultural: Any lasting biological suppression of perennial pepperweed requires establishment and persistence of desirable plants that are capable of competing successfully with perennial pepperweed in managed ecosystems. Competitive ecotypes of native species are suggested. An example might be the use of saltgrass in halomorphic wetland areas [103]. Perennial pepperweed is, however, highly competitive, as evidenced by its ability to establish and spread in vigorous, well-managed alfalfa or tall wheatgrass stands, and its reputed ability to displace quackgrass [97].
In order to give perennial grass plants a chance to biologically suppress perennial pepperweed, repeated applications of selective herbicides may be necessary to help grasses establish. The choice of perennial species for revegetation of seasonally dry meadows with salt affected soils in the Intermountain Area is limited, and tall wheatgrass is the most widely used species [99]. Young and others [99] compared seedling establishment of tall wheatgrass on sites where perennial pepperweed was controlled with 2,4-D or chlorsulfuron. Perennial pepperweed was controlled with applications of 2,4-D, and tall wheatgrass seedlings established on some of these 2,4-D-treated plots. Plots treated with chlorsulfuron remained virtually free of perennial pepperweed, but no seedling establishment of tall wheatgrass occurred. Even when these plots were seeded for 4 consecutive years after herbicide application, tall wheatgrass seedlings never established. The plots remained weed free except for occasional perennial pepperweed plants until the 4th year, when Russian-thistle (Salsola kali), lambsquarters (Chenopodium album) and summer-cypress (Kochia scoparia) plants established in the treated area. The authors speculate that the apparent persistence of chlorsulfuron residues may be heightened by the high pH of the salt affected soils.
Where perennial pepperweed was initially controlled with applications of 2,4-D a few seedlings of tall wheatgrass were initially present, but no tall wheatgrass plants were present the 2nd year after seeding. Where perennial pepperweed was initially controlled with 2,4-D and followed with a lower rate of 2,4-D over tall wheatgrass seedlings the spring after the seeds were planted, good stands of tall wheatgrass established.
Results from competition tests performed with perennial pepperweed and cheatgrass presented by Blank and others [9] (see Successional Status) suggest that a similarly aggressive, densely rooted native grass may successfully compete with perennial pepperweed. Information about which species can best compete with perennial pepperweed and/or prevent new invasions is needed [69].
