More info for the terms: allelopathy, association, competition, cover, density, fire management, frequency, interference, invasive species, litter, natural, presence, resistance, restoration, selection, succession
Impacts: Many environmental and economic losses have been attributed to diffuse knapweed infestations. Examples include replacing wildlife and livestock forage on rangeland and pasture [19,36,63,68,101,128], depleting soil and water resources [19,85,101], displacing native species on wildlands [137], reducing biodiversity [19,101], reducing land value [19,85], and disflavoring milk [59].
The presence of knapweeds may be a symptom of range degradation. Diffuse knapweed fills niches created by soil disturbance and can also invade good condition range in the absence of grazing [50,68,100]. Diffuse knapweed invasion can be insidious or rapid and conspicuous [50].
In a study conducted in British Columbia, neither diffuse nor spotted knapweed inhibited the growth or survival of conifer seedlings (lodgepole pine and Douglas-fir) [79]. Similarly, diffuse knapweed did not affect seed weight in antelope bitterbrush at British Columbia and northern Washington sites [47]. Diffuse knapweed does, however, possess several traits that give it an advantage over perennial grasses such as intense competitiveness, rapid growth rates, large seed output, and extended growing periods. Continuous seed rain and sequential seedling emergence allow diffuse knapweed to occupy more microsites for seed germination, and to maximize site dominance and eventually form monotypic stands [100]. Even under good range conditions, bluebunch wheatgrass may offer little resistance to knapweed invasion. Diffuse and spotted knapweed growing at moderate densities among bunchgrasses in British Columbia were more vigorous than when growing alone. Inhibition of the bunchgrasses may not occur until threshold densities of knapweed are reached [2].
Diffuse knapweed may suppress other vegetation by allelopathy [16,26]. Diffuse knapweed contains varying concentrations of phytotoxic secondary compounds [64,91,106]; however, the importance of allelopathy has been challenged since concentrations of these compounds in soil are usually below phytotoxic levels [44,73]. Allelopathy may be part of a more complex interference strategy that includes other specialized mechanisms unknown to the plant communities that it invades [14,15].
Control: Lasting control of diffuse knapweed requires proper land management to maintain desired vegetation. It is important to define land use objectives before developing management plans for invasive plants. Killing the target plant is not usually an adequate objective. An understanding of basic diffuse knapweed biology (see the "Botanical and Ecological Characteristics" section of this report) will help land managers choose appropriate control tools and determine proper timing of their application according to the plant's life cycle, as part of a long-term control program [83]. More information on diffuse knapweed's requirements for litter cover, soil moisture, and nutrient needs for establishment and spread can enable managers to develop more effective integrated management programs [57].
For diffuse knapweed, a biennial or short-lived perennial that reproduces by seed, control is very effective during the 1st season of growth when the plant is in the rosette stage and prior to the development of viable seed. A plan to prevent new seed production (e.g. killing the plant or destroying the aboveground portion prior to seed set) can contain existing infestations. To deplete the existing seed bank, areas must then be monitored 2 to 3 times a year, for several years, and any new rosettes destroyed. It is important to document the location and densities of any diffuse knapweed stands or individual plants in order to record the rate of spread of the infestation and to know where to look for emerging seedlings and rosettes in following years [16,137]. Steps must then be taken to prevent reinfestation by cooperating with managers of adjacent land, and land along shared transportation and water corridors, and by being aware of and preventing potential seed dispersal vectors. For instance when diffuse and spotted knapweed were first found in Alberta in 1974, an eradication program was launched that included cooperation between provincial and local government agencies and landowners. By 1985, the infestation was reduced to scattered plants and remains so, to date. Early detection and public awareness were keys to their success [4,5].
Integrated management: The use of multiple control methods is important when implementing any weed management system [83], because multiple approaches can create a cumulative stress on the plant, thus reducing its ability to flourish and spread. A combination of methods also provides some redundancy, in case one type of control treatment is ineffective [16]. With combinations of treatments, timing is critical and must be customized to the plant community, present and desired, and to site conditions [21]. Procedures that increase bare ground on rangeland without replacement by desirable species are not recommended [22].
Integrated management includes a long-term commitment to replace weed-infested plant communities with more desirable plant communities in a way that is complementary to the ecology and economics of the site. The methods selected for control of diffuse knapweed on a specific site will be determined by the land use objectives, environmental factors, economics, the extent and nature of the infestations, and the effectiveness of the control techniques on diffuse knapweed [83]. Sheley and others [102] suggest using a generalized objective such as developing an ecologically healthy plant community that is weed resistant and meets other land-use objectives such as livestock forage, wildlife habitat, or recreation. A weed-resistant plant community is comprised of diverse species that occupy most of the niches [22]. Once the desired plant community has been determined, an integrated weed management strategy can be developed to direct succession toward that plant community by identifying key mechanisms and processes directing plant community dynamics (site availability, species availability, and species performance) and predicting plant community response to control measures [98].
Prior to deciding which control measures are most appropriate, a land manager should: 1) inventory and assess the land to determine the size of the infestations; 2) assess non-target vegetation in the management area; 3) determine soil types, climatic conditions, and important water resources; and 4) determine the limitations of various control methods [83]. Cooperation between all private and public landowners and government agencies that manage land in the area is also necessary for a successful weed management program [22]. Additional components in any integrated management program are sustained effort, monitoring and evaluation for several years, and the adoption of improved strategies [99].
Some examples of combined approaches are presented within the following sections. Managers are encouraged to use combinations of control techniques in a manner that is appropriate to the site objectives, desired plant community, available resources, and timing of application.
Prevention: Preventing the spread of diffuse knapweed is the most economically and ecologically effective management strategy [99]. Prevention is achieved by minimizing soil disturbance on range and other noncrop lands, early detection and treatment of newly established plants, eradication of small infestations before they spread, containing large infestations, and preventing seed dispersal. Seeding desirable perennial grass species on areas disturbed by logging, fire, construction, mining or other activities can help prevent diffuse knapweed invasion [22]. Renney and Hughes [84] suggested in 1969 that much of the knapweed infestation could be contained if transportation corridors (highways, roads and trails) could be rid of the plants.
Proper grazing management is essential to the maintenance of a competitive, desirable plant community that can slow diffuse knapweed encroachment [22,57]. To minimize weed invasion, grazing systems should alter the season of use, rotate or combine livestock types and pastures, and allow grazed plants to recover before being regrazed [22]. In eastern Washington, the establishment of diffuse knapweed was enhanced when defoliation of bluebunch wheatgrass exceeded 60%, suggesting that defoliation above this level reduced the competitiveness of the grass. Diffuse knapweed density did not initially increase on a similar crested wheatgrass site, but after a year it increased when crested wheatgrass was defoliated by 80-100%. Although this study indicates that moderate defoliation does not accelerate diffuse knapweed invasion, disturbances associated with grazing, such as trampling and exposed mineral soil, were not examined [101].
Public awareness of the identity and characteristics of diffuse knapweed and support of local weed management programs can help prevent seed dispersal. Driving, walking, biking and trailing animals through infested areas must be avoided. When vehicles have been in weed-infested areas, it is important to wash the undercarriage before driving through uninfested areas [22]. Additionally, use only certified weed-seed free seed and hay for livestock before entering the backcountry, and avoid grazing livestock on knapweed-infested sites during flowering and seeding. When this cannot be avoided, it is important to hold livestock for 7 days before moving to uninfested pastures [22,127,138].
Physical or mechanical: Physical and mechanical approaches to diffuse knapweed control include hand pulling, digging, tilling, disking, and cutting or mowing. Physical removal of, or damage to diffuse knapweed plants may offer some degree of control depending on the timing and frequency of treatment, the condition of desired vegetation and the degree of soil disturbance imposed by the treatment itself. The Salmon River Restoration Council provides an example of nonchemical spotted and diffuse knapweed control in the Salmon River watershed in northern California, using physical and mechanical control techniques such as hand pulling and digging, propane torching, mulching with black plastic (solarization), and mowing [46]. See their website, SRRC, for detailed information on this program.
Control of diffuse knapweed by hand pulling is feasible for scattered diffuse knapweed plants or in areas where other control methods are not feasible and sufficient hands are available. It is important to remove the entire taproot with as little soil disturbance as possible [137]. Diffuse knapweed rosettes cut just below the crown regrew 38% of the time, while only 4% of those cut 2 to 4 inches (5-10 cm) below the crown resprouted [85]. Pulling works best when done 3 times per year. Begin by removing diffuse knapweed plants in spring, taking care to get a "lethal portion" of the root. This is easiest done when soil is moist. Pull again in June to remove bolting plants before they flower and set seed. Finally, pull plants just before seed dispersal, taking care to remove plants from the site and dispose of them in a manner that ensures seeds are not dispersed [137]. After 5 years of this regimen using volunteers in Oregon to control small populations of diffuse knapweed scattered amongst native plants, average density of diffuse knapweed plants was reduced 98%. About 10 person-hours are then required each season to monitor and remove the few dozen plants that sprout from the seed bank. For larger infestations, a combination of chemical and mechanical control can be used. In Oregon and Colorado, diffuse knapweed was sprayed with picloram in the spring, followed later in season by mechanical removal of plants that were missed with herbicides, with good results [96,136]. In some cases, however, hand pulling may not be effective. On a Colorado rangeland, hand pulling twice a year failed to control diffuse knapweed probably because the root tended to break off near soil surface. Additionally, plants on nearby experimental plots were allowed to seed, and just a few diffuse knapweed plants can repopulate a large area [95]. Hand pulling twice for 2 consecutive years in west-central Colorado was expensive and provided only 0 to 35% diffuse and spotted knapweed control, respectively, after 3 seasons, and increased bare ground [21].
Mowing diffuse knapweed can reduce seed production or alter phenological development, and can reduce weed competition during establishment of newly seeded grasses. A long-term program in which only bolted plants are cut for several consecutive years can reduce number and cover of diffuse knapweed plants, or in some cases it can severely damage or disturb surrounding vegetation and make the area more susceptible to knapweed infestations [16,95,128,138]. Because diffuse knapweed is an obligate outcrosser, seed production can be greatly reduced when diffuse knapweed is mowed prior to flowering [40]. Mowing diffuse knapweed in British Columbia at the bud stage and again at flowering reduced the number of plants producing seed by 77 to 99% compared to unmowed plants. Mowing treatments also reduced germination of seeds by about 79%. Energy remaining in the cut plants may be adequate for seeds to develop. Plants mowed early in the growing season produce few viable seeds; however, mowed plants usually resprout and flower again [128]. Seeds are then produced late in the season and are, therefore, likely to escape attack by biocontrol insects. In Washington state, 22% of plants mowed to a 2-inch (5 cm) height each month of the growing season (April through October) were still growing 4 years later [85].
Diffuse knapweed is intolerant of cultivation and irrigation and is generally not considered a problem on cultivated land [36,108,112,128,129]. Cultivation in combination with reseeding competitive perennial grasses may minimize reinvasion by the knapweeds [22,55].
Fire: See Fire Management Considerations.
Biological control: Biological control of invasive species has a long history, and there are many important considerations to be made before the implementation of a biological control program. The reader is referred to other sources [38,67,90,118,133] and the Weed Control Methods Handbook [117] for background information on biological control.
Biological control efforts for diffuse and spotted knapweed began in 1970, and since that time 13 biocontrol agents have been released in North America [54]. Several introduced biological control agents occur in high numbers at sites in Washington, Oregon, and Montana, where diffuse knapweed populations appear to be decreasing [105]. The combined effects of 2 knapweed seedhead flies (Urophora spp.) on diffuse knapweed has reduced the dry weight of the attacked plants by 74%, reduced the average seed weight by 18%, and reduced seed production by 95% [3,118]. At 1 site in British Columbia, the combined attack by the 2 Urophora species and the root beetle Sphenoptera jugoslavica resulted in a 98% reduction in seed numbers [37]. Although the insects reduce seed numbers, diffuse knapweed plants still produce enough seed to maintain population levels [3,137]. Throughout the northwestern United States, S. jugoslavica is well established on diffuse knapweed and causes noticeable stunting of plants, but no measurable impact on plant density, while the weevil Larinus minutus is having a serious impact on plant growth and density at many locations [111]. It appears that none of these agents, alone or in combination, effectively controls diffuse knapweed populations. They may, however, be useful in integrated control programs by weakening the plants and/or reducing seed output enough to make the plants more susceptible to herbicides, prescribed fires, or other control techniques [16,81,83].
Site characteristics may be an important consideration in the successful establishment of biocontrol agents [113]. For example, seedhead flies may be most effective under site conditions that are marginal to diffuse knapweed survival [11]. Infection of diffuse knapweed by mycorrhizal fungi increased the suitability for infestation by S. juogoslavica [35]. Other considerations for biological control include not only the potential effects on nontarget plant species, but also the complex indirect effects agents can have on native communities, as exemplified by the case study of spotted knapweed gall flies and deer mice [76]. For more detail, see spotted knapweed.
Story and Piper [111] provide brief information, and Turner and others [118] provide more detailed information on individual insect control agents for diffuse knapweed:
NameType States established or recovered on diffuse knapweed [111] Additional references Sulfur knapweed moth
(Agapeta zoegana) root-boring moth CO, MT, UT, WY [65] Broad-nosed seedhead weevil
(Bangansternus fausti) weevil CA, OR [111,118] Broad-nosed seedhead weevil
(Bangansternus fausti) root-boring/gall weevil CO, MT, OR, UT, WA, WY [105,132] Lesser knapweed flower weevil
(Larinus minutus) seedhead weevil CA, CO, MT, NV, OR, UT, WA, WY [43,53,85] Spotted knapweed seedhead moth
(Metzneria paucipunctella) seedhead moth MT, OR, WA [111,118] Gray-winged root moth
(Pterolonche inspersa) root-boring moth OR [17] Bronze knapweed root borer
(Sphenoptera jugoslavica) root beetle CA, CO, ID, MT, OR, UT, WA, WY, BC [37,78,83,105] Banded gall fly
(Urophora affinis) gall-forming seedhead fly CA, CO, ID, MT, OR, UT, WA, WY, BC [3,33,37,52,81,105,111,130] UV knapweed seedhead fly
(Urophora quadrifasciata) gall-forming seedhead fly CA, CO, ID, MT, OR, UT, WA, WY, BC [3,33,37,52]
In addition to insect agents, 2 fungal pathogens are known to damage diffuse knapweed under certain conditions. Puccinia jaceae attacks the leaves, and Sclerotinina sclerotiorum attacks the crowns of diffuse knapweed [77]. These fungi are still being studied and are not cleared as biocontrol agents [28,104,105].
Grazing: The use of grazing animals to control invasive rangeland species is discussed by Olson [72]. Control of diffuse knapweed populations with grazing has received little attention. It is often suggested that grazing is not effective for diffuse knapweed because diffuse knapweed is unpalatable [138] and because ground disturbance created by grazing animals creates ideal seedbeds for further invasion [16]. Piper and others [77] suggest that livestock grazing of diffuse knapweed in early spring can reduce seed production. One study in Colorado found that cattle readily grazed diffuse knapweed and negatively influenced diffuse knapweed population dynamics [9]. Diffuse knapweed is more likely to be grazed by domestic sheep during the rosette through bud stage (when it is green and succulent), or when it is the only plant available (when associates are dormant) [85]. Roche and Roche [85] suggest that methods of utilizing diffuse knapweed can be patterned after programs designed for spotted knapweed and yellow starthistle. Timing relative to the development stages of both the weed and associated vegetation is critical to achieve the desired selectivity. Early and late-season grazing appear to be most effective for spotted knapweed control with sheep (early season to reduce flower production and late season to reduce density of young plants) [31]. Olson and others [74] found that 3 summers of repeated sheep grazing negatively impacted spotted knapweed but minimally affected the native grass community.
Chemical: Herbicides are effective in gaining initial control of a new invasion or a severe infestation of diffuse knapweed, but are rarely a complete or long-term solution to weed management [13]. Herbicides are more effective on large infestations of diffuse knapweed when incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations [13,22]. Control with herbicides is temporary, as it does not change conditions that allow infestations to occur [137]. See the Weed Control Methods Handbook for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.
Diffuse knapweed plants are easily killed by any of several properly used herbicides. However, because diffuse knapweed produces abundant, long-lived seed, the impact of nonresidual herbicides is reduced in the long-term [86]. As of 1999, several herbicides are registered for control of diffuse knapweed on rangeland, with varying degrees of residual activity for control of later germinants. In order of decreasing residual effects, the following herbicides control diffuse knapweed: picloram, clopyralid, clopyralid + 2,4-D, and 2,4-D [85], and glyphosate. The effectiveness of a residual herbicide varies with application rate and method, soil texture, soil organic matter, and precipitation pattern [86]. Best results from herbicides are usually when the knapweed is in the rosette stage [16,85]. Backpack sprayers or wick applicators are recommended over spray booms or aerial applicators to minimize damage to nontarget plants [16].
Several researchers have compared different rates, application times and combinations of herbicides for diffuse knapweed control [84,94,95,125,131]. Others have examined herbicide use combined with reseeding of desirable plants [24,36,41,60], effects on nontarget plants [96], as well as herbicide compatibility with biocontrol agents [67,81] and grazing [60]. Some researchers recommend mowing or burning prior to herbicide application to increase rates of efficacy [22,95,96].
Fertilizer may effectively stress diffuse knapweed by enhancing competition where conditions are drier than optimal; it may not be effective under moister conditions [11,128]. It has also been suggested that inducing bolting under field conditions by application of gibberellic acid can be timed so that fewer seeds are produced and winter kill is increased. Gibberellic acid can also be used to synchronize the appropriate stages in the life cycles of diffuse knapweed and biocontrol agents. However, induction of bolting of this type under field conditions remains to be demonstrated [123].
A study designed to compare 5 control treatments on diffuse knapweed (burning, cultivation, picloram, seeding of smooth brome (Bromus inermis) (another weedy, exotic species), and nitrogen fertilization, alone and in combination) in northeastern Washington was carried out for 8 years. When precipitation was at or above normal, the combination of herbicide and fertilizer produced maximum weed control and forage production, whereas fertilization alone stimulated both grass and knapweed, producing more knapweed than grass. Picloram alone decreased diffuse knapweed for 2 years, with knapweed returning the 3rd year, and grass production remaining higher on all sprayed plots through the 5th year. As single treatments, burning and cultivation provided only fleeting control of diffuse knapweed, with weed production equal to or greater than untreated controls after the 1st year. When all 5 treatments were combined, grass production peaked the 3rd year after treatment and then declined during 3 subsequent drought years. Diffuse knapweed reestablished completely on all of the plots 8 years after treatments, in the absence of grazing or clipping. The authors attribute reinvasion to 3 years of drought and to small plot size, which allowed reseeding from adjacent plots [85]. In a similar study in west-central Colorado, researchers compared hand pulling, mowing, herbicides, and the root weevil Cyphocleonus achates alone and in combination for the control of spotted and diffuse knapweeds. The only increases in grass cover were with treatments including herbicides. Herbicides provided the most cost effective and efficacious control of the knapweeds over multiple years with the greatest increase in grass cover. Insects alone and combined with herbicides may prove cost effective for long term management of knapweed if insects establish and maintain suppression of weed populations [21].
Cultural: No matter what method is used to kill diffuse knapweed plants, reestablishment of competitive plant cover is imperative for long-term control [22,85]. Fertilization and reseeding with competitive, adapted species is often necessary in areas without a residual understory of desirable plants [83]. Revegetation with aggressive desirable species has been shown to inhibit reinvasion of knapweeds [22], especially with the help of effective biological control agents and carefully prescribed grazing practices [85]. Vegetative suppression is applicable both after knapweed control and before knapweed establishment [108,110].
No single species will suppress diffuse knapweed on all sites at all times. Species effectiveness depends on site conditions including soil type, moisture, slope, and aspect [22]. Species that remove water from the rooting zone of diffuse knapweed during seedling establishment are most effective [16,108]. Larson and McInnis [55] found some wheatgrasses (Triticeae) capable of decreasing knapweed density in northeastern Oregon. On British Columbia rangeland, 11 years after treatment with picloram, diffuse knapweed density was high in non-seeded plots, moderate (1/3 density of control) in plots seeded to Russian wildrye (Psathyrostachys juncea), and very low in plots seeded to crested wheatgrass (Agropyron cristatum) [11]. Hubbard [41] also found crested wheatgrass effectively suppressed the invasion of diffuse knapweed. In a 2-year study in Oregon, a diffuse knapweed infestation was disked in the spring and seeded to 'Covar' sheep fescue (Festuca ovina), 'Ephraim' crested wheatgrass, 'Paiute' orchardgrass (Dactylis glomerata), and 'Critana' thickspike wheatgrass (Elymus lanceolatus). Orchardgrass and thickspike wheatgrass controlled knapweed establishment during both years of the study [56]. At one site in northeastern Washington, 'Durar' hard fescue (Festuca trachyphylla) limited diffuse knapweed reinvasion more effectively than did smooth brome or orchardgrass [85]. A long-term study to identify the species best suited to seeding semiarid rangeland sites in northeast Washington indicated that hard fescue was the most aggressive competitor, and that crested wheatgrass taxa provided the highest yields [32].
While these aggressive species can be effective at suppressing diffuse knapweed, it is important to consider the implications of using 1 exotic to suppress another. Native species may be best for maintaining or achieving biodiversity that is site specific. Unfortunately, there is little success reported for suppressing invasive species with native species. Idaho fescue seedlings were planted on preserve land in Oregon; however, survival was low, and the low density plantings failed to further reduce knapweed numbers [137].
The same cultural practices will have different effects on knapweed suppression under different climatic regimes. Crested wheatgrass provided very good long-term suppression in a region of British Columbia that receives 8 inches (200 mm) mean annual precipitation, but poor suppression on a site with 13 inches (330 mm) mean annual precipitation [11]. Site preparation prior to seeding will also affect results. Fagerlie [24] suggests that treatment with picloram in association with grass seeding is more successful for grass establishment than disking for seedbed preparation. Herbicide selection is also an important consideration, since seeded species will vary in susceptibility to different chemicals [24]. Maxwell and others [60] found spraying with picloram to be successful at controlling knapweed, while interseeding had little impact on knapweed cover and grazing negated treatment effects.
