SPECIES: Centaurea diffusa

• Introductory
• Distribution and occurrence
• Botanical and ecological characteristics
• Fire ecology
• Fire effects
• Management considerations
• References

INTRODUCTORY

• AUTHORSHIP AND CITATION
• ABBREVIATION
• SYNONYMS
• NRCS PLANT CODE
• COMMON NAMES
• TAXONOMY
• LIFE FORM
• FEDERAL LEGAL STATUS
• OTHER STATUS

DISTRIBUTION AND OCCURRENCE

• GENERAL DISTRIBUTION
• ECOSYSTEMS
• STATES
• BLM PHYSIOGRAPHIC REGIONS
• KUCHLER PLANT ASSOCIATIONS
• SAF COVER TYPES
• SRM (RANGELAND) COVER TYPES
• HABITAT TYPES AND PLANT COMMUNITIES

The following table reflects estimates of diffuse knapweed acreage as reported by surveyed states or provinces in 1988 and again in 2000 (from [20]):

Although inventories are more common and more accurate in the year 2000 than in 1988, 50% of these states reported only 50% accuracy, while 31% reported 51 to 75% accuracy, and 2 states reported 75 to 100% accuracy [20].

The following lists reflect ecosystems and cover types in which diffuse knapweed is known or thought to be invasive. Diffuse knapweed occurs in some midwestern and eastern states and provinces, primarily along roadsides and in "waste places", but it is unclear, from the available literature on these areas, which ecosystems and cover types it occurs in. These lists are not, therefore, exhaustive, as the plant may be invasive in other types not listed.

In Utah, the antelope bitterbrush/bunchgrass shrub steppe is highly susceptible to invasion by diffuse knapweed [85].

In Colorado, diffuse knapweed invades the shortgrass steppe along the Front Range, including the foothills. Adjacent montane zones and the lower elevation pinyon-juniper-oak (Pinus-Juniperus-Quercus spp.) brush zones are also susceptible [85]. Diffuse knapweed is also found on upland sites in pinyon and juniper woodlands in the interior west [114].

In Montana, diffuse knapweed infestations are found primarily on grasslands and fringe forest areas near Helena, Big Timber, and Ennis [49]. It is well adapted to open-forested areas, especially after logging or other disturbances [27].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

• GENERAL BOTANICAL CHARACTERISTICS
• RAUNKIAER LIFE FORM
• REGENERATION PROCESSES
• SITE CHARACTERISTICS
• SUCCESSIONAL STATUS
• SEASONAL DEVELOPMENT

Once it is established diffuse knapweed can form monotypic stands. The competitive ability of diffuse knapweed has been attributed to its being adept at depleting soil moisture [108], to allelopathy [14,15,26], and to other competitive or interference mechanisms [14,15].

Pollination: Diffuse knapweed is primarily insect pollinated [39,129]. Honeybees, bumble bees, and digger bees are frequent diffuse knapweed flower visitors [39]. Fertilization in diffuse knapweed requires cross-pollination between flowers on different plants. This can limit the reproductive success of isolated individuals, but it also promotes genetic diversity and may thereby improve competitive ability. Watson and Renney [128] reported that diffuse knapweed is self-compatible, but the results of Harrod and Taylor [39] refute this assertion.

Seed production: Average seed production by diffuse knapweed is reported by area as follows:

Considerable variation in seed production was observed between sites and years in northeastern Washington; nonetheless, seed production was estimated to be 1,000 times what would be necessary to maintain observed levels of infestation [93].

Seed dispersal: Dispersal of diffuse knapweed seed is mainly by wind. Seeds usually remain in the flowerheads after they mature and break away from the receptacle. Dispersal in the vicinity of the parent plant is facilitated by horizontally placed seedheads that open at the top and release seeds as dehydration occurs and plants sway in the wind. Dispersal over longer distances occurs when plants are broken off at ground level and tumble in the wind, dispensing seeds individually from the small opening at the top of the seedheads [128]. This technique adapts well to "hitchhiking" on the frames of vehicles and colonizing the bare shoulders of roads. Seeds may also be transported in mud adhering to vehicles or shoes [129]. Plants bearing seeds may also be carried in rivers and irrigation systems, thus colonizing the banks of waterways [85]. In British Columbia, logging trucks, off-road vehicles, and trail bikes have greatly contributed to the spread of both spotted and diffuse knapweed [112].

Seed banking: Evidence suggests that diffuse knapweed seed germination is distributed over time [69]. This may be considered as evidence of seed banking, although the length of time that diffuse knapweed seeds remain viable in the soil seed bank is undetermined.

Viability: Seed viability information for diffuse knapweed is scarce. In British Columbia, diffuse knapweed seed removed from seedheads at maturity exhibited 40% germination; 25-day-old seed stored under dry conditions exhibited 68% germination; and seeds that overwintered in the seedhead under field conditions exhibited 88% germination [128]. Watson [129] reports successful (93-95%) laboratory germination of 20-month-old seed stored at room temperature [138]. Another laboratory germination test showed greater than 95% viability of diffuse knapweed seeds, although germination values observed in the field were <70% [93].

Germination: Diffuse knapweed seeds germinate in spring (May/June) or late summer/early fall (August/September), and develop into low-lying, tap-rooted rosettes given sufficient moisture [11,68,128]. Diffuse knapweed seeds germinated under a wide range of environmental conditions simulated in the laboratory. Germination of over 80% occurred between 55 and 82 °F (13-28 °C) at optimum moisture levels [128]. Diffuse knapweed seeds require more than 55% soil moisture to initiate germination, with optimum emergence between 65% and 70% [107]. Diffuse knapweed seeds germinate best on the soil surface, with emergence rate decreasing as seeding depth increases, and little to no emergence below 1 inch (2.5 cm) [107,128]. Spears and others [107] found diffuse knapweed seeds germinated equally well over the range of 0 to 100% canopy cover. Nolan and Upadyaya [69] observed 3 types of germination behavior in diffuse knapweed with respect to light conditions: nondormant seeds that can germinate in the dark; light-sensitive seeds that germinate after exposure to red light; and light-insensitive dormant seeds that do not respond to exposure to red light. They concluded that bunchgrass rangeland and other open canopy conditions provide favorable light conditions for diffuse knapweed seed germination.

Seedling establishment: Stannard [108] reports high seedling vigor in diffuse knapweed, while Myers and Berube [68] indicate that the greatest mortality in diffuse knapweed occurs between the seedling and rosette stages. Seedling mortality is highest during the summer and is largely related to moisture availability [68,80,93,128]. Once diffuse knapweed seedlings establish as rosettes, they become very drought resistant [11]. Crowding of plants is also a factor in seedling and juvenile mortality, with mortality rates highest among the smallest rosettes, and declining with size under crowded conditions. The proportion of diffuse knapweed plants that flower and produce seed each year (and subsequently die) increases with available growing space [80]. Additionally, Powell [80] observed that the majority of diffuse knapweed rosettes that died during the summer at 1 site in British Columbia were infested with the larvae of the introduced biocontrol beetle Sphenoptera jugoslavica, and suggested that the damage caused by the larvae increased the susceptibility of rosettes to interference-related mortality. Amount and pattern of rainfall are important for diffuse knapweed survival. Wetting increases germination, but when followed by drying (e.g., the wet-dry cycle occurring with the summer convective storms in British Columbia ranges) seedling survival is diminished [11]. High spring precipitation appears to favor diffuse knapweed seedling establishment [101].

Asexual regeneration: Diffuse knapweed can sprout from the root crown after top-kill [85,128]. Other methods of asexual regeneration are not known to occur in diffuse knapweed [128].

Diffuse knapweed is tolerant of a wide range of precipitation and temperature conditions but does best in semiarid and arid conditions, and is most competitive in areas receiving between 12 and 17 inches (305-432 mm) of annual precipitation [11,36,87,128]. Diffuse knapweed is susceptible to flooded or waterlogged conditions, and infestations stop abruptly with an increase in soil moisture near temporary and permanent streams. Irrigation can also eliminate diffuse knapweed [68]. Diffuse knapweed is not competitive in moist microsites such as gullies, depressions, and poorly drained soils [11,85]. Diffuse knapweed seeds require conditions near field capacity for at least 4 days to begin germination. Seedling root growth in diffuse knapweed may be sensitive to saline conditions (electrical conductivity of 4 dS m^-1). Water stress (osmotic potential of -0.5 mP or less) may delay germination and impact seedling root growth [45].

In eastern Washington, diffuse knapweed grows on all aspects and slope positions, from flat to over 60% [85,87,115]. In the Gilpin range, British Columbia, knapweed is commonly found on south-facing slopes below 3,000 feet (900 m) [63]. The following table provides some elevation and precipitation ranges for diffuse knapweed as reported by state or province:

Diffuse knapweed is commonly found on well-drained soils such as sandy or gravelly loams or loamy fine sands, with coarse fragments from 0 to over 80% [86,87]. It is less competitive on shallow soils (<15 inches (38 cm) deep) and very coarse textured soils such as sand or loamy coarse sand, although it may thrive on these sites when disturbance removes other vegetation [87]. It grows best on fertile, well-watered Cryoborolls, mesic Argiudolls, and mesic Hapludolls in open and uncultivated sites with summer drought [36,99].

FIRE ECOLOGY

• FIRE ECOLOGY OR ADAPTATIONS
• POSTFIRE REGENERATION STRATEGY

Fire regimes: Diffuse knapweed occurs primarily in bunchgrass and open ponderosa pine forest community types in the northwestern United States and southern British Columbia. The historical fire regimes of these communities were relatively frequent, low-severity surface fires. Diffuse knapweed did not occur in these communities at the time in which these fire regimes were functioning, but has established since fire exclusion began. It is unclear how this type of fire regime might affect diffuse knapweed populations. It is also unclear how the presence of diffuse knapweed might affect these fire regimes, though it has been observed that closely related species, such as spotted knapweed, do not carry fire as readily as grasses [61,134]. If this holds true for diffuse knapweed, dense infestations may affect fire regime characteristics such as frequency and severity by altering fuel characteristics of invaded sites.

The following table provides fire return intervals for plant communities in which diffuse knapweed may occur. For more information on fire regimes in these communities, see the FEIS review for the dominant species listed here. If you are interested in fire regime information for a plant community not listed here, see the complete FEIS Fire Regime Table.

FIRE EFFECTS

• IMMEDIATE FIRE EFFECT ON PLANT
• DISCUSSION AND QUALIFICATION OF FIRE EFFECT
• PLANT RESPONSE TO FIRE
• DISCUSSION AND QUALIFICATION OF PLANT RESPONSE
• FIRE MANAGEMENT CONSIDERATIONS

Reproduction by abundant seed may give diffuse knapweed an advantage in fire-prone environments. Spotted knapweed seeds demonstrate some resistance to high temperatures [1]. This may also be true for diffuse knapweed, in which case diffuse knapweed seeds may persist in the soil and germinate after fire; however, research is needed to test this assertion. Where diffuse knapweed plants are left standing, seeds may be present above ground as well. This aerial seed bank may or may not be an advantage, depending upon fire characteristics. Renney and Hughes [84] suggest that burning shows some promise for diffuse knapweed control, because viability of seeds held above ground in the seedhead is considerably reduced by heat, though they give no reference to experimental evidence for this conclusion. Watson [129] notes that seed collected from diffuse knapweed plants in an area burned by a mid-August wildfire was not viable. A review by Harrod and others [40] cites unpublished data (location not given) suggesting that fire might reduce the ability of diffuse knapweed to produce seed in the current year, since many bolting stems appeared to be reverted back to a rosette stage after fire. The authors further suggest that this might allow grasses (which appear to be stimulated by fire) to gain a competitive advantage [40].

A review by Carpenter and Murray [16] suggests that burning may be an effective means of controlling diffuse knapweed in areas where seasonal or occasional fires are part of the natural ecosystem. Watson and Renney [128] cite Popova (1960) as reporting that fire provides effective control of diffuse knapweed, with vigorous grass regrowth after burning in the Crimea.

Postfire colonization potential: Diffuse knapweed may have the potential to establish and spread following fire. Fire provides an ideal seedbed by removing shade and exposing mineral soil. Therefore, if diffuse knapweed was present on or near the site prior to the fire, there is potential for its establishment. It is a good idea to survey the surrounding area for diffuse knapweed skeletons that may contain seed that could be dispersed through tumbleweed action. Diffuse knapweed is one of the introduced species mentioned as "taking over large tracts of logged, burned, or otherwise disturbed lands in British Columbia" [119].

The USDA Forest Service's "Guide to noxious weed prevention practices" [120] provides several fire management considerations for weed prevention in general that apply to diffuse knapweed. Wildfire managers might consider including weed prevention education and providing weed identification aids during fire training; avoiding known weed infestations when locating fire lines, monitoring camps, staging areas, helibases, etc., to be sure they are kept weed free; taking care that equipment is weed free; incorporating weed prevention into fire rehabilitation plans; and acquiring restoration funding.

When planning a prescribed burn, inventory the project area and evaluate the cover and phenology of any diffuse knapweed present on or adjacent to the site, and avoid ignition and burning in areas at high risk for diffuse knapweed establishment or spread due to fire effects. Avoid creating soil conditions that promote weed germination and establishment. Discuss weed status and risks in burn rehabilitation plans.

MANAGEMENT CONSIDERATIONS

• IMPORTANCE TO LIVESTOCK AND WILDLIFE
• PALATABILITY
• NUTRITIONAL VALUE
• COVER VALUE
• OTHER USES
• IMPACTS AND CONTROL

Miller [63] observed California and Rocky Mountain bighorn sheep, white-tailed deer, mule deer, and elk consuming diffuse and spotted knapweed in the Gilpin range, and in the Robson/Syringa Park area in British Columbia. Knapweeds are important forage for these animals in the winter and early spring. In the Gilpin range, knapweed rosettes comprised 80% of the diet of California bighorn sheep as the snow receded in January and February, and knapweed seedheads were the most common component of their diet when snow depth exceeded 8 inches (20 cm). When snow did not restrict availability, knapweed rosettes and bluegrass comprised 90% of the diet of mule and white-tailed deer during February and early March. In the Robson/Syringa Park area, Rocky Mountain bighorn sheep utilized knapweed seedheads and basal rosettes throughout the year, while local deer and elk populations foraged on knapweed rosettes in late fall/early winter, and again when snow cover receded and spring greenup commenced. The impact of knapweed consumption on the welfare of these animals, and the effects of heavy utilization of rosettes need further examination [63]. Harris [34] notes that deer in British Columbia began eating knapweed seedheads as winter browse after the establishment of the Urophora spp. seedhead flies, and that almost all the nutrition in these seedheads comes from these insect larvae.

Diffuse knapweed is a source of pollen and nectar for honey bees during mid- to late summer when other sources are in short supply, and it is sometimes eaten by pest grasshoppers during outbreaks [25,85]. At high densities, grasshoppers may consume large amounts of knapweed and reduce seed production [25]. Birds and rodents, including chipmunks, use diffuse knapweed seeds for food [86,129]. Chipmunks probably cache some seed for later use [86].

Palatability of diffuse knapweed to a foraging animal is probably more closely related to its availability relative to other forage plants [85,129].

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:

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].

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