Gather Background Information
The first step in an IVM program is to gather information on the life cycle and habits of the noxious weed.
Spotted knapweed, Centaurea maculosa, and diffuse knapweed, Centaurea diffusa, share similar aspects in their biology, distribution, and control mechanisms and will be considered one group unless otherwise noted. Russian knapweed, Acroptilon repens, due to its differences in biology, will be discussed separately or in comparison with spotted and diffuse knapweeds.
Spotted knapweed is an aggressive biennial, or more commonly a short-lived perennial that has slender, erect stems. It grows up to 60 inches tall and its branched stems bear pinkish-purple flowers at the tips. Floral bracts have a short central spine and a distinguished dark spot at the tip, giving the flower head a "spotty" appearance. Spotted knapweed flower heads are not prickly to the touch and are usually larger than diffuse knapweed. Flower heads open soon after they mature, releasing seeds when the plant is stirred by the wind. Pale green leaves are alternately arranged and 1 to 3 inches long (Whitson et al. 1996; Higgins and Schirman 1977).
Diffuse knapweed is normally a biennial or short-lived perennial with long, fibrous taproots. It is generally shorter than spotted knapweed, growing up to 40 inches tall, and has a single upright stem with many branches. Stems have fine short hairs, giving the plant a gray appearance. Leaves are small, alternately arranged, and finely divided. Diffuse knapweed produces white, occasionally pink or purple flowers at the tip of branches. Floral bracts do not have a pappus (a parachute-like structure that facilitates seed dispersal by wind), and the terminal spine reaches about 1/3 inch long, making them sharp to the touch, with 4 to 5 pairs of shorter, lateral spines. Seeds are tightly held in the flower head, allowing seeds to be spread large distances when the plant stem breaks off and becomes a tumbleweed (CO State University 1988; Beck 1994; Higgins and Schirman 1977).
Russian knapweed is haploid (it has half the number of chromosomes than other Centaurea plants) and is classified under a separate genus Acroptilon. Russian knapweed is a long-lived perennial that forms dense clones from black, lateral roots. Vertical roots can reach a depth of over 8 ft. Erect stems are thin, branched, and 18 to 26 inches tall. Young stems are covered with woolly hairs, which rub off in time, giving older stems a darker brown appearance. Silvery buds bloom into pink or purple flowers and turn straw-colored at maturity. Flower heads are urn-shaped, single, and found at the tips of branches. Grayish-white leaves are deeply lobed near the base, and entire or serrate on the upper branches. In addition to its unique black roots, Russian knapweed can be distinguished by its floral bracts, which are green at the base and lightly hairy at the tip (Roche and Roche 1991; Whitson 1987; PIBS 1984; Zimmerman 1996; Whitson et al. 1996; Watson 1980).
Knapweeds are aggressive competitors and reduce biodiversity by outcompeting native vegetation. In certain areas such as Oregon state, knapweed infestations have reduced appraised land values. They form dense infestations in disturbed as well as undisturbed areas, cultivated fields, grain fields, pastures, along roadsides, and ditches. Studies show that spotted knapweed may influence surface runoff and sediment yield. Runoff and sediment yields were greater in knapweed areas compared to native bunchgrass vegetation. This indicates that soil and water losses can be caused by spotted knapweed infestations. Spotted knapweed may also cause soil erosion, decrease biodiversity, and reduce forage for wildlife and livestock. (Lacey and Olsen 1991).
Diffuse knapweed reduces forage potential because of its fibrous foliage, prickly spines, and bitter taste.
Russian knapweed contains allelopathic chemicals which can suppresses other plant species, forming monoculture knapweed stands. Infestations reduce yields of desired plants and decrease the production quality of rangelands. Because Russian knapweed is also toxic to horses, infested hay has lower feeding and market value. Horses with prolonged consumption of Russian knapweed can develop chewing disease or equine nigropallidal encephalomalacia (ENE). ENE is a permanent disease caused by lesions in the brain. Symptoms include the inability to eat or drink, aimless or awkward movement, and spontaneous activity. Symptoms may occur after ingestion of 60-200% its body weight for at least 30 days. Russian knapweed is not toxic to animals such as sheep and dogs, and cattle have grazed infested pastures with no evidence of toxicity (Rees et al. 1996; SBNM 1997; Watson 1980; Lacey and Olsen 1991; Panter 1991).
Knapweeds were accidentally introduced from Eurasia in the early 1900s, probably as a contaminant of alfalfa seed. Today, knapweeds are found in states west of the Rocky Mountains, with infestations in every county of Montana, Idaho, Washington, and Wyoming. Currently in the Western United States, spotted knapweed infests over 7 million acres, diffuse knapweed covers over 3 million acres., and Russian knapweed infests roughly 1.4 million acres (Sheley et al. 1998; Roche and Roche 1991).
Diffuse knapweed infests roadsides, waste areas, plains, and dry rangelands and prefers well-drained, light-textured soils. Spotted knapweed can establish in any type of disturbed soil. Spotted knapweed prefers similar soil types as diffuse knapweed and sites with more moisture. Russian knapweed forms colonies in cultivated fields, orchards, pastures, and roadsides (Rees et al. 1996 Whitson et al. 1996).
Diffuse and spotted knapweeds reproduce by seeds while Russian knapweed spreads locally by its lateral roots. Knapweeds flower from July to October and bolt in early May. A mature spotted knapweed plant can produce up to 25,000 seeds that are dispersed by infested hay, wind, water, vehicles and equipment, animals, or shoes. Seeds can germinate in the fall or spring and germination greatly decreases if seeds are buried deeper than 2 inches (SBNM 1997; Spears et al. 1980).
Diffuse knapweed seedheads are urn-shaped, and partially closed; ideal for gradual dispersion over long distances. The main stem of diffuse knapweed, unlike spotted knapweed, can easily break off at ground level after seed production, turning into a "tumbleweed" and further spreading seeds. Knapweeds can tolerate a wide range of conditions, from irrigated to arid lands and from sandy to silty soils. Spotted knapweed requires higher amounts of moisture than diffuse knapweed.
Russian knapweed is commonly found in areas near a water source, such as river bottoms or areas that are irrigated. In Washington state the indicator species for sites susceptible to Russian knapweed invasion is basin wild rye (Elymus cinereus). Basin wild rye occurs in 75% of sites invaded by Russian knapweed. (Watson 1980; CO State Univ. Ext. 1988; Roche and Roche 1991).
Spotted and diffuse knapweeds are prolific seed producers and are quick to colonize new sites. They tolerate a wide range of conditions and climate and can successfully invade non-disturbed areas. Sheley et al. (1997) indicates that non-disturbed, moist areas are more susceptible to knapweed invasions than non-disturbed dry areas.
Russian knapweed is allelopathic, inhibiting the growth of neighboring plants. Unlike spotted and diffuse knapweeds, areas previously infested with Russian knapweed require tillage or exposure to moisture for two growing seasons before vegetation can re-establish. Although Russian knapweed is not the most abundant of the knapweeds, it is the most persistent (Sheley et al. 1997; Bottoms et al. 1995; Roche and Roche 1991).
Some questions, such as those below, can only be answered on site.
Set Management Objectives
Set Realistic Goals
for Your IVM Program
The answers to the following questions can help you set realistic objectives and goals.
Reduction - reducing the area covered by knapweeds, or reducing its dominance. This strategy can also be used against new or established weeds, but it requires more resources and more time than containment.
The "Bradley Method" (see Appendix 2), developed in Australia, is a simple yet innovative strategy for natural areas that combines containment and reduction.
Eradication–completely eliminating the weed from the management area. This strategy usually consumes the greatest amount of time and resources and is applicable mainly to newly-invading weeds that are confined to a limited number of small areas.
Establish Monitoring Programs
When planning a monitoring program, keep in mind the context of your target weed: is it invading or has it already invaded?
Locate and record knapweed infestations on a map. (Chapter 2 of the University of Northern Iowa IVRM Technical Manual contains a detailed discussion on how to map and inventory vegetation - see Bibliography). Note particularly sensitive areas on the map, such as critical habitat for threatened or endangered species, agricultural production areas, or areas subject to frequent disturbance and thus prone to invasion. Update maps at regular intervals.
Focus monitoring efforts on sites where knapweed problems are most likely to occur (see Distribution). Encourage public sighting and reporting through an education or incentive program (see Educate Vegetation Management Personnel and the Public).
Prioritize the sites you will work on. Make a realistic assessment of your weed management resources, keeping in mind the goals of your project and the cost of a follow-up program after any treatments. Without follow-up, your control efforts will be wasted. It is better to thoroughly control a weed at one or two sites than to use up resources to incompletely control the weed at many sites. If the weed is very widespread, try to determine where it poses the most serious economic, social, or environmental problem and concentrate on those areas.
Plan monitoring and treatment efforts to coincide with critical life stages of the weed. To use your resources efficiently, try to include monitoring with other planned activities in the area.
Maintain records of your monitoring activities. Creating standardized forms will make data collection easier and help remind you to gather all the information you need. Forms work best if they include labeled blanks for all pertinent information and allow the user to check or circle rather than having to write words or numbers (See Appendix 3 for some examples of forms).
Include information such as the name(s) of the person(s) collecting the data, the location, and date of monitoring; a qualitative description of the vegetation, such as the names of the plants or types of plants (native vegetation, annual/perennial weeds, trees, etc.) and stage of growth (germinating, flowering, setting seed, etc.); a quantitative description, such as percent cover, plant density, size of the patch, or if possible, the number of plants.
Note special conditions such as unusual weather events and record treatment history, including information on treatment applications (who, when, where, how, cost, difficulties, and successes). This will allow you to evaluate and fine-tune treatments.
Set Treatment Thresholds
Setting treatment thresholds includes prioritizing and balancing treatments with resources. Weeds will be treated when populations increase beyond a predetermined level. This level will largely depend on the characteristics of the site and weed. In some cases the level may be no weeds at all, and in other cases the number of weeds you can tolerate may be much greater.
What is the size of the weed population? The opportunity for control is related to the infested area. Small patches can be more easily controlled than large infestations.
What is the level of the threat? Is the knapweed population changing? Is it in an area where soils are frequently disturbed? Does it threaten agriculture, pastures, or rangeland? Is it encroaching on critical habitat for a rare, threatened, or endangered species? Is it displacing the best examples of native communities?
What resources are available? Do you have the resources required for carrying out your goal?
With the advent of herbicides, prevention as a weed management technique has often been neglected; however, it is a practical, cost-effective, and extremely important part of noxious weed control.
General Weed Prevention Measures
(Adapted from Fay et al. 1995).
Revegetation - Follow-up Weed Prevention
Establishing dense, competitive vegetation can help permanently replace weeds. Revegetation is critical in preventing weed infestations in areas where the soil has been disturbed or the vegetation removed. Maintenance of a healthy, competitive grass cover can reduce the rate of knapweed re-infestation and spread. However, when knapweed areas are killed, revegetation from the native seedbank or deliberate planting programs are often necessary or weeds will easily re-invade. In some situations you may be able to encourage desirable vegetation that is already in place; but because of the aggressive nature of knapweed, it is more likely that you will need to thickly sow seeds of desirable, competitive plants.
Native, perennial grasses, in conjunction with other control methods, can weaken knapweeds by reducing shoot and root growth. Reseeding of rangelands commonly involves competitive rhizomatous perennial grasses. For example, crested wheatgrass has been a successful competitor against knapweeds in Canada, reducing the rate of vegetative spread, limiting weed density, and reducing weed seed production. Competitive plants can stress knapweeds in dry areas by reducing available soil moisture. Studies by Steiger and Muller-Scharer (1992) indicate that spotted knapweed is susceptible to competition from the grass, Festuca pratensis, reducing the number of leaves and shoot and root growth. This response to competition corresponds to its distribution in open, disturbed areas with little plant cover. Populations of Russian knapweed might be kept low due to competition with wheatgrass or other hardy perennial grasses (Bottoms et al. 1995; Berube and Myers 1982; Watson 1980; Sheley 1997).
Apply Management Methods
No individual method will control knapweeds in a single treatment; diligence and persistence will be required over a number of years to subdue this weed. The treatment methods described in this section will help you to design an integrated program that will suit the circumstances of your particular situation.
Biological control does not aim to eradicate weeds, but to keep them at low, manageable levels. After their introduction, biocontrol agents can take 5 to 10 years to become established and increase to numbers large enough to reduce the density of the target weed. Once established, effective biological controls provide an inexpensive, long-term, and non-toxic means to control weed populations. It is important to match the insect to the weed management site. Insects have specific requirements for growing and thriving. Understanding these requirements will help you find a hospitable release site and allow you to integrate the insects into your other weed control efforts. When you release biocontrols, continue using other control methods on the perimeter of the release site, but avoid using them where they might adversely impact the insect population. Natural enemies are highly sensitive to pesticides, and should be released in areas without disturbances or pesticide applications.
The information provided below is only a summary. For more information consult Biological Control of Weeds in the West (see Bibliography) or contact commercial weed biocontrol insectaries (see Insectaries).
To date, 12 beneficial insects have been released in a program to introduced a complex of natural enemies against spotted, diffuse, and Russian knapweeds.
Research continues to search for other possible biocontrol agents for the knapweeds. The following natural enemies are still undergoing rigorous testing and will not be available for at least 5 years. Aceria acroptiloni, a mite native to Crimea, develops in the flowers of Russian knapweed. Attacked plants show symptoms such as red swellings on the underside of leaflets. Heavy infestations can stunt the plant and reduce its number of leaves. The mite has not been introduced to the United States. A. centaurea, is a leaf-galling mite from Europe that feeds on rosettes and shoots of Centaurea plants. Signs of mite attack are clusters of light green galls. This arachnid is under quarantine and recommended for further studies to help control knapweeds. Another mite, A. thessalonicae, a bud-galling mite is a newly described species that causes broom-like deformations on diffuse knapweed. In high numbers, it may also reduce plant growth, reduce seed production and kill rosettes. Further studies are needed to determine the efficacy of this mite as a biocontrol agent against diffuse knapweed. In addition current studies have identified three insects that have potential as biocontrol agents against Russian knapweed: Aceria acroptiloni, Aulacida acroptilonica, and Puccinia acroptili (Rosenthal 1996; Zimmerman 1996).
A number of pathogens are under study for control of knapweeds and may reach the commercial stage in the future.
Spotted knapweed is more palatable in late spring or early summer and repeated grazing can reduce flower stem production. Diffuse knapweed seed production can be reduced when grazed during the bolting stage for 10 days, and again after 14 days for an addition 10 days. Although grazing diffuse knapweed can reduce seed production, it can also cause diffuse knapweed to become a short-lived perennial and when grazing is removed, populations often return to its former levels (Beck 1997; Coombs 1998).
Goats and sheep are economical and they do not pose the environmental dangers of applying chemicals. In addition to their value for weed control, sheep can also be used for income from the sale of their wool. If confined, Angora and Spanish goats will trample or browse virtually any vegetation within a fenced area. Desirable trees or shrubs can be protected with light-weight flexible fencing reinforced with electrified wire (Olkowski and Olkowski 1992; Daar 1983).
Consider the impact of grazing on biocontrols, if present. Long-term grazing can be detrimental to seedhead insects because of the removal of seedheads. Also, grazing can delay flowering times and cause asynchrony between the insect and knapweed life cycle. Grazing is compatible with root feeding biocontrols.
Knapweed responses to physical, mechanical, or chemical treatment may vary according to species and environmental conditions—particularly moisture availability. For example, in an unusually wet year, diffuse knapweed densities actually increased after 3 mowings. On the other hand, moist soil can facilitate the growth of healthy, competitive perennial grasses (Beck 1997).
Russian knapweed often invades riparian areas, where its ability to recover from treatment is enhanced by moist soils. Russian knapweed is a perennial with an extensive root system that can resprout from root fragments. Therefore many physical control methods are not highly effective. Since Russian knapweed can reproduce from cut rhizomes, cultivation and handpulling is not effective and often contributes to its spread (Roche and Roche 1991; Sheley 1997).
It can be very difficult to control spotted and diffuse knapweeds by hand pulling alone; but it can be an effective component of an integrated control strategy. Hand pulling requires a commitment of at least 7 or 8 years to ensure that follow-up removal of new seedlings and resprouts do not re-establish. A major advantage of hand removal is that it occurs before plants set seed and thus reduces the soil seedbank over time. Some workers may suffer allergic reactions from touching knapweeds and inhaling pollen. Skin irritations can be minimized by wearing gloves and a long-sleeved shirt.
When beginning a hand removal project, flag the treated areas so they can be identified for follow-up treatments in subsequent seasons. It is easiest to work in relatively small areas of infestation. When faced with dense and/or extensive stands of knapweeds, it is best to divide them into grids (with flags, stakes, etc.) so that workers can thoroughly weed smaller areas before moving onto the next grid. The grid system also facilitates dividing work activities between those pulling and those removing the debris.
Pulling or digging out knapweeds is best done in the spring during the rosette or early bud stage, before flowers appear, and when soils are moist. Since roots can extend a few feet into the soil, be sure to pull out the entire "carrot-like" taproot, otherwise if the plant breaks at the root collar near the surface, it may resprout. Beck (1997) noticed that diffuse knapweed usually will not resprout if the root collar is removed. However, spotted knapweed resprouts more easily than diffuse knapweed. For small infestations, use an asparagus pick or a dandelion pick that has forked prongs. Use a shovel or pulaski (a long-handled hoe) to dig out larger plants with larger roots. Continual monitoring and treatment will catch resprouts and young plants (Renkin 1997).
Once pulled, bag plants and dispose of them properly. Plants can be piled and burned, buried in a landfill, buried in a deep, covered pit, or thoroughly composted in a hot composting system to kill seeds. Normal fires are not long or hot enough to destroy knapweed seeds. The first and second years of a hand-pulling program will involve intensive removal because 1st year rosettes will then mature. During the 3rd and 4th years, you should see a decrease in knapweed numbers and pulling efforts will be noticeably easier. Hand pulling is ineffective against mature Russian knapweed because of its deep roots. It can also resprout from root fragments. Although hard to find, the early rosette stage of Russian knapweed can be pulled. For identification of young rosettes, Weeds of the West provides excellent color photos of spotted, diffuse, and Russian knapweed (Payton et al. 1986; Whitson 1997; Renkin 1998).
Controlled burns can potentially help control knapweeds given sufficient fire intensity. However, it is difficult to find fuel conditions needed to carry a sustained consuming fire that will cause substantial damage. Knapweeds are not very flammable and many areas do not contain sufficient amounts of fuel, resulting in low temperature fires and patchy, discontinuous burns. Even when the surface area of the plant is killed, knapweed can often resprout from root reserves. Fires might actually be detrimental in knapweed control, creating disturbances favorable to knapweed establishment and growth. On the other hand, a combination of fire and herbicide treatment resulted in greater grass cover than non-burned, herbicide treated sites (Renkin 1997; Whitson 1997; Xanthopoulos 1988; Roche and Sheley 1982; Sheley et al. 1998).
Flaming with a propane-fueled weed burner can be used to quickly heat the foliage of young knapweed seedlings or rosettes (it is not very effective on mature plants). The flame sears the plant, raising the temperature of the cell sap, causing cells to rupture and the plant to dehydrate and die within a few hours. Flaming is done on green plants; not on dead foliage. Plants can be seared at any time before flowering. Flaming is useful in clearing small areas such as irrigation ditches, roadsides, or areas close to fences or other structures (Whitson 1997).
In IVM programs, herbicides are considered transition tools that enable the manager to suppress weeds and replace them with desirable, competitive vegetation. Thus, it is important to select the least-toxic, low-residual herbicide that is effective against the target weed, and to apply them in a judicious manner.
The following links discuss the primary considerations when using herbicides: Use Herbicides Properly, Herbicide Information Resources, Criteria for Selecting an Herbicide, and Application Methods.
Applying herbicide to plants when knapweed is most susceptible (preferably before seeds are produced) is crucial to the effectiveness of the treatment. Perennials such as Russian knapweed are susceptible to herbicides shortly before or after the first major killing frost in the fall. At this time, nutrients are translocated and stored in the root systems (Whitson 1998).
Beck, K.G. 1994. Diffuse and spotted knapweed: biology and management. Colorado State University Cooperative Extension 3.110. 2pp
Beck, G. 1997. Pers. Comm. Extension Weed Scientist. Colorado State University.
Berube, D.E. and J.H. Myers. 1982. Suppression of knapweed invasion by crested wheatgrass in the dry interior of British Columbia. Journal of Range Management 35(4): 459-461.
Bobylev, M.M., L.I. Bobyleva, and G.A. Strobel. 1996. Synthesis and bioactivity of analogs of maculosin, a host-specific phytotoxin produced by Alternaria alternata on spotted knapweed (Centaurea maculosa). Journal of Agricultural Food Chemistry 44: 3960-3964.
Bottoms, R.M., T.D. Whitson, and D.W. Kock. 1995. Chemical and biological control techniques for Russian knapweed. NCWSS Proceedings 50: 34-38.
Czembor, E. and G.A. Strobel. 1997. Limitations of exotic and indigenous isolates of Fusarium avenaceum for the biological control of spotted knapweed-Centaurea maculosa. World Journal of Microbiology and Biotechnology 13: 119-123.
CO State University. 1988. Non-chemical alternatives for managing selected plant species in the Western United States. Colorado State University Cooperative Extension. XCM-118. 11pp.
Coombs, E.M. 1995. Biological control of weeds project summaries. Oregon Department of Agriculture Commodity Inspection Division, Noxious Weed Control Program. Technical Bulletin 96-1.
Daar, S. 1983. Using goats for brush control. IPM Practioner 5(4): 1-2.
Dewey, S.A. and J.M. Torell. 1991 What is a noxious weed? In: James et al., Noxious Range Weeds. Westview Press, Boulder, CO.
Engeland, R.1986. Hand-pulling of diffuse and spotted knapweed. Citizens for Environmental Quality of Okanogan County. 10pp.
Fay, P.K., T.D. Whitson, S.A. Dewey, and R. Sheley, eds. 1995. 1995-1996 Montana-Utah-Wyoming Weed Management Handbook. Coop. Ext. Serv., Montana State University, Bozeman, MT. 245 pp.
Fay et al. 1991. Chemical control of Centaurea maculosa in Montana. pp. 303-315. In: James et al., Noxious Range Weeds. Westview Press, Boulder, CO.
Fuller, T.C., and G.D. Barbe. 1985. The Bradley method of eliminating exotic plants from natural reserves. Fremontia 13(2): 24-25.
Harris, P. and J.H. Myers. 1984. Centaurea diffusa Lam. and C. maculosa Lam. s. lat., diffuse and spotted knapweed (Compositae). pp 127-137. In: Kelleher and Hulme, Biological Control Programmes against Insects and Weeds in Canada 1969-1980. Commonwealth Agricultural.
Higgins, R.E., and R. Schirman. 1977. Know and control spotted and diffuse knapweed. Current Information Series No. 362. University of Idaho College of Ag. Coop. Ext. Serv. Agricultural Experiment Station. 3pp.
Hubbard. W.A. 1975. Increased range forage production by reseeding and the chemical control of knapweed. Journal of Range Management 28(5): 406-407.
Kennett, G.A., et al. 1992. Effects of defoliation, shading, and competition on spotted knapweed and bluebunch wheatgrass. Journal of Range Management 45: 363-369.
Jordan, K. 1995. Host specificity of Larinus minutus Byll. (Col., Curculioidae), an agent introduced for the biological control of diffuse and spotted knapweed in North America. Journal of Applied Entomology 119: 689-693.
Jacobs, J.S., R.L. Sheley, and B.D. Maxwell. 1996. Effects of Sclerotinia sclerotiorum on the interference between bluebunch wheatgrass (Agropyron spicatum) and spotted knapweed (Centaurea maculosa). Weed Technology 10: 13-21.
Jacobs, J.S., R.L. Sheley, and B.D. Maxwell. 1997. Yellow starthistle population dynamics model. Colorado Weed Management Association 1997 Annual Conference Proc. Granby, CO 80446-1910.
Lacey, C.A., et al. 1988. Bounty programs—an effective weed management tool. Weed Technology 2: 196-197.
Lacey, J.R., and B.E. Olson. 1991. Environmental and economic impacts of noxious range weeds. pp.5-15. In: James et al., Noxious Range Weeds. Westview Press, Boulder, CO.
Lang, R. 1997. Diffuse knapweed and spotted knapweed. Biological control: a guide to the natural enemies in North America. http://nysaes.cornell.edu/ent/biocontrol/toc.html. (visited Feb. 5, 1998).
Lindquist, J.L., B.D. Maxwell, and T. Weaver. 1996. Potential for controlling the spread of Centaurea maculosa with grass competition. Great Basin Naturalist 56(3): 267-271.
Kennett, G.A., et al. 1992. Effects of defoliation, shading and competition on spotted knapweed and bluebunch wheatgrass. Journal of Range Management 45: 363-369.
Maddox. D.M. 1979. The knapweeds: their economics and biological control in the Western States, USA. Rangelands 1(4): 130-141.
Maddox, D.M. 1979. The knapweeds: Their economics and biological control in the Western States, USA. Rangelands 1(4): 139-141.
Maddox, D.M. 1982. Biological control of diffuse knapweed (Centaurea diffusa) and spotted knapweed (C. maculosa). Weed Science 30: 76-82.
Maxwell, B.D. and R.L. Sheley. 1997. Noxious weed population dynamics education model. Weed Technology 11:182-188.
Maxwell J.F. et al. 1992. Effect of grazing, spraying, and seeding on knapweed in British Columbia. Jour. of Range Mangmt. 45: 180-182.
Maxwell, B. " YST software." Personal e-mail (9 Feb 1998).
Muller-Sharer, H. 1993. The biological control of Centaurea spp. in North America: do insects solve the problem? Pesticide Science 37: 343-353.
Olkowski H. and B. Olkowski. 1992. Using animals for weed management. Common Sense Pest Control 8(2): 5-13.
Olson, B.E. and J.R. Lacey. 1994. Sheep: a method for controlling rangeland weeds. Sheep Research Jour., Special Issssue: 105-112.
Popay, I. and R. Field. 1996. Grazing animals as weed control agents. Weed Technology 10: 217-231.
Panter, K.E. 1991. Neurotoxicity of the knapweeds (Centaurea spp.) in horses. pp.316-324. In: James et al., Noxious Range Weeds. Westview Press, Boulder, CO.
Payton, G. et al. 1986. Hand-pulling of diffuse and spotted knapweed. Citizens for Environmental Quality of Okanogan County. Columbia Publishing, Oroville, WA. 10pp.
Pearson, W. 1998. Pers. Comm. County Weed Extension Agent. Stillwater County, Columbus, Montana.
Randall, J.M. and J. Marinelli eds. 1996. Invasive Plants: Weeds of the Global Garden. Brooklyn Botanic Garden , Inc. Brooklyn, NY. 108pp.
Rees, N.E., P.C. Quimby, Jr., G.L. Piper, E.M. Coombs, C.E. Turner, N.R. Spencer, and L.V. Knutson, eds. 1996. Biological Control of Weeds in the West. Western Society of Weed Science, USDA/ARS, Montana Dept. Agric., Montana State University, Bozeman, MT.
Renkin, R. 1997. Pers. Comm. Management Biologist. Branch of Natural Resources. Yellowstone National Park, WY 82190.
Roche, B.F., and C.T. Roche. 1991. Indentification, introduction, distribution, ecology, and economics of Centaurea species. pp 274-291. In: James et al., Noxious Range Weeds. Westview Press, Boulder, CO.
Rosenthal, S.S. 1996. Aceria, Epitrimerus and Aculus species and biological control of weeds. In: Lindquist, Sabelis, and Bruin. Eriophyoid Mites - Their Biology, Natural Enemies and Control. Elsevier Science B.V.: 729-737.
Rosenthal, S. et al. 1991. Biological control of Centaurea spp. pp 292-302. In: James et al., Noxious Range Weeds. Westview Press, Boulder, CO.
Scotts Bluff National Monument. 1997. Centaurea diffusa. http://www.npsc.nbs.gov/resource/othrdata/exoticab/scotcent.htm (visited Dec. 1997).
Sheley, R.L., J.S. Jacobs, and M.L. Carpinelli. 1998. The distribution, biology, and management of diffuse (Centaurea diffusa) and spotted knapweed (Centaurea maculosa). Weed Technology (In Press).
Sheley, R.L., and J.S. Jacobs. 1997. Response of spotted knapweed and grass to picloram and fertilizer combinations. Journal of Range Management 50: 263-267.
Sheley R.L., B.E. Olson, and L.L. Larson. 1997. Effect of weed seed rate and grass defoliation level on diffuse knapweed. Journal of Range Management 50: 39-43.
Sheley, R.L., and B.F. Roche 1982. Rehabilitation of spotted knapweed infested rangeland in northeastern Washington. Abstr. of papers, Western Society Weed Science, Denver, CO. In. Sheley, R.L., J.S. Jacobs, and M.L. Carpinelli. 1998.
Sheley R.L., T.J. Svejcar, and B.D. Maxwell. 1996. A theoretical framework framework for developing successional weed management strategies on rangeland. Weed Technology 10: 766-773.
Steinger, T. and H. Muller-Scharer. 1992. Physiological and growth responses of Centaura maculosa (Asteraceae) to root herbivory under varying levels of interspecific plant competition and soil nitrogen availability. Oecologia 91: 141-149.
Spears, B.M., S.T. Rose, W.S. Belles, 1980. Effect of canopy cover, seeding depth, and soil moisture on emergence of Centaurea maculosa and C. diffusa. Weed Research 20: 87-90.
Story, J.M. and R.M. Nowierski. 1985. Population increase of the seed head fly in Western Montana. Montana Agricultural Research Summer 1985: 8-10.
Story, J.M., K.W. Boggs, and D.R. Graham. 1989. Effects of nitrogen fertilization on spotted knapweed and competing vegetation in western Montana. Journal of Range Management 42(3): 222-225.
Strang, R.M., K.M. Lindsay, and R.S. Price. 1979. Knapweeds: British Columbia’s undesirable aliens. Rangelands 1(4): 141-143.
University of Northern Iowa. 1993. Integrated Roadside Vegetation Management Technical Manual. Produced by the Roadside Management Program. To obtain, call Kirk Henderson at 319-273-2813.
Watson, A.K. 1986. Host range of, and plant reaction to, Subanguina picridis. Journal of Nematology 18(1): 112-120.
Watson, A.K. 1980. The biology of Canadian weeds. 43. Acroptilon (Centaurea) repens (L,) DC. Canadian Journal of Plant Science 60: 993-1004.
Whitson, T. 1998. Establishing a sustainable vegetation ecosystem to replace noxious weeds. Abstract from Science in Wildland Weed Management Symposium. Denver, CO.
Whitson, T.D. 1997. Pers. Comm. Control measures for knapweeds. Extension Weed Specialist. Box 3354 University of Wyoming, Laramie, WY 82071.
Whitson, T.D. et al. (Eds.). 1996. Weeds of the West. The Western Society of Weed Science and te Western United States Land Grant Universities Cooperative Extension Services. Newark, CA. 630pp.