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Integrated control is most effective against diffuse knapweed. Central to the strategy is the prevention of seed formation, which can be accomplished by grazing, mowing, hand-pulling and biological control. Any control method must be integrated with a programme of revegetation to prevent re-infestation. As knapweeds thrive in direct sunlight, they can be suppressed by shading from competing vegetation (Kennett et al., 1992; Woo et al., 2002).
Grazing by sheep and goats can suppress knapweeds (Olson and Wallander, 2001). Continual grazing of the tops of young plants can retard plant development, seed formation, and gradually deplete root reserves. As animals usually prefer to eat nearby grasses instead of knapweeds, grazing is most effective against knapweeds when the livestock is enclosed in a fenced-off, weedy area (Beck, 1994; Olson and Lacey, 1994).
Diffuse knapweed should be grazed during the bolting stage for 10 days, and again after 14 days for an additional 10 days to reduce seed production. Although grazing 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 their former levels (Popay and Field, 1996).
In knapweed-infested rangeland, any patches of bare ground can lead to knapweed invasions (Lacey et al., 1990). Thus, reseeding of rangelands with competitive perennial grasses is important. Crested wheatgrass (Agropyron cristatum) has been a successful competitor against knapweeds in dry areas of Canada, reducing the rate of vegetative spread, limiting weed density and reducing weed seed production (Berube and Myers, 1982). Crested wheatgrass is more competitive than Russian wildrye (Elymus junceus [Psathyrostachys juncea]) or lucerne (Maxwell et al., 1992) but orchardgrass (Dactylis glomerata) and thickspike wheatgrass (Agropyron dasystachyum) are more competitive than crested wheatgrass (Larson and McInnis, 1989). Meadow fescue (Festuca pratensis) and rough fescue (F. altaica) are more competitive than bluebunch wheatgrass (Pseudoroegneria spicata) (Lacey et al., 1990; Steinger and Muller-Schärer, 1992).
Manipulation of plant ecology is important for the long-term suppression of knapweed. The kinds and numbers of plants at a site change slowly over time. Initial colonizers, usually seedy, weedy and fast-growing, are replaced successively with other plants, until a final set of plants known as the climax vegetation predominates. Soil analyses have shown that the perennial grasses that dominate climax populations on grazing lands are associated with low available nitrogen. Early colonizers such as knapweed do well in nitrogen-rich soil, but cannot compete with perennial grasses in a low nitrogen environment (Sheley et al., 1996a, b; Herron et al., 2001).
When the soil is rich in nitrogen, aggressive plants such as knapweed, which appear early in the succession, will out-compete perennial grasses. Controlled colonization can be used to force plant succession towards the preferred climax vegetation. This strategy is to plant simultaneously intermediate and late colonizers in the natural plant succession scheme. For instance, Herron et al. (2001) found that plantings of annual rye and bottlebrush squirreltail (Elymus elymoides) can deplete the soil of nutrients, especially nitrogen. As a result, late seral plants such as bluebunch wheatgrass (Pseudoroegneria spicata) will out-compete the early colonizer knapweed. Low nitrogen availability also makes knapweed more susceptible to root-feeding biological control agents (Steinger and Muller-Schärer, 1992).
A more short-term approach is to fertilize or combine fertilization with herbicides. Nitrogen fertilization has been used with mixed results. When fertilizer is applied to areas with a substantial grass understorey, grass yields increase on those sites (Sheley and Jacobs, 1997). Where knapweed has already invaded, nitrogen fertilization with no other treatment may favour knapweed over native perennial grasses (Prather and Callihan, 1991; Velagala et al., 1997; Sheley et al., 1998; Herron et al., 2001).
Effects of fertilization may be complicated by moisture conditions at the site. Fertilization in dry areas may remove moisture from the soil, leading to increased competition of drought-tolerant grasses such as crested wheatgrass (Berube and Myers, 1982). However, Myers and Berube (1983) found that the addition of nitrogen had no effect on biomass of either diffuse knapweed or competing grasses in dry areas of British Columbia, Canada.
Control of diffuse knapweed by hand-pulling is feasible for scattered diffuse knapweed plants. It is important to remove the entire taproot with as little soil disturbance as possible. Plants should be pulled in the spring and when the soil is moist, and again in June to remove bolting plants before they flower and set seed. Finally, plants are pulled just before seed dispersal, taking care to remove the plants from the site. Hand-pulling is tedious and not always effective (Roche and Roche, 1999).
Mowing decreases flower and seed production, and in the long-term it may affect knapweed densities. It can also reduce weed competition during a revegetation programme. Mowing diffuse knapweed in British Columbia, Canada, at the bud stage and again at flowering reduced the number of plants producing seed by 77-99%. Mowing treatments also reduced seed germination by 79%. Plants mowed early in the growing season produce few viable seeds; however, mowed plants usually resprout and flower again. In some instances, diffuse knapweed densities increased after a single mowing (Watson and Renney, 1974; SBNM, 1997).
Cultivation and irrigation will kill diffuse knapweed. However, cultivation causes soil disturbance and may not be a management option on rangelands. In a location where cultivation is acceptable, regular deep ploughing can slice knapweed roots and bury seeds, thus weakening the plant. Cultivation in combination with reseeding competitive perennial grasses may minimize re-invasion by the knapweeds (Roche and Roche, 1999).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Many economic losses have been attributed to diffuse knapweed infestations. Examples include the loss of forage on rangeland and pasture, depletion of soil and water resources, displacement of native species on wildlands, reduction of biodiversity, reduced land value and increased road maintenance costs. Costs for forage loss alone in Washington state, USA, have been estimated at about $1-3 million each year (Lacey and Olson, 1991; Roche and Roche, 1999; Zouhar, 2001).