Rhaponticum repens

The control strategy for R. repens and many perennials involves stressing the plant by mowing or defoliation, causing depletion of nutrients in the root system. Plants should not be allowed to produce seed and root fragments should not be spread to other locations following treatment (Zouhar, 2001). R. repens has a competitive advantage in a variety of environments and will continue to persist unless it is suppressed long enough to allow the introduction and establishment of desirable species (Bottoms et al., 2001). The number of shoots produced increases with light intensity (Roche et al., 1986).
Any treatment that provides control of R. repens must either release competitive species present in the understory or be combined with reseeding before long-term sustainable control can be achieved (Whitson, 1999;Whitson, 2001). Plants that compete well include smooth brome (Bromus inermis), thickspike wheatgrass (Elymus lanceolatus), crested wheatgrass (Agropyron cristatum) and Russian wild rye (Psathyrostachys juncea). Sod-forming perennials such as r thickspike wheatgrasses compete better than bunch grasses (Benz et al., 1999;Beck, 2003).
Animal grazing is not very effective. Animals will not graze on it when other vegetation is available because of its bitter taste (Olson and Lacey, 1994;TD Whitson, Extension weed specialist, University of Wyoming, Laramie, WY 8207 1, USA, personal communication, 1997). The weed is also poisonous to horses, producing a neurological condition called equine nigropallidal encephalomalacia. Symptoms include the inability to eat or drink and aimless or awkward movement. R. repens is not toxic to sheep and dogs and cattle have grazed infested pastures with no evidence of toxicity (Lacey and Olson, 1991;Panter, 1991).
Mechanical Control
R. repens often invades riparian areas, where its ability to recover from treatment is enhanced by moist soils. R. repens is a perennial with an extensive root system that can resprout from root fragments. Therefore, many physical or mechanical methods are not highly effective. Hand pulling is generally ineffective against mature stands because of its deep roots and its ability to resprout from root fragments. Hand pulling or cutting the plants to kill the tops will starve the roots if done repeatedly, but this might not be a practical solution. Hand pulling might be successful for the early rosette stage (TD Whitson, Extension weed specialist, University of Wyoming, Laramie, WY 8207 1, USA, personal communication, 1997).
Cutting, mowing or disking infestations several times annually will control the existing top-growth and prevent seed production. Cutting or mowing three times a year depletes nutrients in the roots, but unless mowing is continued, plants will recover. Mowing may also not be possible in environmentally sensitive areas (Zouhar, 2001). Cutting is slightly less effective than pulling since cutting does not remove any portion of the root. Pulling plants two to three times annually contained, but did not eliminate, an infestation in Washington (Carpenter and Murray, 2002). Cutting roots for 3 years to a depth of 30 cm can destroy the root system in the top metre of soil. Root fragments up to 40 cm long are killed by burial below 30 cm, indicating some control by deep ploughing (Watson, 1980).
Thick mulches of straw and manure have suppressed small patches of the weed. Sheet metal and paper have also been used successfully to mulch small patches, and black plastic might be effective (Zouhar, 2001).
Unlike other knapweeds, R. repens can survive and grow in tilled soils. In fact, tilling breaks up the roots into fragments and facilitates the spread of R. repens. As root fragments are killed if buried below 25 cm, repeated deep ploughing and cutting of the roots over a period of 3 years may destroy the top layers of the root system. Even when dead R. repens can leave allelopathic residues in the soil and prevent the growth of other plants for two growing seasons. To alleviate the effect of residual plant chemicals, remove or plough the treated R. repens plants and later re-seed to facilitate grass establishment (Watson, 1980;TD Whitson, Extension weed specialist, University of Wyoming, Laramie, WY 8207 1, USA, personal communication, 1997).
Fire is not very effective at controlling R. repens (Bottoms and Whitson, 1998;Zouhar, 2001).
Chemical Control
In general, herbicides should only be used as a last resort and as part of an integrated vegetation management programme. Herbicides are expensive and do not provide lasting control. They can also cause negative impacts on water quality, and select for resistant weeds that may be worse than the original problem (Powles and Shaner, 2001). The persistent herbicides clopyralid and picloram often recommended for control are not metabolised by grazing animals and are not destroyed by composting (Bezdicek et al., 2001;Houck and Burkhart, 2001).
If herbicides are used, they should be applied when R. repens is most susceptible and before seeds are produced;perennials are most susceptible to herbicides in the autumn (Whitson, 1999). The most effective herbicides are picloram and chlopyralid (Benz et al., 1999;Whitson, 2001).