Field evaluations of herbicides for the control of Centaurea diffusa lam. and C. maculosa lam.

• Main Author: Douglas, Brian John
• Language: English
• Published: University of British Columbia 2010
• Online Access: http://hdl.handle.net/2429/24580

Diffuse knapweed (Centaurea diffusa) and spotted knapweed (C. maculosa) are widespread in the southern interior of British Columbia and are rapidly spreading to colonize new areas. The use of herbicides for control of these knapweeds has been restricted because of the high cost of and environmental impact resulting from applications of picloram. Phytotoxic residues of picloram have caused damage to cultivated plants in B.C. and at present the only effective alternate herbicide is 2,4-D. Field studies were conducted in the Thompson Valley to evaluate picloram, triclopyr, 3,6-dichloropicolinic acid and chlorsulfuron for control of diffuse and spotted knapweeds. Triclopyr and chlorsulfuron provided control of both species only for the season of treatment. Picloram and 3,6-dichloropicolinic acid controlled spotted knapweed for at least 300 days and diffuse knapweed for at least 420 days. Both herbicides also provided residual control of knapweed seedlings. The addition of Citowett Plus (50% alkyl aryl polyethylene glycol) at 0.1% (v/v) to solutions of chlorsulfuron did not improve control of diffuse knapweed with chlorsulfuron. Chlorsulfuron had some soil residual activity against diffuse knapweed seedlings. The experimental sulfonyl urea, DPX-T6376 applied at 20 to 160 grams/ha controlled existing diffuse knapweed plants and emerging seedlings in the season of treatment. A minimum effective rate of 3,6-dichloropicolinic acid for control of existing diffuse knapweed plants and residual control of seedlings for the season of treatment was 150 g/ha. Rates of 3,6-dichloropicolinic acid below this provided poor control of emerging seedlings and many diffuse knapweed plants were able to recover from the treatment. The effects of picloram, triclopyr, 3,6-dichloropicolinic acid and chlorsulfuron on seed germination, radicle elongation and seedling growth of diffuse knapweed, spotted knapweed, bluebunch wheatgrass and crested wheatgrass were evaluated in Petri plates. Chlorsulfuron reduced the radicle elongation and seedling growth of diffuse and spotted knapweeds. The effects of the pyridine herbicides varied with concentration and herbicide. === Land and Food Systems, Faculty of === Graduate