Effectiveness of Herbicides Applied Mid-Summer or Fall on Canada Thistle Control

Canada thistle (Cirsium arvense) is a widespread noxious weed found throughout the northern half of the United States. Field studies were established on Canada thistle in several states in the upper Midwest to measure effectiveness of herbicides applied at various application times including mid-summer and fall. Milestone® herbicide at 5 and 7 fluid ounces per acre (fl oz/A) was applied at various growth stages from May through early November (Figure 1).

Plot design was a randomized complete block with three replications of each treatment. Evaluations of herbicide treatments were made one year following treatment. Control was determined from visual estimates of Canada thistle shoot growth reduction relative to the non-treated control.

Figure 1: (left to right) Rosette to early bolt (May); bud (June/ July); flower (July/Aug); seed dispersal (mid- to late-August); green basal growth to late fall dormancy (September to early November). Photos by C. Duncan

Figure 1: (left to right) Rosette to early bolt (May); bud (June/ July); flower (July/Aug); seed dispersal (mid- to late-August); green basal growth to late fall dormancy (September to early November). Photos by C. Duncan

RESULTS

Figure 2: Canada thistle control at four application timings evaluated one year after application (2 locations SD/MN). Early bolt stage SD data only.

Figure 2: Canada thistle control at four application timings evaluated one year after application (2 locations SD/MN). Early bolt stage SD data only.

Milestone® herbicide at either 5 or 7 fl oz/A applied in June/ July (bud to flower growth stage) and throughout summer and early fall provided significantly better control one year after treatment than applications made in May (rosette to early bolt). Control declined slightly with applications made at the seed dispersal/dormant growth stage compared to bud/flower-stage, but was greater than 80 percent with Milestone at the 7 fl oz/A rate (Figure 2).

Milestone applied in fall resulted in greater control when applied in September compared to late October or November, indicating that new basal growth in September may be the optimal time for fall applications (Figure 3).

Figure 3: Canada thistle control at three fall application timings evaluated one year after treatment (3 locations ND, SD, NE).

Figure 3: Canada thistle control at three fall application timings evaluated one year after treatment (3 locations ND, SD, NE).

CONCLUSIONS

Results of these studies indicate that the optimal time to control Canada thistle plants and stop seed production is to apply Milestone herbicide when Canada thistle plants are at bud to early flower growth stage. However, good control of Canada thistle plants can be achieved at later growth stages. Apply the higher rate of Milestone at 7 fl oz/A at late flower to early seed dispersal to achieve greater than 80 percent control. Fall herbicide applications are effective if green growth is present on Canada thistle stems and basal rosettes. Milestone at 7 fl oz/A was more consistent than lower rates of Milestone at seed dispersal and late fall application timings.

REFERENCES

  • Almquist TL, RG Lym. 2010. Effect of aminopyralid on Canada thistle (Cirsium arvense) and the native plant community in a restored tall grass prairie. Invasive Plant Science Management 3:155-168

  • Burch PL, RG Wilson, ES Hagood, RA Masters, and VF Carrithers. 2006. Canada thistle management with Milestone herbicide. Weed Science Soc. of Am. 46th Conf. In Proceedings. New York, New York.

  • Deneke D, M Moechnig, D Vos, J Alms. 2012. Optimal herbicide application timing for Canada Thistle Control. In Proceedings, Western Society of Weed Science. p. 22-23.

  • Dow AgroSciences LLC. Internal field data.

 

®™ Trademarks of Dow AgroSciences, DuPont or Pioneer and their affiliated companies or respective owners.

Milestone herbicide is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. When treating areas in and around roadside or utility rights-of-way that are or will be grazed, hayed or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas or rotating the treated area to sensitive crops. See the product label for details. State restrictions on the sale and use of Milestone apply. Consult the label before purchase or use for full details. Always read and follow label directions. ©2019 Corteva