Q: How do I get rid of prostrate knotweed in each of my major crops?

A: In Ontario, prostrate knotweed is more frequently found in Huron County and on Brookston clay-loam soils under no-till or minimum-till production systems (Frick, 1990). It can thrive in compacted soils where other plants will struggle to grow well. Below is a breakdown of management strategies that have provided the most success to date.

A mature plant in mid-September.
photo: Supplied

CORN: Prostrate knotweed is typically not a problem in corn because primary and secondary tillage remove the weed. Atrazine will provide good control when applied prior to the emergence of prostrate knotweed. Once emerged, Distinct can provide good control in conventional corn, while glyphosate (360 g/l) applied at two l/ac. is about the best you can do in glyphosate-tolerant crops (Hefty and Hefty, 2009).

SOYBEANS: Metribuzin (e.g. Sencor) has provided the best control of prostrate knotweed in Ontario trials (Hamill et al., 2000). Only one replicated study in Ontario has evaluated post-emergence control of prostrate knotweed. Pinnacle SG was the only herbicide to provide over 80 per cent control. Classic and FirstRate suppressed prostrate knotweed (~70 per cent control) and Reflex provided roughly 60 per cent visual control.

A young seedling with linear cotyledons at the ideal stage for control with labelled herbicides.
photo: Supplied

CEREALS: Prostrate knotweed isn’t usually a problem in cereals as it doesn’t tend to affect either grain yield or harvesting ease. It’s likely not a weed worth altering your herbicide program for since a number of the common herbicides will provide some level of control.

photo: Source: Pesticide Label Search — Health Canada, //pr-rp.hc-sc.gc.ca/ls-re/index-eng.php. Accessed: December 3, 2016

Citations

• Frick, B.L. 1990. Studies on the control of problem weed species in conservation tillage systems. Accessed: December 3, 2016.
• Hamill, A., Doucet, C., Ferguson, G., Weaver, S., Sikemma, P. and B. Deen. 1999. Control Systems for Problem Weeds in Field Crops. Ontario Research Enhancement Program. Report No. OREP-1999/18.
• Hefty, B. and D. Hefty. Weed of the Week #588 — Common knotweed. Accessed: December 3, 2016.

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