Guidelines for Fall Weed Control

When it comes to perennial weed control, most growers, agronomists and weed scientists agree that persistence is key. If you let your guard down even once, perennial weeds will become more problematic the next year.

Perennial weeds are more easily controlled in the fall than in the spring. Cooler temperatures will trigger movement of sugars down to their roots for over-winter storage. Applying a systemic herbicide in the fall promotes movement of the active ingredient down to the roots providing effective control.

The following is a summary of control guidelines for the most asked about weeds.

1. Canada Thistle: Single applications do not provide long-term control as the root system can survive even when the shoots and leaf tissue has been killed. Effective control requires multiple applications¹.
   
   

   Table 1. Application timing and rate of glyphosate needed for the control of Canada Thistle.

   Product	Rate (L/ac)	Application Timing
   glyphosate	1 - 1.5 L/ac	early bud stage

   
   
2. Perennial Sow Thistle: Fall herbicide applications are the most effective method for the control of perennial sow thistle. However, field experience has shown that perennial sow thistle is less sensitive to glyphosate than Canada Thistle and therefore requires a higher rate. Field experiments conducted in Michigan showed that applications made to the rosette stage in the fall gave better control then when perennial sow thistle was at the bud stage⁵.
   
   

   Table 2. Application timing and rate of glyphosate needed for the control of Perennial sow thistle.

   Product	Rate (L/ac)	Application Timing
   glyphosate	2 L/ac	rosette to early bud stage

   
   
3. Field Bindweed: Field bindweed is effectively controlled with glyphosate when applied in the fall (i.e. September/October). Field experiments in Ontario show adequate control with glyphosate applied at a rate of 1.5 L/ac or higher. A fall application of glyphosate should provide acceptable control for the following growing season, but continuous fall applications will be necessary for long-term management. Tillage prior to a glyphosate treatment will reduce field bindweed control and can cause an increase in the number of buds produced at the nodes of crowns, roots, rhizomes and stems ^3,4.

   Should I Add Banvel II? The addition of Banvel II to glyphosate does provide better initial control of field bindweed. However evaluations made the following spring showed little benefit to addition of Banvel II.

   Table 3. Application Rate (L/ac), percent control achieved and price ($/ac) of glyphosate and glyphosate + Banvel II for the control of field bindweed.

   Product(s)	Rate (L/ac)	% Control (spring)	Price ($/ac)
   glyphosate	1.5 L/ac	92.5%	$13.50
   glyphosate + Banvel II	0.68 L/ac + 0.5 L/ac	99%	$21.76

   Source: Summary of 1 Field Trial by François Tardif, 2004
   

4. Prickly Lettuce: Glyphosate will provide effective control. However, It is difficult to determine whether fall applications are the most effective way to deal with this weed since it germinates over an extended period of time in the fall and spring. Therefore it is possible for a second flush of weeds to germinate after the initial application, particularly when temperatures remain warm. There may be more value in pre-plant spring versus fall applications for the control of prickly lettuce.
   
   

   Table 4. Application timing and rate of glyphosate needed for the control of Prickly Lettuce

   Product	Rate (L/ac)	% Control
   glyphosate	2 L/ac	92%

   
   
5. Dandelion: Numerous products and tank-mixes provide adequate control of dandelion when applied in the fall. Deciding on which one to use will depend on the weed spectrum, cost and rotational flexibility of the specific herbicide or tank-mix.
   
   

   Table 5. Application Rate (L/ac), percent control achieved and price ($/ac) of numerous herbicides for the control of dandelion.

   Product(s)	Rate (L/ac)	% Control	Price ($/ac)
   Amitrol 240	3 L/ac	96	$20.22
   glyphosate	2 L/ac	92	$18.00
   glyphosate + Banvel II	1 L/ac + 0.25 L/ac	91	$16.82
   glyphosate	1.5 L/ac	90	$13.50
   Banvel II	0.5 L/ac	89	$15.64
   2,4-D Ester	1 L/ac	89	$7.09
   glyphosate + 2,4-D Ester	1 L/ac + 0.5 L/ac	89	$12.55
   Amitrol 240	2 L/ac	88	$12.15
   glyphosate + Amitrol 240	1 L/ac + 1 L/ac	86	$21.76

   Summary of 2 trials by Peter Sikkema.

6. Common milkweed: The most effective way to control this species is with a pre-harvest application of glyphosate. Once the combine header cuts down milkweed it will not generate enough foliage for post harvest applications to be successful.
   
   

   Table 6 Application timing and rate of glyphosate needed for the control of Common milkweed.

   Product	Rate (L/ac)	Application Timing
   glyphosate	1 L/ac	pre-harvest (Bud Stage)

Water Volumes: Glyphosate

It has been well documented that the effectiveness of glyphosate will increase as spray volume decreases². Therefore a water volume of 5-10 gpa (20-40 L/ac or 50-100 L/ha) is recommended. Higher water volumes will increase the chance of salt antagonism of glyphosate and lowers the concentration of glyphosate within each spray droplet².

References:

• Bussan, A.J. and W.E. Dyer. 1999. Herbicides and Rangeland. In: Biology and Management of Noxious Rangeland Weeds. Oregon State University Press, Corvallis OR, pp 116-132.
• Ramsdale, B.K., Messersmith, C.G. and J.D. Nalewaja. 2003. Spray Volume, Formulation, Ammonium Sulfate and Nozzle Effects on Glyphosate Efficacy. Weed Technology. 17:589-598.
• Sandberg, C.L. and W.F. Meggitt. 1977. A summay of field studies for the control of field bindweed with glyphosate. Proceedings of the NCWSS Conference. Vol 32. pp 113.
• Wiese, A.F. and D.E. Lavake. 1981. Control of Field Bindweed with Postemergence Herbicides. Proceedings of the NCWSS Conference. Vol 36. pp 35-36.
• Zollinger R.K. and J.J. Kells. 1986. Perennial sow-thistle (sonchus arvensis) control in crop production systems. Proceedings of the NCWSS Conference. Vol 41. pp 62-63.

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