Perfect Spray Timing for Better Dandelion and Canada Thistle Control

/_uploads/images/ts-weeds-WCAN.jpgSpraying multiple effective modes of action is one of the best ways to control tough and herbicide-resistant weeds.1 But for some weeds, when you spray is even more important than what you spray. Here are some strategies to help you perfect your application timing and get better control of perennials like dandelion and Canada thistle in the spring and fall.




For both dandelion and Canada thistle, expect to achieve only topgrowth control in the spring. The weeds in your field are the overwintering perennial plants from last year, and they will have already flowered and set seed by the time you make a pre-seed burndown.2

Your spring weed control strategy should focus on reducing competition for the incoming crop and removing top growth to starve the root system.

Canada thistle is vulnerable to herbicides in the bud-to-early-bloom stage, when it’s using its nutrient reserves to power a flush of new growth in late May to mid-June. Applying herbicides during this window will help to remove top growth and force the plant to use its root reserves.3

The dandelion seeds produced by the over-wintering plants will germinate in late June to early July – after many farmers have completed their in-crop herbicide application. Control this population with a post-harvest glyphosate application.




“Dandelions are easier to control in the fall than in the spring,” says Johnathan Dierking, Monsanto Weed Management Technology Development Representative in Wisconsin. “If you wait until spring, dandelions will be much hardier after storing up moisture and nutrients in their root systems from overwintering.”4

Fall is also the ideal time to control Canada thistle. Like dandelion, it stocks up its root reserves for the winter. Systemic herbicides can move through the plant with the sugars being sent down to the roots.3 Applying a full-rate of glyphosate when the plants are actively growing will do the most damage to the roots.

Post-harvest weed staging Canada thistle and dandelion can survive several light frosts, so they can be treated later into the fall. Watch out for hard frosts and droughts – they’ll stop weed growth, and glyphosate will not control perennials at the root level.2

Target weeds Regrowth required Rate (L/ac) Tillage interval Comments
Canada thistle 3 to 4 new green leaves 0.67 – 1.33 Minimum 5 days Use higher rates for larger plants. Ensure adequate growth and good environmental conditions.
Dandelion <15 cm diameter
>15 cm diameter
0.67 – 1.0
1.0 – 1.33
Minimum 5 days Use higher rates for larger plants. Apply pre-bloom to full-flower for best results. Dandelions covered in straw will likely not be controlled.

/_uploads/images/ts-weeds-WCAN2.jpgCULTURAL CONTROL STRATEGIES

Grow competitive forages like alfalfa in your crop rotation.

Several studies have shown that alfalfa hay in rotation can substantially reduce population densities of Canada thistle and other weed species within a three-year time frame.5


Use selective, post-harvest tillage.

For dandelions, tillage must be deep enough to cut the root 4 inches below the crown. Heavy-duty cultivators and deep tillers offer the best control.6


Using tillage to control Canada thistle is a risky strategy. Tillage equipment can spread root pieces across the field, and new plants can generate from pieces as small as 3/8 of an inch long. You may need to till or mow frequently, over multiple seasons, to remove top‑growth and starve Canada thistle’s root reserves.5


Canada thistle can be tilled five days after a post-harvest glyphosate application, and dandelion seven days after application. Following this window will ensure sufficient herbicide translocation for optimum control.



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1 Kniss, Andrew. “Want to reduce herbicide resistance? Spray more herbicides!” Control Freaks, Wyoming Weed Science 17 Nov. 2014. Web. Accessed 4 Apr. 2016

2 “Controlling Dandelion In The Fall.” Manitoba Agriculture, Food and Rural Development. Web. Accessed 1 May 2017.,controlling-dandelion-in-the-fall.html

3 Gover, Art, et al. “Managing Canada Thistle.” Pennsylvania State University - Department of Horticulture, College of Agricultural Sciences. 2007. Web. Accessed 1 May 2017.

4 “Fall is Best Time to Control Dandelions in No-Till Fields.” Roundup Ready Plus. 1 Nov. 2012. Web. Accessed 28 Apr. 2017.

5 “Canada Thistle and its Control.” Saskatchewan Ministry of Agriculture. November 2008. Web. Accessed 30 Apr. 2017.

6 “Dandelion.” Manitoba Agriculture, Food and Rural Development. Web. Accessed 1 May 2017.,dandelion.html