With lateral roots that go as deep as two metres and seed production approaching 10,000 per plant, perennial sow thistle is extremely difficult to eradicate. Its impact can be minimized, however. Management that reduces top growth will lower seed production and root mass over time.
But be forewarned. If you decide to take a year off because it seems like you have the species under control, it can very quickly recover and return to its original level of domination. Below is a game plan for a corn, soybean and wheat crop rotation.
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Timing of herbicide applications: In timing studies, auxin-inhibiting herbicides (e.g. Lontrel 360, 2,4-D and dicamba) demonstrated higher levels of control and greater reductions in root mass when applied to larger rosettes (>nine leaves) and plants approaching the early-bud stage than on smaller (two- to four-leaf) rosettes.
Influence of tillage: Timing of tillage operations at the seven- to nine-leaf rosette stage is optimal. This reduces the reproductive capabilities of the root. To manage perennial sow thistle through tillage only requires tilling several times throughout the season since new vegetative shoots will almost certainly emerge.
Rate of glyphosate: If growing glyphosate-tolerant (Roundup Ready) corn or soybean, glyphosate is the most effective herbicide at controlling top growth of perennial sow thistle with the higher labelled rate giving more consistent control.
Corn: Marksman, Peak and Distinct have been the most effective herbicides at controlling top growth of perennial sow thistle when at the nine-leaf or greater-rosette stage at the time of application.
Soybean: Many post-emergent herbicide options in non-GMO or “conventional” soybean have proven to be inconsistent in their effectiveness at controlling perennial sow thistle top growth. Classic or FirstRate have been the most consistent at suppressing top growth, although the level of control won’t blow you away.
Winter wheat: Several herbicides are good at controlling top growth of perennial sow thistle, but vegetative shoots can continue to emerge from underground rhizomes well into June, i.e. after most herbicides can be applied safely. Although delaying herbicide applications until as late as possible (flag leaf) may seem like a good strategy to control as many emerged shoots as possible, the approach is flawed due to greater risk of crop injury. For this reason, some farmers prefer either to control perennial sow thistle with glyphosate prior to planting winter wheat or utilize pre-harvest applications.
After wheat harvest: Fall herbicide applications are beneficial in reducing root mass and delaying emergence the following spring but one should still expect to see perennial sow thistle the following season. Glyphosate alone has proven to be as good as any herbicide applied in the fall but some prefer to tank mix with 2,4-D.
Have a question you want answered? Hashtag #PestPatrol on Twitter to @cowbrough or email Mike at [email protected].
