Best Practices for Controlling Perennial Weeds in Soybean

Controlling perennial weeds, which live for two or more years, such as quackgrass, dandelion, Canada thistle, field bindweed, yellow nutsedge, milkweed, and perennial sowthistle, requires a different strategy than when dealing with annual weeds.

Perennial weeds can reproduce via seed the same as annual plants, but they also spread through vegetative reproduction. Vegetative reproduction can be in the form of above-ground stems or stolons, but mostly through “growing points” that are underground in the form of rhizomes, tubers, and root buds that must be eliminated for complete control. This combination of seed and vegetative reproductive capacity makes perennial weeds extremely hard to control.

📸: BASF Contributing Writer/Featuring common milkweed seed. Well adapted to disperse by wind, seed from common milkweed, Canada thistle, and dandelion can travel long distances and remain viable for many years. Seed can remain dormant during poor growing conditions but are ready to germinate once conditions are favorable.

Understanding the flow of nutrients in perennials 

The phloem vascular tissue in a plant moves stored energy or food reserves from the below-ground roots, rhizomes, and tubers of perennial plants upward to support above-ground growth in the spring and summer. In the fall, sugars produced by the leaves are moved downward via the phloem and into the below-ground storage for the next year's growth.

• Herbicide timing and selection. Understanding that the flow of nutrients is downward in the fall, herbicides are most effective below ground in perennial weeds when they are able to take advantage of the phloem flow direction. Systemic herbicides can be very effective on perennial plants in the fall but tend to be unsuccessful when applied in the spring and early summer. Among the list of common herbicides to control perennial weeds are glyphosate herbicide, dicamba herbicide, 2,4-D herbicide, and clopyralid herbicide. Most often, multiple applications are required for complete control.

• Using tillage to control perennials: Tillage can both help or hurt perennial weed control, depending on the situation.

1. Tillage can hurt control by cutting perennial roots, furthering the spread of the weed, and also reducing the movement of systemic herbicides. It is generally recommended to avoid tillage both seven days prior to and seven days after the application of a systemic herbicide.
2. Tillage can help control new seedlings that still have small root systems. Some shallow-rooted perennials, such as curly dock, are controlled quite well with tillage, as curly dock has limited below-ground reproduction. Vigorous and deep tillage, such as moldboard plowing, can help to deplete the underground food reserves.

• Prevention: Perennials take advantage of poor crop canopy and bare spots in a field, so maintaining a competitive crop that can close the row early will help to compete with the establishment of perennial weeds. Keep perennials on adjoining roadsides and fence lines under control, as that is often the entry point to your field. Also, spot-treat any isolated patches of perennial weeds in your field to prevent spreading.

• Biological control: There has been some success with biological control using insects or plant diseases, but they need to be host-specific and also released into a field. Research using insects, including gall fly and stem weevil, to control Canada thistle has not been met with much success as they only affect seed production. More recently, a rust fungus has shown some promise for Canada thistle. For controlling leafy spurge, flea beetles whose larvae feed on the root of leafy spurge, causing root rot, have been used. Gall midge, which reduces seed production, has also been used on leafy spurge. Purple loosestrife, a perennial that grows in very wet areas as well as standing water, has been managed with a number of leaf-feeding beetles.

📸: @dadalia via Canva/Featuring Canada thistle (Cirsium arvense).

Specific perennial weed best control recommendations

• Canada thistle (Cirsium arvense): Listed as a noxious or invasive weed across the midwestern states because of its potential to spread and reduce yields, Canada thistle roots can grow as deep as fifteen feet. Glyphosate herbicide alone or tank mixes with dicamba herbicide , 2,4-D herbicide, or clopyralid herbicide are all good on Canada thistle, but expect that multiple applications will be required to exhaust root viability.

• Field bindweed (Convolvulus arvensis): Field bindweed, also known as Creeping Jenny, has the ability to rapidly grow and extend its vine-like stems, out-competing any crop. Glyphosate herbicide tank mixes with dicamba herbicide are the most effective. 2,4-D herbicide is not effective on field bindweed.

• Common milkweed (Asclepias syriaca): Glyphosate herbicide is the most effective herbicide for common milkweed control when applied pre- or post-harvest. Apply herbicides when in the late bud-to-flower stage. Glyphosate herbicide tank mixes with dicamba herbicide or 2,4-D herbicide are also very effective and generally better than glyphosate herbicide alone.

• Dandelion (Taraxacum officinale): Dandelion tap roots can grow as deep as eighteen inches underground, easily protected from tillage. Dandelion is rarely a problem in continuously tilled fields but can quickly form dense infestations in reduced and no-till fields. Glyphosate herbicide plus 2,4-D herbicide or dicamba herbicide is effective and generally the go-to mix. Chlorimuron herbicide is also effective in soybeans, while mesotrione herbicide is effective in corn.

• Yellow nutsedge (Cyperus esculentus): Yellow nutsedge can reproduce from seed, underground tubers, and rhizomes. A sedge is neither a broadleaf nor a grass, so the herbicide options are different than for most perennials. Halsufuron herbicide in corn and sufentrazone herbicide or bentazon herbicide in soybeans have good activity. The go-to perennial weed herbicides, including glyphosate herbicide, dicamba herbicide, and 2,4-D herbicide, do not offer good yellow nutsedge control.

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📸: @VitalisG via Canva/Featuring a close up of common milkweed seed (Asclepias syriaca).

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