Late-Season Weed Management

Updated August 01, 2023

Eric Jones

Assistant Professor and SDSU Extension Weed Management Specialist

Additional Authors: Philip Rozeboom

At this time during the growing season, many herbicides cannot be applied to crops in the reproductive growth stage. However, weed management can still be implemented.

Control weeds adjacent to fields.

Common ragweed and kochia growing along a road ditch.
Figure 1. Common ragweed and kochia growing along a road ditch adjacent to crop fields. While these weeds won’t interfere with crop production, their seeds can emigrate into the field and have to be controlled in subsequent growing seasons.

Fence lines and road ditches adjacent to crop fields are usually inhabited by weeds (Figure 1). While these weeds will not reduce crop yield or harvest efficiency, the seeds produced by these weeds can encroach on crop fields.

Mowing road ditches is a viable option for control if the weeds have not started to produce seeds. If viable seeds are present, weeds should not be mowed, as the mower discharge can move the plant residue, including the seeds, into the crop field.

Spot spraying in road ditches and fence lines is a viable option as well. The most-viable herbicide option is dependent on the weed species present. For example, if kochia or waterhemp inhabit these areas, glyphosate will likely not be a viable option due to their widespread resistance to glyphosate.

Remove small, isolated weedy patches from the field.

Weeds that are mechanically removed should not be returned to the soil surface to eliminate the possibility of re-rooting (Figure 2). The mechanically excised weeds should be removed from the field and destroyed (Figure 3). Even if there is one weed that is present in the field, that one weed has the potential for thousands of offspring, and those offspring could possibly be resistant to previously effective herbicides (Figure 4).

Waterhemp plant re-rooting on soil surface.
Figure 2. A waterhemp plant that has been mechanically excised at the soil surface but has rerooted at the soil surface near the cut and initiated new vegetative growth. This weed may survive to produce viable pollen or seeds.
Pile of burned weed carcasses.
Figure 3. Carcasses of weeds removed from a field and destroyed via burning.
Lone weed in a soybean field
Figure 4. A lone weed in a soybean field will likely not impact crop yield negatively. But do you want to control the potential thousands of offspring produced by this plant and risk the possibility of it being herbicide-resistant? Or would you rather take the time to remove it from the field and avoid future problems? Courtesy: C. Cahoon and A. York, North Carolina State University

Start planning next year’s weed management.

The 2023 growing season presented a lot of challenges for South Dakota farmers. Crop planting was delayed, and hot, dry conditions started early in the season and have been persistent. Weed control ranged from poor to good with both pre and postemergence herbicide applications due to the hot, dry conditions. While every growing season is different and presents a unique set of challenges, prepare for the worst conditions, but hope for the best conditions.

  • Determine herbicide choices and application timings.
    Crop rotation will dictate what herbicides can be applied next season. If possible, try to diversify the herbicide groups used each season. Pre and postemergence applications of a single herbicide active ingredient should be avoided. Herbicides from different herbicide groups can be applied as tank mixtures to increase the spectrum of weed control and delay the selection of herbicide-resistant weeds. Additionally, consider herbicide applications (fall or early spring) prior to crop planting. The preplant herbicide applications are made before many weeds begin to germinate and, therefore, have more time to be transported via moisture (such as snow melt or rain) to the soil layer where weed seeds will be germinating. If the 2024 growing season also presents hot and dry conditions where the preemergence herbicide application does not provide adequate control, plan on making an early postemergence application to control the early emerging weeds with a follow-up application later in the season.
  • Implement non-herbicide management tactics.
    While non-chemical management tactics are not as effective as herbicides, the effectiveness of control increases when several tactics are implemented together. Non-herbicide management tactics include planting date, prescription tillage (where applicable), narrow row spacing, fertilization (increase crop vigor), and crop rotation. Many of these non-herbicide management tactics can be implemented readily with current crop production practices.

Related Topics

Corn Weeds, Soybean Weeds, Wheat Weeds

Related Content

Green waterhemp plant next to a black, ink pen. Green soybeans in the background with brown soil in the foreground.

Waterhemp is flowering. No seeds present yet.

Waterhemp is one of the most-difficult weeds to manage in South Dakota row crops. While herbicide resistance is a great factor in the difficulty of managing this weed, the biology of the species plays an important role as well.

Left: Kochia growing abundantly in an untreated soybean field. Right: Kochia weeds burned down in a soybean field managed with an herbicide mixture.

Glufosinate + lactofen is likely an effective mixture to manage kochia postemergence in soybean

Kochia is a difficult to manage weed that infests crops across South Dakota. Mixing effective herbicides is a tactic that can reduce selection pressure on resistant weeds and increase the effectiveness of weed management.

Green plant with bright yellow flowers throughout.

Leafy Spurge: Do I spray now or wait until fall?

In June, leafy spurge is one of our most problematic rangeland, pasture, and roadside weeds. It is also one of South Dakota’s noxious weeds that landowners are required by law to control. So should you spray it now or wait until fall?