Late-Season Weed Control

Preventing Weed Seed Dispersal is Key

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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A waterhemp escape looms above a soybean field. Late-season weed control is essential to keeping your weed seedbank under control. (DTN photo by Pamela Smith)

ROCKVILLE, Md. (DTN) -- Those waterhemp escapes towering over your soybeans are more than just a mar on the landscape -- they are the source of weeds for years to come.

"Pigweed species can produce anywhere from a couple hundred thousand to a million seeds per plant," said Purdue University weed scientist Bill Johnson. "So just allowing a few weeds in an 80-acre field to go to seed can result in an almost catastrophic situation the next year."

Weed control failures become quite visible in farm country this time of year, particularly in soybean fields where the plants loom above the canopy.

In Indiana, waterhemp, giant ragweed, Palmer amaranth, marestail and even lambsquarters and velvetleaf have made their presence known in crop fields, ditches, field edges and fencerows, Johnson said.

Weed scientists are urging growers not to give up on these mature weed escapes.

"The potential to spread this problem at harvest via the combine is great, so anything that can be done to control the pigweeds prior to crop harvest is imperative," Pennsylvania State University weed scientist Bill Curran warned growers in a university newsletter.


Right now, Indiana growers are facing the results of poor waterhemp control in 2015, Johnson said. During the soggy months of May and June last year, many farmers had to abandon crop fields that were too wet to plant, spray or till.

Waterhemp moved in and thrived and left an enormous seedbank that will haunt farmers for many years to come, Johnson said.

"We're talking about years up to decades of survival, depending on the weed species," he said. "Waterhemp tends to survive longer than Palmer amaranth, but a lot more of the Palmer seed will germinate right away next year."

Waterhemp seeds will still germinate, but a larger percentage will stay dormant, waiting for another year to sprout and rob yields from you, he added.

For more information on different weed species and their seed survival, consult weed guides from your local land-grant universities, such as this one from Michigan State University:….


For the most part, chemical options are limited and unhelpful at this time of year, Johnson said. Most postemergence herbicides are only labeled for use in soybeans up to the R2 growth stage. After weeds breach 6 inches in size, herbicide control becomes highly variable and unreliable, as well.

To add to the problem, many weed species are resistant to a number of herbicides, and the list grows every year.

In Indiana, glyphosate and ALS-resistance is extremely common in waterhemp, ragweed and Palmer amaranth populations. Many waterhemp populations are also resistant to PPO herbicides. Down south, Palmer amaranth has developed resistance to this class of chemistries as well, Johnson said.

"There are some postemergence grass herbicides that can go on pretty late and will reduce seed set, but for broadleaves, once you can no longer spray the beans, the only resort is to remove them by hand," he said.

How you approach hand removal also depends on the weed species.

Waterhemp and Palmer amaranth plants are already producing seeds at this time of year. "They set seed over a long period of time," Johnson said. "You could go out now and grab some waterhemp and Palmer amaranth plants over the next six to eight weeks, and be able to rub viable seeds out of the seed heads."

Marestail produces seed a little later in the season, typically in mid-to-late August, he added. Giant ragweed seed likewise isn't viable until later in the season, when it turns brown.

As a result, you could safely lay most marestail and ragweed plants down in the field after pulling them and move on. But for Palmer amaranth and waterhemp, most weed scientists recommend a "bag and burn" approach, where you ensure that the weeds and the seeds they carry are physically removed and destroyed.

With funding from the Pennsylvania Soybean Board, Penn State weed scientists are handing out 40-gallon paper bags to farmers in an effort to stem the Palmer amaranth infestations racing through the state's fields. The bags have instructions printed on them directing farmers to bag, burn or bury mature pigweed plants.

If an infestation is too severe for hand-weeding, Curran recommends more extreme steps.

"With small, severe infestations, you may consider destroying the crop and the weeds by mowing and/or herbicide application," he wrote. "Dicamba plus or minus 2,4-D are probably the preferred products. On dairy farms, perhaps the soybeans (or corn) could be harvested for silage, which may occur prior to Palmer amaranth seed production. Harvesting all plant material and ensiling should also kill some of the weed seeds that could be present as we move into the fall."

Don't forget to grab a couple samples from those plants before you destroy them, Johnson added. Some land-grant universities offer plant diagnostic services that include molecular tests for herbicide-resistance.

"For the common types of resistance, some of those molecular assays can be turned around pretty quickly," he said.

The University of Illinois' Plant Clinic has a new service that allows for the quick testing of waterhemp samples for both glyphosate and PPO-resistance. Last year, the clinic screened around 1,350 plants from 338 fields in five states. You can read more about the service in this University of Illinois Pest Bulletin posting:….

You can find Curran's article on controlling late-season Palmer weeds here:… and another Penn State article on controlling waterhemp escapes here:….

You can find an Ohio State University article on late-season waterhemp control here:….

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Emily Unglesbee