Weed Seed Dispersal

It’s well worth the trouble to remove late-season weed escapes, particularly waterhemp, marestail and Palmer amaranth, from your fields. Weed control, management, ecology, and minutia The Muddy Boot Weed Seed Dispersal Method Tall waterhemp is one of the most problematic weed species throughout the Midwest and has now arrived and spread to eight counties in Upstate New York.

Late-Season Weed Control

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.”

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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: http://bit.ly/….

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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.”

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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.

Weed Seed Dispersal

I was out in the field today and drove by this fallow field and did two things. First, since I’m a weed scientist, I stopped to take a photo of this weedy streak. Second, because I know the site manager and know he hates weeds, I called to give him a hard time about “missing one”.

Any guesses on what happened here? It might help if I told you I’m facing south and this area often has a prevailing wind out of the northwest during winter and early spring. Another clue would be the construction site to my back with minimally managed weeds. One more clue might be the fence behind me still loaded up with Russian thistle carcasses and debris.

Here’s a link to the UC-IPM Pest Note (Pub 7486) on Russian thistle. That document indicates ” Russian thistle normally matures in late summer. The seed is spread when mature plants detach at the base and are blown along by the wind. A large Russian thistle plant may produce more than 200,000 seeds. In spring, months after their dissemination, it is possible to trace the paths of tumbleweeds across plowed fields by the green trails of germinating Russian thistle seedlings .”

I think we have a winner! Looks like a handful of Russian thistles, one of our common “tumbleweed” species in California went rolling across this landscape earlier in the spring scattering seeds as it made its merry way to the next fenceline.

Good photo. Probably one of the “tumbleweeds” meandering across the field and spreading its seed on the way. FYI, Harry Agamalian has a similar great photo meandering across a seedling vegetable field in the Salinas Valley.

The Muddy Boot Weed Seed Dispersal Method

Tall waterhemp is one of the most problematic weed species throughout the Midwest and has now arrived and spread to eight counties in Upstate New York. Waterhemp can spread from field-to-field and farm-to-farm on equipment, clothing, application equipment, or via water from over flooded ditches and rivers. Following a recent field day event we wanted to demonstrate the amount of weed seed that could travel back with you.

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Boots that were considered “clean” were not as clean as we had thought (Figure 1). A knife was used to clean the boots and break up any hard clots that were present. Once the boots were clean, tweezers were used to separate the weed seeds from the dirt (Figure 2). The pigweed/waterhemp seed was then separated from other weed seeds that were present, and pigweed seeds were counted (Figure 3). The clods of dirt were also checked, and one pigweed seed was found stuck to a clay particle (Figure 4).

Figure 1: Muddy boots – Photo: Josh Putman Figure 2: Tweezers used to separate weed seed from dirt – Photo: Josh Putman Figure 3: Seeds were separated and counted; 17 total pigweed seeds – Photo: Josh Putman Figure 4: One pigweed seed hidden in a clay particle – Photo: Josh Putman

An estimate of a 3 year establishment of waterhemp assuming 50% of the seeds were waterhemp and 100% were waterhemp was then calculated, respectively. The calculations are seen below:

16 pigweed seeds + 1 pigweed seed hiding in soil = 17 pigweed seeds from 2 boots.

Assuming only half of those are waterhemp and it can produce 250,000 seeds per female plant: 17/2 = 8.5 X 250,000 = 2.125 million seeds the following year in a field.

Assuming every seed on the bottom of the boots are waterhemp: 17 X 250,000 = 4.250 million seeds the following year.

Assuming 75% survival rate and reproduction in year 2: 4.250 million X 75% = 3.1875 million plants X 250,000 seeds per plant = |

**796,875,000,000 seeds going into the soil in year 3 (potentially)

In conclusion, correct and early identification is very important; learn the correct characteristics of the plants (Figure 5) and seeds. Proper cleaning and sanitation of equipment, clothing, and vehicles can help prevent spreading. Intense management and continuous scouting are vital to eradication of this weed species. Mechanical control such as plowing can bury the seed deep which might decrease seed bank numbers. And, if in doubt, contact your local CCE specialist for help with identification or other management practices.