After 2019 Floods, Weed Complications Abound

Bins bursting with corn spewing into floodwaters just on the other side of the Missouri River in southwest Iowa. Photo: John Wilson/Nebraska CropWatch

Bins bursting with corn spewing into floodwaters just on the other side of the Missouri River in southwest Iowa. Photo: John Wilson/Nebraska CropWatch

Floods and excessive rainfall in the Midwest and Midsouth in 2019 plagued farmers with a wide range of snags and setbacks, some of which could extend into 2020 and beyond.

The extent and duration of this flooding go beyond anything considered ordinary. For example:

  • On Missouri’s northwest border with Kansas, some crop fields were still underwater in December of 2019. There’s also concern about the possibility of “fallow syndrome” raising its head in 2020. Essentially, prolonged flooding wipes out beneficial soil microbes needed for root development, among other things.
  • In west-central Arkansas, numerous fields sustained substantial damage due to erosion and a thick layer of sand deposited by the floods. In places, the ground may be beyond repair.
  • In Iowa, proper timing of field operations was nearly impossible, and one result is significantly more weed pressure going into 2020.
  • Inside the Mississippi River levee in Mississippi, floodwaters hung around well into July on many bottomland fields.

According to USDA, farmers went with prevented planting on 19.4 million acres in the U.S. in 2019, compared to 2 million in 2018. USDA says it’s the worst planting season on record.

The effects vary by location, of course, but weed control and management decisions present challenges at any of the areas cited above. Here are outlooks and potential options in four states – Missouri, Mississippi, Arkansas and Iowa.


“We’re seeing a wide range of impacts,” Kevin Bradley, a weed scientist at the University of Missouri, said of the high water, which was responsible for about 1 million acres of prevented plantings in his state.

The best-case scenario for the fields involved, according to Bradley, is where waters receded relatively quickly. Growers replanted some of those fields, while others went fallow. “In any case, a lot of those fields got very weedy, particularly the ones that were never planted. Expect an abundance of weed seed there.”

Worse-case scenarios happened in fields that remained underwater for long periods.

“The flooding deposited heavy loads of silt,” Bradley said. “So, all of a sudden, you’re dealing with a different soil texture, and you’ll likely have a different soil pH. That affects the likelihood of herbicides persisting and causing injury.”

Farmers whose fields were underwater for weeks or months might also have to deal with fallow syndrome in 2020, particularly if they’re going into corn, according to Greg Luce, grain crop specialist, the University of Missouri.

The condition occurs when populations of soil microbial fungal organisms called mycorrhizae die off due to the lack of living plant roots in a field. In a healthy symbiotic relationship, roots of plants (including many weeds) provide carbohydrates for mycorrhizae. Mycorrhizae, in turn, help roots absorb nutrients, especially phosphorus. That is one reason why people in the field often label fallow syndrome as phosphorus deficiency (which actually is the case).

To rebuild mycorrhizae populations and begin to restore soil health, University of Missouri Extension recommended planting a cover crop on fields that have been underwater. “It takes plant roots to keep these mycorrhizae alive. There needs to be something growing in that field every year,” Luce said.

He adds that not all plants are equal for mycorrhizae. “Brassicas (mustards), pigweed and lambsquarter, for instance, do not have a good mycorrhizae relationship.”

Luce also advises producers to consider adding phosphorus when going to corn. “For soybean, if the field was flooded and water was on it for a long time, the rhizobia bacteria could be depleted. For soybean, inoculating them after a flood is a recommended practice.”

As for weeds, Bradley urges an aggressive approach.

“In the Midwest, a fall herbicide application is somewhat common to help farmers get ahead in the spring and keep things clean,” he explained. “Unfortunately, we didn’t have many fall herbicide applications in 2019 because of the late harvest.

“So, try to be as timely as possible this year and use residuals. It’s going to be a numbers game with weeds and you’ll be contending with more of them. Our recommendations for weed management aren’t changing. It’s just that there could be a lot more weeds than usual to deal with out there.”


Flooding along the insides of the levees along the Mississippi River and backwater from the Yazoo and Sunflower rivers – which persisted until July – forced significant acreage into prevented plantings, according to Jason Bond, weed scientist with Mississippi State University.

“The backwater was devastating, but if you had a lot of land inside the levee, you had as big a catastrophe as anyone,” he noted. “But other than some sedimentation, I didn’t see much damage to fields. The flood came up slow and went down slow. But we really don’t know what the long-term effects will be”

Bond says when the water finally receded in late summer, many farmers were able to prepare fields in the fall to destroy weeds before they went to seed. However, fields that farmers were unable to work “completely grew up.”

Mississippi reported over 600,000 acres of prevented plantings in 2019.


Fields in the state most affected by the flooding were located along the Arkansas River from Little Rock to the Oklahoma line, according to Jason Kelley, wheat and feed grains Extension agronomist, University of Arkansas.

“I don’t know what producers are going to do with some of these fields,” he said. “There’s sediment. In one area, the levee blew and gouged out a big path through some fields. Whether those fields will ever go back into production, I don’t know. It would take a lot of money and a lot of effort.”

Fields inside the Mississippi River levee on the Arkansas side sustained damage due to localized flooding and persistent rain, Kelley said. “Plenty of corn was planted but didn’t make it. It probably went to another crop.”

Kelley said producers who had flooded fields are “nervous about what weed species are going to show up in their fields. We know from experience that when a field gets flooded, you’ll likely see weeds that you didn’t have the year before.”

USDA reports 1.3 million acres of prevented plantings in Arkansas.


According to Bob Hartzler, Extension weed specialist at Iowa State University, proper weed control in 2019 was nearly impossible because of constant deluges. “If you applied your preemergence herbicide in a timely fashion, there was so much rain that residual activity was cut by 50% or so. It was a tough year all around.”

Escaped weeds resulted in larger deposits of weed seed in soil seed banks going into 2020, Hartzler added. “The bigger the seed bank, the more inputs it takes to get acceptable control.”

For producers considering tillage to bury weed seeds, Hartzler advises caution, plenty of it.

“If you’re in no-till or have practiced good weed control, then a year of tillage would help mitigate the impact of escapes,” he observed. “However, if you continually use tillage, you’re just re-cycling the weed seeds.”

Instead, Hartzler urges producers to focus on their preemergence program.

“Look at the preemergence as the heart and soul of your weed management program,” he recommends. “Use the postemergence products to clean up what survives, rather than vice versa, which is sort of what we got in the habit of doing during the Roundup Ready era. We need to flip the way we go after the weeds.”

Iowa reported over 460,000 acres in prevented plantings in 2019.