Waterhemp continues to be one of the most common yield-robbing weeds found in corn and soybean fields across the country. Unfortunately, it can look similar to other weeds migrating into Minnesota, such as Palmer amaranth. This, along with the spread of resistant waterhemp, means growers may have a harder time identifying and controlling the weeds.
Tim Dahl, a Syngenta agronomy service representative based in Peterson, Minn., says it can be difficult to tell the two weeds apart – but there are differences to look for.
“Generally, waterhemp is shorter and thinner than Palmer amaranth,” Dahl said. “It will also have thinner leaves that don’t end in a sharp point or spine.”
One of the easiest ways to tell the two apart is to look at the length of the petiole. With Palmer amaranth, the petiole can be longer than the actual leaf. With waterhemp and pigweed species, the petiole is usually around half the length of the actual leaf.
As herbicide-resistant waterhemp reaches its flowering stage, the similarities to Palmer amaranth become more apparent.
“Their flowers can look very similar, but female Palmer heads will be larger and more robust,” Dahl said. “At the flowering stage, the best way to tell them apart is to look at the base of the flower where Palmer will have spikes.”
Dahl said the best approach for managing waterhemp is to use a herbicide program with multiple effective sites of action (SOAs). This will help control the current population and thereby prevent resistance from developing.
“Syngenta has a model, which shows that using the same single SOA year after year will allow resistance to develop in only a few years’ time,” Dahl explained. “However, using as many as four SOAs can delay resistance development by as much as two decades or longer.”
“In corn, it’s imperative to use a two-pass application system of Acuron® or Acuron Flexi herbicides, or follow them with Halex® GT herbicide plus atrazine or dicamba as a post-emergence herbicide,” he said. “This kind of program could put as many as five sites of action in a field. That’s a lot for a weed to try and overcome.”
Dahl breaks it down similarly for soybeans.
“A pre-emergence application of Boundary® 6.5 EC or BroadAxe® XC herbicides is a great foundation,” he explained. “Each of these offer two effective SOAs, which can increase to as many as four SOAs when used in a program with Flexstar® GT 3.5 herbicide.”
Dahl said due to the nature of waterhemp, extra steps may be needed to ensure control.
“Waterhemp can germinate over a very long period of time,” he explained. “In many fields we add Dual Magnum® with our post-emergence applications to extend the length of residual control.”
Strategies that overlap multiple effective SOAs reduce the chance of building up the weed seed bank, meaning waterhemp is controlled in both the short and long term.
It is also important to consider when herbicides are applied. How close the application is to planting, the length of time between pre- and post-emergence applications and weed height all matter.
“Weed height is very important,” Dahl said. “Post-emergence applications should be sprayed according to the height recommendation on the herbicide label.”
For most labels, this is a maximum of 4 inches tall for the tallest weed in the field.
Dahl also said that just using herbicides isn’t enough, and a true weed management strategy requires other agronomic practices.
“Cover crops, tillage, narrow rows, and increased plant populations as appropriate are all integral parts of a comprehensive weed management strategy,” he explained. “This practices not only help with weed control, but they also help protect the herbicides from resistance so they can be as effective as possible for as long as possible.”