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Is worrying about waterhemp keeping you up at night? This aggressive herbicide-resistant weed species is certainly a valid cause for concern. But here’s the good news: You can put your fears to rest with integrated weed control tactics that can reduce the impact of waterhemp in corn and soybeans.
In this article, we provide an overview of four integrated management strategies and how they work to control waterhemp populations. These strategies include using cultural practices, crop rotation, cover cropping, and herbicides to combat this troublesome weed threat.
📸:John D. Byrd, Mississippi State University, Bugwood.org / Featuring an image of waterhemp foliage.
Waterhemp is one of the most widespread and damaging weeds affecting Midwest growers. A member of the pigweed family, waterhemp can quickly overtake cropland, growing at a nightmarish rate of approximately 1"–1.25" per day during the growing season.
Waterhemp's vigorous, rapid growth rate and prolific seed production have helped the weed develop its infamous resistance to a number of herbicide families, including HPPD-inhibiting herbicides.
Three main growth characteristics have contributed to its rapid herbicide resistance development:
These factors show why it is imperative to interrupt waterhemp’s life cycle with proactive management tactics and reduce the proliferation of seeds with resistant genes.
Soybeans are especially susceptible to yield losses from waterhemp pressure, with waterhemp competition resulting in yield reductions of up to 44%. The effects on corn yields are not far behind, with corn yield losses of up to 36% occurring in fields where waterhemp has invaded.
Crops planted in late May or early June are more likely to fall victim to waterhemp since this timeframe coincides with the height of waterhemp emergence in the Midwest. Planting your crops earlier can lessen the impact of waterhemp since it is a late-emerging species. With good stand establishment, your crop will be more competitive against this weed.
Identifying waterhemp is the first step to controlling it. Continually scouting for waterhemp throughout the growing season is critical, considering it has an emergence period that lasts nearly all season for Midwest growers.
Some key features of waterhemp include:
📸:Howard F. Schwartz, Colorado State University, Bugwood.org and Bruce Ackley, The Ohio State University, Bugwood.org/ Featuring a side-by-side comparison of waterhemp (left) and Palmer amaranth (right).
Learning how to distinguish waterhemp from other species of pigweeds and similar broadleaf weeds can help you create a targeted weed management plan for the best control. Waterhemp is frequently mistaken for other pigweed species such as Palmer amaranth, redroot, prostrate, and tumble pigweeds, so understanding specific features can help you tell these weeds apart through various growth stages.
For example, both waterhemp and Palmer amaranth are dioecious, meaning male and female flowers appear on separate plants. However, this flowering habit differentiates them from redroot, prostrate, and tumble pigweeds, which are monoecious, with each plant bearing both male and female flowers.
Additionally, both waterhemp and Palmer amaranth have those smooth, hairless stems, unlike other pigweed species whose stems typically feature fine white hairs, making them look like unkempt cousins who need a shave.
So, with all these similarities, how can you tell waterhemp and Palmer amaranth apart? Though their flower stalks are similar, waterhemp flower heads are not as long or thick as those of Palmer amaranth. Waterhemp’s flower clusters are also more widely spaced than those of Palmer amaranth. Additionally, those smooth, hairless stems have different petiole lengths; waterhemp has shorter leaf petioles, and Palmer amaranth has petioles (especially on more mature leaves) that are as long or longer than the leaf blades.
Understanding these key features can help you accurately differentiate between waterhemp and other pigweeds.
A combination of cultural practices, crop rotation, cover cropping, and the effective use of herbicides can help provide the best control of waterhemp populations. Below, you will learn how these four practices work together to help you keep waterhemp in check.
Tillage can be an effective step in your weed control system. While waterhemp can remain viable in the soil for up to five years, depending on the seed depth, emergence only occurs at soil depths of 0.25"–0.5". So, for example, using moldboard plowing can greatly reduce waterhemp populations the following season by effectively burying weed seeds deeper than they can survive.¹ Moldboard plowing is typically more effective than chisel plowing for reducing waterhemp emergence because plowing creates more disturbance and inversion of the soil (subsequently burying and disrupting the seed bank) compared to chiseling, which is less disruptive, so choosing the right tillage option for your field is key.²
Additionally, don’t overlook good old-fashioned manual removal to help reduce waterhemp populations. The key here is to find and destroy plants when seed is not yet developed (or at least when the seed head is still green and soft versus dark and hard).
📸:Bruce Ackley, The Ohio State University, Bugwood.org/Featuring a side-by-side comparison of male (left) and female (right) waterhemp seedheads.
If you discover mature seed heads, bag them on-site, remove them from the area, and dispose. While this method feels painstaking, leaving waterhemp escapes unattended can be even more of a pain in the long run, so catching small populations before they mature and spread can definitely be worth the effort. A zero-tolerance policy for waterhemp seed production is important to keep it from getting out of hand in your field.
Crop rotation is a key tool in your weed management tool belt. To combat waterhemp, a corn-soybean rotation is ideal to help you maximize the timing and effectiveness of herbicide control. For example, although late-emerging waterhemp populations may not impact your corn yield enough to justify herbicide treatment, targeting these escapes is critical to reducing the soil seed bank for the following soybean crop.³
Planting cover crops is another method of waterhemp control that can help you make the most of herbicide applications. Additionally, when combined with narrow-row spacing, cover crops have been found to significantly suppress waterhemp pressure.
A 2019–2020 field study conducted by Iowa State University revealed that waterhemp emergence was reduced by 87% when corn crops were managed with an aggressive weed control program, and followed by a rye cover crop and a narrow-row soybean crop. These results were compared with systems that had marginal weed control in corn, did not use cover cropping, and planted soybean in 30" rows.⁴
To outmaneuver waterhemp’s herbicide resistance, select herbicides with multiple modes of action and layering residuals, along with integrated weed control methods.
Since waterhemp has a long emergence period, the best continuous control of this weed can be achieved by layering residuals at planting and again tank-mixing with post-herbicides. Overlapping residuals even prior to planting can suppress waterhemp populations by halting emergence from the get-go. Before mixing components, always conduct a compatibility jar test and consult respective product labels for specific mixing instructions. The most restrictive label applies.
There are a number of options for waterhemp control in corn and soybeans on the market.
In corn, Sharpen® herbicide or Verdict® herbicide can be used as a burndown, as well as as a preplant, or preemergence application. Outlook® herbicide and Zidua® SC herbicide can be used for pre-plant, preemergence or postemergence herbicide treatment, while Armezon® PRO herbicide, Armezon® herbicide, Clarity® herbicide, Liberty® herbicide, or Status® herbicide offer good postemergence weed control.
In soybeans, waterhemp can be managed with burndown applications of Sharpen® herbicide, Verdict® herbicide, or Zidua® PRO herbicide, while Engenia® herbicide, Liberty® herbicide, Outlook® herbicide, or Zidua® SC herbicide can be used as a preemergence or postemergence application.
Learn more about controlling herbicide-resistant weeds and how to make the most of non-chemical and chemical weed control methods:
Experts are available to help you make your decisions. Reach out to your seed retailer, a nearby extension office agent, or a seed company professional like your regional BASF representative.
Before mixing components, always conduct a compatibility jar test. Always consult respective product labels for specific mixing instructions. The most restrictive label applies.
Always read and follow label directions. Sharpen, Verdict, Outlook, Zidua, Armezon, Clarity, Liberty, Status, Engenia and Grow Smart are registered trademarks of BASF. © 2024 BASF Corporation. All Rights Reserved.
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