Fighting Waterhemp

 

Waterhemp is a tall summer annual and a common Midwest weed, thriving in wet areas but is adaptable to various conditions. It is an invasive weed in both corn and soybean fields. Yield losses may range from 11% to 75%. It has separate male and female plants, producing over 1 million seeds per plant.

Waterhemp seedlings have light green to red-pink stems. Cotyledons (first leaves) are oval to lance-shaped, hairless, red-green above, and dark red below. The first true leaves are oblong, dark green, shiny on top, reddish-pink underneath, and shallowly notched at the tip. Mature plants can be 2.5–12 feet tall, upright and branching. Waterhemp has a deep taproot with secondary fibrous roots, competing with corn and soybean crops for water and nutrients (Cornell University).

The pigweed family includes waterhemp, Palmer amaranth, and redroot pigweed species. Palmer amaranth has longer leaf stalks than the leaf blade, broader leaves, sometimes with white chevron markings while waterhemp leaf stalks are shorter and leaves lack chevrons. Redroot pigweed has hairy stems, whereas waterhemp stems are smooth and glossy.

Waterhemp is a late-emerging species, often appearing after crops are established, and thrives in agricultural fields, stream banks, and waste areas. Its adaptability to a wide range of soil pH (4.5–8) and nutrient-rich soils makes it highly competitive (Cornell University). Waterhemp emerges throughout the season, with later emergence increasing the waterhemp seedbank. Seeds remain viable for several years, with about 10-12% surviving after 3-4 years. Waterhemp grows rapidly, nearly 1 inch per day, and may reach 12 feet tall. Dense waterhemp infestations significantly reduce soybean and corn yields. Shallow tillage and soil disturbance favor shallow seed germination.

Water hemp has a lot of genetic diversity so it is adaptable and its pollen spreads long distance on the wind which favors plants becoming herbicide resistant. Waterhemp has developed resistance to many herbicide groups, including glyphosate (Group 9), ALS (Group 2), PPO (Group 14), and triazines (Group 5). In many cases, waterhemp has multiple herbicide modes of resistance, including three-way resistance, which complicates weed control.

Combining soil-applied and postemergence herbicides, promoting crop canopy, and using cultivation in organic fields are somewhat effective. Farmers can manage resistant waterhemp populations by diversifying weed control over 3-4 years to deplete seedbanks and manage resistant populations. The use of both pre-emergence and post-emergence herbicides are critical in corn and soybean fields.

Effective waterhemp control relies on a combination of postemergence herbicides like glufosinate (Liberty) and 2,4-D choline, along with layered residual herbicides to manage resistant populations. Us Enlist, Xtendflex, or Liberty Link soybean resistant varieties so that glufosinate, dicamba or 2-4D can be used.

For soybeans, glufosinate is highly effective against waterhemp, especially when applied under warm, sunny conditions with good coverage of all growing points. It is a contact herbicide and works best with ammonium-containing adjuvants, applied when temperatures are above 75°F to ensure active weed growth. The herbicide 2,4-D choline is effective on smaller waterhemp plants (3 inches or less) and can be tank-mixed with glufosinate for taller or dense patches, enhancing control. Glyphosate alone is largely ineffective on waterhemp but may provide synergy for grass control when mixed with glufosinate.

Layering residual herbicides helps manage waterhemp’s extended emergence period and resistant populations. Preemergence options include: S-metolachlor (Dual II Magnum, Warrant, Zidua), Dimethenamid-P (Outlook), Acetochlor (Warrant), and Metribuzin, sulfentrazone, flumioxazin. Applying a second residual application about 30 days after planting can extend seedling control and reduce reliance on postemergence herbicides (Michigan State University).

Waterhemp populations are increasingly resistant to glyphosate and ALS-inhibiting herbicides. To maintain effectiveness: 1) Rotate herbicide modes of action, 2) Use tank mixes with multiple effective active ingredients, 3) Apply full labeled rates rather than reduced rates in mixtures, and 4) Integrate nonchemical tactics such as cover crops, inter-row cultivation, and harvest weed seed control.

Combining chemical and cultural strategies improves long-term control. Cover crops like cereal rye can suppress early waterhemp emergence. Forages such as alfalfa reduce annual weed pressure. Targeted tillage or inter-row cultivation can manage emerged weeds. Field edge management and mowing prevent seed return to the soil. The goal is to reduce flowering and seed production if possible (Michigan State University). In summary, use glufosinate and 2,4-D choline for postemergence control, timed according to weed size and weather conditions.

Use layered residual herbicides to cover waterhemp’s long emergence period. Rotate herbicide modes of action and integrate nonchemical practices to prevent resistance and to reduce the seedbank. Monitor fields closely and adjust herbicide programs based on local resistance patterns and weed pressure (Michigan State University).