What About Waterhemp?

Pigweeds may steal more headlines, but its cousin waterhemp is serious news, too. It can grow an inch per day and threaten yields considerably. One plant can drop as many as 1 million seeds, which stay viable in the soil for up to four years. 

According to Purdue University Extension, serious infestations require three to four years of intensive weed management practices to deplete the seedbank adequately. Waterhemp is a highly adaptable weed that’s “not likely to go away in the near future,” even after taking an integrated approach, Purdue researchers conclude. 

Reid Smeda, University of Missouri weed scientist, agrees: Farmers dealing with waterhemp should adopt a zero-tolerance approach. 

“In today’s environment, there’s no such thing as an economic threshold for control of resistant weeds,” he says. “Zero tolerance for adding weed seed to the soil should be every farmer’s goal.” 

An integrated, zero-tolerance approach includes the following practices: 

  • Start with a pre-emergence residual herbicide followed by postemergence applications as needed. 
  • Treat any new infestations early and control seedlings before they reach 4 inches tall. 
  • Always use full, labeled rates and herbicides with multiple sites of action and different chemistry classes. 
  • Be aware that there is documented waterhemp resistance to multiple chemistry classes, including ALS inhibitors, PPO inhibitors, glyphosate, triazines and HPPD inhibitors. Consult the International Survey of Herbicide Resistant Weeds for a state-by-state look at resistance. 
  • Rotate crops and plant cover crops. 
  • Consider manual removal of escaped weeds. 

Waterhemp is a member of the amaranth family, but seedlings have some distinct characteristics that set it apart from other amaranth species, including pigweed. Look for first true leaves that are longer and more spear-shaped than pigweeds. Seedlings will be hairless, with waxy or glossy leaves. 

When plants are mature, a distinguishing feature is the collection of small, shiny black seeds only found on female plants. Rub mature flowers between two fingers to reveal them. 

Photo Credit: ©2011 AgStock Images / Arlyn Evans

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