Keeping Watch On Resistant Weeds

Recently, farmers have seen broadleaf weeds such as Palmer amaranth and waterhemp develop resistance to previously effective herbicides. These superweeds pose a threat to production and yields. To help you stay on top of managing these yield-robbing threats, we’ll identify some of the worst offenders, the damage they can cause and how you can control them.

Palmer amaranth

After emerging as a new weed threat during the late 1980s, Palmer amaranth has developed increased resistance in just 30 years, adapting to five different herbicide sites of action. The first glyphosate-resistant case of Palmer amaranth was confirmed in Georgia in 2005. Changing conditions have led to additional instances being reported in the southwestern United States as well as the Midwest. 

Palmer amaranth is classified as a dioecious plant, which means male and female flowers grow on separate plants. This inherent genetic diversity elevates the spread of herbicide resistance and adaptive traits that boost Palmer amaranth’s survival in farm fields. 

Aggressive and strong, Palmer amaranth devastates crops. Over the course of a growing season, the presence of this weed at a rate of 2.5 plants per foot of row may reduce soybean yield by up to 79%.1 And with just one female Palmer amaranth plant being able to produce 600,000 seeds, it’s easy to understand why this weed is so problematic.


Glyphosate-resistant waterhemp was first identified in Missouri more than 16 years ago. Since its discovery, it has spread to more states and has exhibited a growing resistance to additional classes of chemicals.

Waterhemp can quickly infest an area with upwards of 20 plants per square foot. It’s known for rapid growth — typically 1 to 1.25 inches2 per day during the growing season — and this growth has been shown to reduce soybean yield by as much as 44%.3

While waterhemp can emerge at any time during the growing season, a higher number of plants emerge later in the season than is typical for most summer annual weeds. The delayed nature of its emergence helps it to avoid many pre-emergent weed applications and may have contributed to its resistance of postemergent herbicide applications, like glyphosate.

A prolific seed producer, waterhemp can produce as many as 250,000 seeds per plant. These seeds can then be easily transported and remain viable in the soil for up to several years.

Like Palmer amaranth, waterhemp is classified as a dioecious plant, which gives the plant greater genetic diversity and adaptability and increases the likelihood of the spread of herbicide-resistant species.