Identifying the Difference Between Pigweed Species and Control

Pre-emergent herbicides and pigweeds:

Group 15 herbicides, sometimes known as the acetamides, were originally brought to market back in the 1970s and 80s to provide pre-emergent residual grass control of grass weeds. Products containing these active ingredients (acetochlor, metolachlor, dimethenamid-P, etc.) are soil-applied and work as weed seedling shoot growth inhibitors. In other words, the weeds must grow through the soil layer containing the herbicide. What became apparent is these herbicides worked well on small-seeded broadleaf weeds.

This group of weeds are generally known as pigweeds, which all produce small black seeds in copious amounts. Several examples of weeds in this group include: tall waterhemp (Figure 1), red root pigweed, Palmer amaranth and smooth pigweed.

When an acetamide is applied, control of pigweed species is generally experienced as long as the plants have not emerged from the soil. However, once these weeds are emerged and growing, control will not be achieved from a stand-alone application of an acetamide product. These species should be sprayed in combination with an effective postemergent herbicide before they reach 4 inches in height. Palmer amaranth and tall waterhemp are especially tough to control after they reach 4 inches in height. This combination would allow for postemergent control of the pigweed as well as residual control from future flushes.

Identifying these species can be tough because as seedlings, they look similar. However, Palmer amaranth and waterhemp have no hairs on the leaves or stem while rough and redroot pigweeds contain hairs. Palmer amaranth has a wider leaf, often compared to a poinsettia leaf, while tall waterhemp has a narrower pointed leaf that gives the plant a less dense foliage pattern compared to Palmer amaranth. The seed head on a female Palmer plant is thicker and denser than that of waterhemp. A Penn State Extension article, “Invasive Pigweeds: Palmer Amaranth and Waterhemp” provides a detailed description of the various species. 

Curran, W.S. and Klodd, A. 2014. Invasive pigweeds. Palmer amaranth and waterhemp. PennState Extension. Pennsylvania State University. https://extension.psu.edu/invasive-pigweeds-palmer-amaranth-and-waterhemp

Web source verified 10/14/19.

Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.

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