Is it waterhemp or is it Palmer?

Agronomists have discovered several new populations of Palmer amaranth across Iowa in the past couple years. These finds have again reminded us the importance of paying attention to those weeds and making sure you know for a fact that the waterhemp is just waterhemp. 

While it is difficult to differentiate between waterhemp and Palmer amaranth during vegetative stages, some characteristics are obvious enough to allow savvy scouts to pinpoint suspect populations early. Early identification allows for eradication efforts prior to seed production, therefore preventing the new problem from increasing within the field. 

Here are a few characteristics to look for when identifying Palmer amaranth in fields:

1. Palmer amaranth is more likely to have ovate or egg-shaped leaves, rather than the long, skinny lanceolate leaves characteristic of waterhemp (Figure 1).
2. Palmer amaranth has longer petioles, the small stalk that attaches leaf blades to the main plant stem; mature leaves will consistently have petioles as long or longer than leaf blade, while waterhemp’s petioles are shorter than blades (Figure 2). This trait is consistent even on quite small plants (Figure 3).
3. Palmer amaranth often has more leaves at the top of the plant, giving it a symmetric or ‘poinsettia-like’ appearance (Figure 4).
4. Palmer amaranth is more likely to have watermarks on leaves (Figure 5).

While scouting this summer, please keep an eye out for populations that could be Palmer amaranth instead of waterhemp. Once flowering, the two species become easier to differentiate and this Encyclopedia article is a good reference for those differences. Contact your local extension field agronomist or ISU extension weed scientist, Wesley Everman, if you find a population or need help with confirmation.