Is that a Palmer amaranth or waterhemp?

Most agronomists are aware of the movement of Palmer amaranth into Iowa.  Although known Iowa infestations are limited to five counties (Harrison, Fremont, Page, Lee and Muscatine) we are confident that it is present in many more counties.  We are pleased by the number of people who have notified us upon encountering suspect Palmer amaranths.  In all cases, the trait that caught their eye was the presence of leaves with long petioles, and on some leaves, the petioles were longer than the leaf blade.  This trait is the most reliable vegetative characteristic to differentiate the two species in our opinion.  However, we were not confident in declaring these plants Palmer amaranth based solely on the petioles because the other characteristics were more ‘waterhemp’ in appearance than we would expect from a Palmer amaranth.  As we’ve stressed before, the floral traits are the only completely reliable trait to distinguish Palmer amaranth from waterhemp.

Long-petioled Amaranthus plant.  Is it a Palmer?

Some people have suggested these plants might be hybrids created by cross-fertilization between waterhemp and Palmer amaranth.  While it is well-documented that Amaranthus species can hybridize, there are biological barriers that minimize the successful fertilization of females of one species with pollen from the other.   Thus, hybrids occur infrequently in wild populations.  We are unaware of any research-based information describing the morphological characteristics of these hybrids, thus it isn’t known whether long petioles are associated with the hybrids.

We encourage everyone to keep an eye out for odd ‘pigweeds’.  Both Palmer amaranth and waterhemp should be flowering soon (if not already) and this eliminates any guesswork in differentiating the two.  Please contact us if you find any suspect Palmer amaranths.