A petiole
Submitted photo
Earlier this week, OSU Extension had our agronomy team meeting up at Becks Hybrids in London, Ohio. After our meeting we headed out to the agronomy plots at the Farm Science Review to see the different trials going on that will be available for FSR attendees to come and see at the agronomy tent.
As we walked up to one of the soybean plots, we were “lucky” enough to see both a Waterhemp and Palmer Amaranth in the soybeans within five feet of each other. Of course, we do not want these treacherous weeds around the farm, but it made for a good learning opportunity for the newer Extension Educators.
Even when we show farmers pictures of weeds, and weeds that have been grown in a greenhouse, sometimes it can be more difficult to identify which pig weed we are dealing with when we are in the field. let’s discuss some of the ways to tell the difference between Palmer Amaranth and Waterhemp.
Palmer Amaranth
Stems and leaf surfaces are hairless
Ovate to egg shaped leaves
Can reach heights of 6-8 feet
Occasional white chevron-shaped water mark on leaves
Occasional spine on the leaf tip
Petiole will be as long or longer than the leaf itself (see picture below)
Long unbranched seed heads (female plants have sharp bracts)
Palmer Amaranth is a big problem for a few reasons. Palmer grows aggressively and can grow 2-3 inches per day, can reduce yield up to 91% in corn and 79% in soybeans, can produce up to 1 million seeds per plant, and is prone to herbicide resistance. For these reasons, it’s imperative that farmers can identify this weed and take action against it.
Waterhemp
Stems are hairless
Lanceolate shaped leaves
Glossy sheen on leaves
Seedhead has multiple branches
Short petioles compared to Palmer Amaranth
Palmer Amaranth and Waterhemp management strategy is going to be very similar for both, but it is important to know what is growing on at your farm. It is also important to know the difference between these weeds when scouting for herbicide resistance. Palmer Amaranth and Waterhemp are both prone to herbicide resistance because of genetic diversity and the same herbicides being used repeatedly.
In the state of Illinois, Waterhemp is resistant to group 15 herbicides, these include products like Dual, Harness, Zidua, and Outlook. Luckily, we do not yet have the same resistance built up within Ohio as in Illinois, but resistance is growing. We can still look to group 15 herbicides to be used as a pre-emergence in the control of Waterhemp, but a pre-emergence application is not enough to use solely for control of these amaranth.
Refer to the 2023 Weed Control Guide publication ANR-789/WS-16/IL15/M171 for guidance on controlling Palmer Amaranth and Waterhemp.
Brooks Warner is the Ag & Natural Resources Educator at OSU Extension Clinton County.
