Scouting fields for Palmer amaranth

The buzz around Palmer amaranth may have quieted it down compared to several years ago, but it is still important to keep your eyes out for this weed. Two new Palmer amaranth sites were confirmed in Johnson and Washington counties just this summer. If a new infestation is found, it is still possible to prevent the establishment and spread of Palmer amaranth in a field. Early identification, coupled with a focus on preventing seed drop and spread is critical. Just like with waterhemp, a single Palmer amaranth plant can be a prolific seed producer. 

Identification: 

In Iowa, there are several pigweed species found across the state, including redroot pigweed, smooth pigweed, Powell amaranth, spiny amaranth, waterhemp, and Palmer amaranth. 

When trying to differentiate between these different species, the first trait to look for is hairs or pubescence on the stems. Redroot pigweed, smooth pigweed, and Powell amaranth all have hairy stems. 

Palmer amaranth, waterhemp, and spiny amaranth have hairless stems. Spiny amaranth can easily be differentiated from waterhemp and Palmer amaranth by the sharp ½ inch spines that are found where the leaves attach to the stem. 

Differentiating between waterhemp and Palmer amaranth can be more challenging prior to flower. In the vegetative stage, traits we can look at to help differentiate between these species include: 

• Leaf shape: Palmer amaranth leaves tend to be wider and ovate, or diamond shaped leaves compared to waterhemp leaves, which tend to be narrower. 
• Leaf watermark: Some Palmer amaranth leaves may have a silverish watermark on them, but not all will. This trait can also be found on some waterhemp leaves. 
• Canopy shape: In general, palmer amaranth tends to have a denser canopy of leaves, which looks similar to a poinsettia plant. Waterhemp has a more open canopy and a less dense leaf arrangement.
• Leaf petiole: The petiole (part that connects the leaf blade to the plant stem) on Palmer amaranth will be longer than the leaf blade. This is one of the more reliable vegetative traits used to distinguish between Palmer amaranth and waterhemp.

When Palmer amaranth starts to flower (July and into August), it becomes much easier to distinguish from waterhemp. Both species have separate male and female plants. If you rub the inflorescence (seedhead) of a plant and find small black seeds, that is a female plant. The male plants only produce pollen. 

To differentiate between Palmer amaranth and waterhemp when they are flowering, Palmer amaranth tends to have thicker inflorescences and longer terminal branches. Additionally, female Palmer amaranth plants have long (1/4 inches in length), sharp, green bracts (modified leaves at the base of the flowers). Bracts on female waterhemp plants are only 1/8 inch long and are not sharp. 

Scouting 

When scouting for Palmer amaranth, target high risk areas, which include farms that use livestock feed or bedding from southern states, fields receiving manure from those farms, and farms where out-of-state equipment has been used. If Palmer amaranth is identified prior to crop harvest, prioritize removing seed-producing plants from fields if possible and minimize spread via the combine by harvesting infested fields last.

Reporting 

If you suspect that you find a plant that may be Palmer amaranth, reach out to your local extension field agronomist.