Purdue weed scientists recommend these simplified steps when identifying pigweeds:
1. Look for hairs or pubescence on the stem to differentiate Redroot/Smooth pigweed from waterhemp and Palmer amaranth.
2. Observe the petiole lengths. The petiole is the structure that attaches the leaf blade to the stem of the plant. Palmer amaranth will have a petiole that is as long or longer than the leaf blade itself (Pic 1 and 2). Common waterhemp will have petioles that are shorter than the leaf blade.
Some other characteristics that will help differentiate Palmer and waterhemp is the growth pattern and female seed head structures. If you look at the main growing point from above the plant Palmer amaranth, it will appear to have a rosette shape with ovate leaves and long petioles, similar to a poinsettia (Pic 3). Palmer amaranth will have multiple seed heads, but will be distinguished with one main seed head that can reach 2 to 4 feet in length. The seed heads of female Palmer amaranth plants will also be spiny, you will know when you grab one.
The presence of spines (hardened seed bracts) in female Palmer amaranth seed heads can cause people to call it spiny amaranth. Spiny amaranth is predominantly a weed of pastures and livestock holding lots and has bushy growth habit. Spiny amaranth has spines throughout its lifecycle especially at all node axils. Spiny amaranth is also a monoecious plant (male and female flowers on one plant), where as Palmer amaranth is dioecious (separate male and female plants) with only females having spiny flowering structures.
Palmer amaranth control and management
The management and control of Palmer amaranth must be aggressively proactive. Producers should also treat all Palmer amaranth with equal vigilance regardless of glyphosate sensitivity. The key to management of Palmer amaranth is to control it at its most vulnerable stage, germination. The use of pre-emerge residual herbicides is critical in both corn and soybeans. There are a large number of corn pre-emerge herbicides that include atrazine premixes that will substantially reduce the amount of plants that emerge and require post-emerge applications.
There are also a large number of post-emerge corn herbicides that will control Palmer amaranth including HPPD inhibitors and growth regulators. In fields heavily infested with Palmer amaranth, it is recommended to grow corn for multiple years as the number of effective herbicides and MOA rotation is far greater than that of soybeans.