Many farmers are aware of a weed species recently introduced to Indiana called Palmer amaranth. This weed could potentially be one of the more difficult weeds to control in recent years. Although Palmer amaranth has not been found in northeast Indiana, it is growing in population in northwestern Indiana and is moving this direction. Palmer amaranth has been found in St. Joseph and Marshall counties, which means it is just two counties over from both Lagrange and Noble counties.
The best way to control the spread of Palmer amaranth is scouting and controlling it early, as it can be very difficult to control as it matures and goes to seed. Travis Legleiter, weed science program specialist and Bill Johnson, professor of weed science at Purdue have done much research on how to best scout and identify Palmer amaranth, something farmers need to start doing now to control it as soon as it gets here, which possibly be within a year or two.
At Purdue’s Palmer amaranth research plots located in Cass County there are already Palmer amaranth plants that have emerged and have quickly reached 2-3 true leaves. For those producers living in the northwestern quadrant of the state, they need to be on high alert for the presence of this weed in their fields. Even if it was not present in their fields last year, they need to be aware of this weed and able to distinguish it from the other pigweed species.
Palmer amaranth is potentially the most aggressive agronomic weed Indiana producers have ever dealt with, and must be managed with an aggressive control program. Seed bank populations will increase quickly in fields where Palmer amaranth is not correctly identified or managed leading to several years of expensive control programs to manage this aggressive weed.
The first key, as alluded to earlier, is the correct identification of palmer amaranth and its very close amaranth relatives: common waterhemp, redroot pigweed, and smooth pigweed. A large majority of the populations discovered last fall had been misidentified as waterhemp for a number of years, which allowed the populations to spread quickly to unmanageable levels.
Differentiating a redroot or smooth pigweed from Palmer amaranth is as simple as looking for fine hairs on the stems and leaves. Redroot and smooth pigweed will have obvious fine hairs on the stems and leaf surfaces, whereas Palmer amaranth will be hairless.
Identifying the differences between Palmer amaranth and common waterhemp (both species are hairless) is much more difficult, especially at a young age. At the cotyledon and first true leaf stage it is even difficult for trained weed scientist to differentiate Palmer amaranth from common waterhemp. A few differences that you can note as these plants begin to put on the second and third true leaves are as follows.
• Singular hair in the leaf tip notch: a characteristic of all pigweed (amaranth) species is a notch in the leaf tip. In the plants examined from and around the state of Indiana it has been consistent that the first, second, and/or third true leaf of Palmer amaranth can have a small singular hair that protrudes from the leaf tip notch. This singular hair often does not occur on all leaves, but is likely to occur on at least one leaf in the early growth stages. This singular hair in the leaf tip notches of common waterhemp seedling as not been observed in the state of Indiana.
• Petiole length of the first true leaves: As noted in Purdue’s Palmer amaranth biology, identification and management publication (WS-51), one of the main characteristic differences in waterhemp and Palmer amaranth is the length of the petioles. In Palmer amaranth plants the petioles can be as long or longer that the leaf blade itself. The Palmer publication includes pictures of mature plants, but this characteristic can be observed in two to four true leaf plants as well. As Palmer plants begin to add the second and third nodes the first true leaf will begin to rapidly elongate its petiole to capture sunlight outside of the shadow of the newly emerging leaves above it.
The petioles of common waterhemp will consistently stay short, and the leaf blades themselves will be elongated to capture more sunlight. The elongating petioles of Palmer amaranth seedlings will also begin give the seedling the characteristic rosette pattern as you look down at the growing point.
These two characteristics are the most evident differences between Palmer amaranth and common waterhemp at the seedling growth stages. As the plants become larger, all of the characteristics noted in the Palmer amaranth publication (WS-51) will become more noticeable and evident.
Although proper identification of seedling plants is necessary in order to make herbicide applications at the 3-6” weed heights that has are recommended for post emergence Palmer amaranth control.