Iowa: Palmer Pigweed in 5 Counties and You Thought Waterhemp Was Bad

    Palmer amaranth was first identified in Iowa in 2013. Currently, we know it is established in 5 Iowa counties (Fig. 1). This past weekend we were made aware of a new infestation of Palmer amaranth in Fremont County, distant from the initial infestation in this county.  In addition, a suspicious looking Amaranthus species was found in Madison County – the owner is allowing a few plants to develop seedheads in order to make a positive identification.


    Figure 1. Known infestations of Palmer amaranth, July 2016. Click image to enlarge.

    A mantra of invasive plant management is ‘Early Detection and Prevention.’ The objective is to identify new infestations as they get started and eradicate the pest before a permanent seed bank is established. In order for this tactic to be effective, everyone involved in Iowa crop production (farmers, industry, landowners, etc.) must learn how to identify Palmer amaranth, and then closely monitor fields and other disturbed habitats for the presence of this new threat.

    The task of identifying new Palmer infestations is complicated by its close resemblance to waterhemp. Because of waterhemps’ omnipresence in crop fields across Iowa, it is easy to ‘tune out’ when encountering another Amaranthusinfested field. Distinguishing waterhemp from Palmer amaranth in the vegetative stage is difficult, this is why it is so important to monitor fields now when these species are entering the reproductive phase.

    From a distance, Palmer amaranth can often be differentiated from waterhemp due to the presence of long (> 12 inches) terminal branches on inflorescences. The inflorescences of Palmer are often more than 3/4 inch in diameter, whereas waterhemp typically has thin (1/4 inch) branches. These two species are dioecious, meaning male and female flowers grow on separate plants. Occasionally, male waterhemp will produce long, thick terminal inflorescences.

    The most reliable trait for differentiating Palmer amaranth and waterhemp is the bracts of female plants. The bracts on Palmer amaranth are up to 1/4 inch in length and extend beyond the other flower parts, giving the seedhead a ‘spiky’ appearance (Fig. 3). These bracts become sharp as they mature, making the seedheads painful to the touch. The bracts on waterhemp do not extend beyond the length of the other parts of the flowers (tepals, seed capsule).

    Figure 3.


    Click image to enlarge. The long bracts of Palmer amaranth are the most reliable trait to differentiate it from waterhemp.

    Why worry about Palmer amaranth when we are already in the ‘fight of our lives’ with its close relative waterhemp? While crops can tolerate moderate infestations of waterhemp without suffering large yield losses, a similar infestation of Palmer amaranth can be devastating to yields. This is why ‘Early Detection and Prevention’ is so important.

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