Several pigweed species (Amaranthus spp.) are common weeds in Iowa corn and soybean fields. In the past, most agriculturists simply referred to these weeds as redroot pigweed. However, casual observations indicate that redroot pigweed is less frequently encountered in production fields than are other Amaranthus species. The most frequently encountered Amaranthus species are redroot pigweed, smooth pigweed, common waterhemp, tall waterhemp, and Powell amaranth.
It is difficult, if not impossible, to differentiate Amaranthus species during vegetative growth stages. Positive identification can only be made when flowers are present. Although not foolproof, the best way to identify Amaranthus seedlings is to know what species were present in the field the previous year.
Why differentiate species that are so similar? Studies at Kansas State University show differential tolerance among the Amaranthus species to postemergence herbicides. For example, the Pursuit label recommends earlier application for tall waterhemp than for redroot and smooth pigweed.