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CommentsThe Polygonum spp. (Knotweeds) with a prostrate or spreading habit have been subjected to various taxonomic classifications through the years. Some authorities regard Prostrate Knotweed as a single polymorphic species with several varieties, while other authorities identify several species of Prostrate Knotweed. Using the classifications of Mohlenbrock (2002) and Yatskievych (2000), the plant in the photograph is probably Polygonum arenastrum, which is by far the most common Prostrate Knotweed in Illinois. Another introduced species, Polygonum aviculare, has leaves that are more narrow (3-6 times longer than wide) and noticeably smaller in size toward the growing tips of the stems, while the leaves of Polygonum arenastrum are wider (2-4 times longer than wide) and remain about the same size along the length of the stems. There is also a native species of Prostrate Knotweed, Polygonum buxiforme, with an appearance that is even more similar to Polygonum arenastrum. For this native species, the 3 outer sepals of the flowers are longer than the 2 inner sepals and they have hooded tips, while the 3 outer sepals of Polygonum arenastrum have flat tips and they are about the same length as the inner sepals. In order to make such distinctions, it is necessary to use a magnifying glass.