It is not easy to sort out the taxonomy of this humble weed; we are going by the listing in the USDA PLANTS database, but the species name is also spelled caespitosum (Classical Latin rather than Medieval Latin), and a whole group of related Polygonum species is often separated into the genus Persicaria, in which case this becomes Persicaria posumbu. It is one of several similar smartweeds that frequently pop up in urban areas. This one is distinguished by its dense spike of tiny pink flowers and pointed leaves without markings; the similar Lady’s Thumb (Polygonum persicaria or Persicaria maculosa) has a dark thumbprint mark on each leaf. Look also for tufts of hairs at the stem joints. These plants were growing by a fence in Beechview, where they were blooming in the middle of July.
