This common weed (or wildflower--depending upon your view) is Lady's Thumb (Polygonum persicaria), it also goes by many other common names such as Spotted Lady's-Thumb, Redleg, Adam's Plaster, Redshank and Persicaria. These little flowers while pretty at first, can fast become a nuisance, they spread rapidly and will take over any area where they are present. I made the mistake of encouraging it, mistakenly thinking it had an interesting flower, now it has choked out many of my other plants and shows no signs of stopping. Seems I will be yanking and pulling this stuff out before there is nothing left of my garden but Lady's Thumb. Polygonum is native to Europe and Asia, it was introduced to the United States, it has established itself in all the mainland states. This troublesome little weed was first discovered in the Great Lake regions in 1843 and is now considered an invasive weed in those parts. This plant can reach heights up to 2 feet, and it has lancet shaped leaves.This plant prefers sunlit moist conditions, but will easily adapt to poorer dirt and shady conditions. There are several species and sub-species of this plant and I am unsure which one is pictured here. One species has a telltale purple splotch on the leaves. It can be found most anywhere, along roadsides, in ditches, along riverbanks or in backyards like the one pictured. This plant contains persicarin and tannins which were used to treat diahrrea and infections. Fresh leaves have been used to stop bleeding.
The nectar of the flowers attracts Halictid bees, wasps, and Syrphid flies primarily. Less comon visitors include small butterflies and bumblebees. Halictid bees also collect pollen occasionally. The foliage is eaten by the caterpillars of some Copper butterflies and several species of Moths, while the flowers and fruit are eaten by the caterpillars of the Gray Hairstreak butterfly. Japanese beetles are also quite fond of the foliage of this and other smartweeds. Mammalian herbivores rarely feed on the foliage of Lady's Thumb because the foliage is pungent, peppery, and slightly bitter. However, White-Tailed Deer may chomp off the tops of young plants upon occasion. The seeds of smartweeds are very popular with waterfowl and granivorous songbirds. The seeds of Lady's Thumb, in particular, are eaten by birds in both upland and wetland habitats. The ecological value of this little plant is rather high, notwithstanding its weedy nature.