”Giant” is relative to the minuscule flowers on most chickweeds: these flowers are still small, but this is a showy chickweed. It generally likes damp environments, but it’s adaptable. These were growing beside a sidewalk in the Central Square district of Mount Lebanon.

Some authorities place this plant in the genus Myosoton—a dispute that has apparently been going on for decades, since it is so listed in the 1951 Check List of the Vascular Flora of Allegheny County. The Wikimedia Commons category for Stellaria aquatica redirects to Myosoton aquaticum, which has a link to the Wikipedia article on Myosoton, which redirects to the article on Stellaria, where Myosoton is not mentioned. We have decided to follow Gray until you botanists sort this thing out. For a description of the species, see the Stellaria aquatica reference page.