Goldmoss (Sedum sarmentosum)
Also known as Stringy Stonecrop or Wild Stonecrop (a name it shares with Sedum ternatum), this little succulent really likes city yards where the soil is a bit dry. But it can seed itself anywhere and grows very fast: this plant grew up in a nursery pot with a Korean lilac, and by the end of May was covered with starry yellow flowers. Often planted as a groundcover; it can be invasive, but its shallow roots make it very easy to yank out if you don’t like it.
This species was introduced from Asia after Gray’s time as a rock-garden specialty, but apparently Pittsburgh is very much to its liking. It does not appear in Gray’s Manual.