This is a very common weed in Philadelphia, often appearing spreading from the cracks in pavement or sidewalks. It often collects small pieces of trash and generally looks scraggly. It gets small flowers on the underside, which, if visible at all from a standing position, look like mere white specks. I had originally misidentified this as Polygonum aviculare, but the flowers are different, and aviculare has a more upright habit, despite them both being called "prostrate knotweed". Both species go by a variety of names, such as "knotgrass", "birdweed", "prostrate knotweed:, "common knotweed" and even the ever popular "pigweed" is applied equally to either species, causing untold difficulty of confusion. I have chosen "prostrate" for Polygonum arenarum because it usually has a truly prostrate habit, though it may lift up. I chose "common" for Polygonum avicular because it tends to form upright mats from trailing stems, even though it is probably less common than Polygonum arenastrum in the city. Arenastrum has the ability to grow from the slightest sidewalk cracks, while Polygonum aviculare seems to prefer a bit of dirt. It does well in lawns and significantly broken sidewalks.