They are "Jupiter's Beard" (Valerinan rubra), which the Sunset Gardening book describes as "rank, invasive, and much maligned." The flowers come in a dark purplish red, white, or the more common bright pink. They are very hardy, drought-tolerant, and "hard to beat for long, showy bloom in difficult situations." I have not seen them sold in stores, but they "self-sow" from dandelion-like seeds that drift on the wind.
A weed is a plant you don't want where it's growing; a flower is something pretty that you do want. People take great pains to grow calla lilies and palm trees, but I pull them out as weeds when they are growing in my lawn. The Jupiter's Beard is a weed when it grows in the iris bed, but a flower when it grows in that strip between fence and sidewalk: the fact that it volunteered to grow and is happy there is a pleasant plus.