Sweet Alyssum (Lobularia maritima)
A popular bedding plant that liberally seeds itself. When seeds get washed downhill, they lodge in sidewalk cracks, where they’re quite happy to grow and bloom all summer and well into fall, producing more seeds to lodge in sidewalk cracks, and making the garden Alyssum one of our more common urban weeds. This plant grew where the sidewalk met a stone retaining wall in Beechview.
From Gray’s Manual:
LOBULARIA Desv. SWEET ALYSSUM
Pod small, orbicular, with only one or two wingless seeds in a cell; valves nerveless, somewhat convex, the margin flattened. Petals white, entire. Cotyledons accumbent. Hairs of the stem and leaves 2-pointed, appressed, attached in the middle. (Latin lobulus, a little lobe, probably referring to the 2-lobed hairs. )
1. L. MARITIMA (L.) Desv. Slightly hoary; leaves linear; flowers small, honey-scented. (Alyssum Lam.; Koniga R. Br.) Often cultivated, and occasionally spontaneous. (Introd. from Eu.)