Horseweed. Conyza canadensis, formerly Erigeron canadensis.
Asteraceae, the daisy, sunflower, or composite family.

Plant: Erect, bristly-hairy plant 1'-6' high, with branching clusters of small, white, cup-shaped flower heads. Annual.
Flowers: White, cup-shaped heads, about 1/4" across, with 25-40 narrow, non-spreading rays surrounding yellow disk flowers. In branching, terminal panicle that may be cymose or racemose.
Leaves: Alternate, narrowly lanceolate to linear, 1"-4", sessile, toothless, or sometimes toothed, with bristly hairs along the margins.
Fruit: A head of achenes with attached downy hairs.
Blooming: July-November
Habitat: Weedy open areas, overgrown fields.

Comments: Native to North America, this weedy plant has spread to Eurasia. An annual, it tends to be a pioneer species on bare ground, and is crowded out as perennials become established. Formerly classed with the fleabanes (genus Erigeron), the plant has recently been moved to a separate genus Conyza.

Where to find it: Appears occasionally in scattered fashion, in open, or newly turned areas.