Goosefoot sp, probably Chenopodium album, Family: Chenopodiaceae, Goosefoots (approx 150 species).

Vernacular names (if Ch. album): Pigweed, Lamb-quarters, Lamb's Quarter, Fat Hen,
Dungweed or simply Goosefoot; other names, depending on species and location.

Alien species (if C. album), probably native of Europe (origin obscure due widespread cultivation).
IPC rating TBD; known to be an aggessive invader of cultivated fields, particularly soybeans and sugar beets.
Difficult to control via chemical means, but can be suppressed in grain fields by appropriate crop rotation.

This plant is edible, allegedly very tasty in salads if greens picked young and tender; older parts can be cooked like spinach.

Specimen found at Patterson Ranch, Santa Clara Cty, CA. Thanks to Dan at CNPS for ID.