Leafy Spurge (Euphorbia esula)

Leafy spurge is the scourge of the West (and other parts of the country). It is a creeping perennial that propagates by a vast, deep root system and seeds. The seed can be propelled up to 15 feet from the mother plant by the three celled seed capsule. (54k)
An identifying characteristic is the yellow-green, heart shaped bracts (90k) that are very prominent during the spring. Also, the plant has a white, milky sap (40k) throughout the plant.
Photos courtesy of the USDA/ARS