Euphorbia esula L.
Leafy Spurge is an erect perennial up to 3' in height. spreading by seed or roots.  Leaves are alternate, long, narrow, 1/4" wide and 2" long. 

Stems are hairless, contain a milky sap and branch at the top.  True flowers are surrounded by large heart shaped floral leaves which turn yellow-green. 

Roots are deep spreading, persistant, brown in color with numerous pink buds.
Photos and information adapted from "Weeds of the West" by Tom Whitson