Hypericum perforatum 

Family: Hypericaceae (St. John's Wort)

Habitat: disturbed areas and grassland

Notes: Invasive non-native. Originally from Europe, this weed nearly took over the Pacific northwest. It is currently under control in that area by a bioagent (in this case an introduced beetle). However, that beetle cannot survive the dry summer heat of the Central Valley and foothills where Klamath Weed continues to be a serious invasive species.