Non native
A volunteer wildflower or weed in a riparian bed. The flower is tiny, tiny and aster-like. The leaves are serrated. The barbed seeds radiate out from a central point. The one plant is a vigorous grower! Over the past month, it has grown to a height of about 2 feet.
Observation and photo by Ricardo Sanchez, sent to nature@nhm.org.
Poison Oak?