Ambrosia psilostachya
 
Perennial Ragweed

Two Staminate Flower Heads, Close-Up

Last Mountain Lake Regional Park
07-Aug-2010

The staminate flower heads form a raceme at the apex of the stem. Below this, the pistillate flower heads are found in clusters in the leaf axils. Each pistillate head consists of a single pistillate floret surrounded by an involucre.  The involucre will eventually harden and form the outer surface of a bur.