Fall Flowers: White Snakeroot, Boneset, and Hyssop Leaf Boneset

The most obvious way to identify a flower is by color and flower shape and, in many cases, that is enough to make a positive identification.  I generally take pictures of flowers while I am hiking and identify them based on the pictures once I get home so I don't have to carry a heavy field guide with me.  I struggled for a long time to identify flowers of similar shape and color because I wasn't taking a picture of the leaves as well.  The leaves can be just as important as the flower itself in identification. 

Fall brings three great examples of the importance of looking beyond the flower itself and considering the whole plant.  The flowers of Ageratina altissima (White Snakeroot), Eupatorium perfoliatum (Boneset), and Eupatorium hyssopifolium (Hyssop-leaf Boneset) look almost identical.  They all bloom in late summer or early fall, so season is not a good indicator of their identity, either.  Here are pictures of the flowers for each plant (click to enlarge): 

 Old Rag Mountain, Saddle Trail.  September 2011.

 Appalachian Trail on Little Antietam Creek, MD.  September 2011.

Sand Flat, Frederick Municipal Watershed, MD.  October 2011.

To tell them apart, one has to look at the leaves (shown in the same order as the flowers):

Ageratina altissima (White Snakeroot).  These are the leaves associated with the first flower picture above.  The leaves are toothed, meaning the edges are jagged; there is a stalk, or petiole, between the leaf and the stem of the plant; and they are about half as wide as they are long.  It is found in woods and thickets. 

Eupatorium perfoliatum (Boneset).  These are the leaves associated with the the second flower picture.  The leaves are toothed, like the A. altissima, but they have a different shape.  They are much longer than wide and they do not have a petiole.  They are also opposite of each other on the stem of the plant.  It is found in moist areas such as swamps and low ground.

Eupatorium hyssopifolium (Hyssop-leaf Boneset).  These are the leaves associated with the third flower picture above.  The leaves are entire, meaning the edges are smooth.  They are whorled around the stem and they are much narrower than they are long.  The petiole is also absent on this plant.  It is found in fields and open places with dry, sandy soil.