Identify that Plant

This plant’s common name is White snakeroot.  Its scientific name is Ageratina altissima.  Here’s another photo of a single plant.  If you look closely, you can see that the lower leaves have stalks while the upper leaves do not.

This closeup view shows an upper leaf with a developing flower stalk from the leaf’s axil.

Next, we have the top of the plant with developing flower clusters . . .

The White snakeroot flowers are beginning to bloom . . .

Look closely at the individual flowers of the White snakeroot.

Each tiny flower grows in a flower head (like “flowers within a flower”) which is typical of a plant in the Asteraceae family.

Now here’s an intriguing piece.  When I studied White snakeroot, I was struck by its flower which immediately reminded me of Boneset (Eupatorium perfoliatum).  Here are two photos of Boneset . . . the first shows the upper part of the plant with its leaves surrounding the stem and with the developing flower clusters:

And this second photo shows the Boneset flower head and flowers up close:

You can see the similarities between the White snakeroot and the Boneset flowers.  When I first learned the scientific name for White snakeroot, it was Eupatorium rugosum.  And Boneset was (and remains) Eupatorium perfoliatum.  With the on-going study and re-classification of plants, White snakeroot was moved from the Eupatorium genus to the Ageratina genus and was given a new species name (altissima).

There are two ragweeds found throughout North America: Common ragweed (Ambrosia artemisiifolia) and Great ragweed (Ambrosia trifida).  Let’s start with Great ragweed (pictured above) and study its parts and life cycle . . .

Great ragweed (Ambrosia trifida)

The lower leaves of Great ragweed have three — sometimes five — lobes (the trifida species name) while the upper leaves of a mature plant are elliptical:

Great ragweed

As the plant begins its reproductive phase, it starts with growing its staminate (male) flower spikes:

Great ragweed

Initially, the staminate flower spikes look fuzzy:

Great ragweed

Here’s a closer view of the developing staminate flower spike. . .

Great ragweed

. . . and a closer view. . .

Great ragweed

. . . and an even closer view — with the staminate flowers open, spread along the spike, and producing pollen:

Great ragweed

Let’s pull back and look at the top of a Great ragweed plant:

Great ragweed

All those staminate flower spikes have elongated and look thin and “weedy” now.  At the base of the staminate flower spike, and from various points where leaves are attached to the plant’s stalk, the pistillate (female) flowers have grown.  They are well-positioned to receive the wind-borne pollen from the staminate flowers:

Great ragweed

After the flowers are pollinated, the Great ragweed plant looks like this:

Great ragweed

You can see the clusters of pistillate flowers — now developing the seeds — and the flower spikes stripped of their dead staminate flowers:

Great ragweed

Here’s a closer view of the pistillate flower cluster (for scale, note the ant along the stem) . . .

Great ragweed

. . . and a final very close shot of the pistillate flower as the seeds are developing:

Great ragweed

Now, let’s turn to Common ragweed (Ambrosia artemisiifolia).  As you look at these photos you can see the similarities between these two ragweeds.  Common ragweed is in the foreground and some leaves from Great ragweed are in the background (in the photo below).  This gives a sense of the relative difference in size between the two ragweeds.

Common ragweed

Common ragweed has deeply cut lobes on all its leaves:

Common ragweed

Here are its developing staminate flower spikes:

Common ragweed

Just as with Great ragweed, the Common ragweed’s flower spikes elongate . . .

Common ragweed

. . . and the flower heads spread along the length of the spike:

Common ragweed

The pistillate flowers, again, are primarily at the base of the staminate flower stalk:

Common ragweed

This final photo is a close-up view of the clustered pistillate flowers on a Common ragweed plant.  The dead stalk to the left is the staminate flower spike.  We are looking down into the pistillate flower clusters.

Common ragweed