Saturday, August 6, 2011

Prairie Ironweed (Vernonia fasciculata)

August already!?  I swear I was enjoying and photographing Trillium and Hepaticas just the other day.  My how time flies when you are having fun!  If you've spent much time outside and kept your eyes focused in on what's in bloom out there you may have noticed the transition that's currently happening.  June and July's summer flowers have given way to the onslaught of those more synonymous with late summer and fall.  Sunflowers (Helianthus spp.), Coneflowers (Rudbeckia spp.), Blazing Stars (Liatris spp.) and other members of Asteraceae are coming to life and coloring our world with yellows, oranges, pinks and purples.  Speaking of purple flowers, a fun and pretty genera of plants is just starting to really come on in Ohio; the Ironweeds (Vernonia spp.).

The Ironweed's are easy to distinguish from everything else by their tall and straight, unbraching stems topped with a beautiful panicle of purple-pink flowerheads.  Despite having four different native species in Ohio, 99% of the time what you see and find is the Common or Tall Ironweed (V. gigantea).  Along the road, in old fields and meadows, along streams and woodland margins; the stuff grows just about every place that has lots of sun and moist soils.  In fact it can almost be considered by some as a bit of an obnoxious weed despite being indigenous to the area.  The other three species are all much, much more rare in Ohio and in fact all are state listed as either threatened, endangered or even believed to be extirpated as in the case of the New York Ironweed (V. noveboracensis), which was only collected once in Ohio in Gallia County well over a century ago.  Missouri Ironweed (V. missurica) is endangered and only currently known from Clark and Lucas counties.  Today's focus is on the fourth species, Smooth or Prairie Ironweed (V. fasciculata) and how to distinguish this rarity from the much more common Tall Ironweed.

Prairie Ironweed is a much more western, Great Plains species of Vernonia that just slips into Ohio at the eastern fringe of its natural distribution.  Listed as threatened in Ohio, it occurs scattered throughout the western half of the state in areas of wet prairie.  Prairie Ironweed's habitat preference is open, wet situations with plenty of sunlight, especially in more deep and rich soils.  It can occasionally be found in marshes as well.  The population I know of grows in a wet ditch along a select stretch of bike path in eastern Clark County.  So let's take a look this versus the common V. gigantea.

Vernonia gigantea flowerheads
Vernonia fasciculata flowerheads

Considering both species will and do grow side by side, habitat differentiation for these plants goes right out the window.  In fact almost all the Ironweeds are known to naturally hybridize with each other, making the I.D. factor difficulty go up tenfold.  Height is the first thing that can help to separate the two.  Tall Ironweed gets its name for a reason, sometimes growing to nearly 7' tall.  Prairie Ironweed rarely gets 5' tall and is generally more in the 2-4' range.  The panicle of flowerheads can also help as well.  As you can see on the left picture above, Tall Ironweed's panicle is more open with spacing in-between the flowerheads while Prairie Ironweed (on the right) is bunched up with almost all the flowerheads densely compacted.  This characteristic can be seen in better detail further down.

V. gigantea inflorescence
V. fasciculata inflorescence

Beginning to take a closer look at the individual inflorescences it can be seen that both differ in the shape, size and color of their phyllaries.  Phyllaries are the little leaf-like bracts that look like little shingles on the involcure which holds all the individual disk flowers together.  On Tall Ironweed the phyllaries are smaller, darker colored (usually a purplish-green) and end in a sharp point.  With Prairie Ironweed the phyllaries are much larger, a distinct mixture of purple and green and are more blunt at the end.

V. gigantea's hairy stem
V. fasciculata's smooth stem

The presence or lack of hairs on the stem can also be used as a distinguishing factor but is one of the weakest areas due to the high level of variability within each species.  Generally Tall Ironweed's stem is covered in a pubescence of fuzzy hairs especially the further down the stem you go.  Prairie Ironweed's stem is almost always glabrous, showing no signs of hair.  I've seen plenty of Tall Ironweed with almost completely smooth stems and hardly any hairs so this is a pretty weak feature to put much faith in.

V. gigantea inflorescence
V. fasciculata inflorescence

Above are a couple more shots of each species flowerheads.  In these pictures it can be seen more clearly just how spaced apart the flowerheads are in the Tall Ironweed against the much more clustered and tightly bunched Prairie Ironweed's.

V. gigantea leaves
V. fasciculata leaves

All members of the Vernonia genera in Ohio have elongated and lance-shaped leaves with serrated margins and alternately arranged but there are varying differences within each species.  The leaves of Tall Ironweed are on average 2-2.5" wide and up to 10" long.  Prairie Ironweed's are significantly shorter and narrower, only averaging 5" long and 1/2" wide.  I've read where Prairie Ironweed leaves undersides can exhibit a habit of pitting; more or less a lot of dark dots.  None of the plants I found had leaves showing that feature but it can be used as an I.D. feature as well.

Prairie on L, Tall on R
Tall leaf on L, Prairie leaf on R

Above are some images comparing the leaves and individual inflorescences/phyllaries of each species side by side.  You can really begin to grasp the size and shape difference between the leaves and especially in the phyllaries.  Sometimes it can be hard to differentiate between similar species without having both to compare at each others side.  Below is another shot of each plants respective phyllaries.  I think the differences can really begin to stick out once you spend enough time comparing the two together.

V. gigantea phyllaries

V. fasciculata phyllaries

Both species are quite pretty to my eyes, coloring the green meadows and fields with tall wands of purple that are covered in all types of butterflies, moths and bees/flies.  While you are not likely to run into much Prairie Ironweed in our state I am just drawn to certain species of plants that can be fun to separate and study.  I plan on making a trip to southern and southwestern Kentucky to study and photograph the other two species of native Ironweeds to complete this treatment.  Perhaps not my most interesting topic ever but it was fun for me and hopefully for those wanting to separate just one of those tricky Asteraceae family members.


  1. Andrew- what a great lok at these two species. I spent last summer updating the northwest and northwest central populations of fasciculata. It is most certainly a facultative wetland species- I only found it in wet situations. Did you look at the undersides of the leaves with a hand lens? Imfound the pitting and absence of hairs on the undersides of the leaves to be quite diagnostic- but only with a 10x lens. The best public area to see this in Ohio is Probably Killdeeer Plains wildlife area- there are thousands of plants there scattered amongst several populations. It's threatened however by cut leaf teasel- a very big invasive problem.

    Happy botanizing


  2. Haha, interest comes from knowing or seeking to know. This post lies at the other end of the spectrum of your talents. Away from the artistic,closer to the analytical..Lovin this post...exactly the kind of thing for anyone interested in understanding the differences..its one thing ,albeit awesome,to show artistic shots or a single shot of a species,but this adds an entire and much needed side by side photo comparison record. Great work.

  3. I've really enjoyed your last two posts. I don't know how you find time to do your homework so well but I'm glad you do. It benefits us all to have somebody so meticulous and passionate about flora as you are.

  4. We have fasiculata and baldwini in Iowa, Western and Baldwins respectfully.I see most of the buterflies visiting this new bloom a lot.

  5. - Thanks, Tom! Appreciate your approval considering your experience with the plant. I did not have my handlens on me (a bad habit I need to break) so I'll have to check back on the pitting aspect of the plants from this population.

    - Michael, glad you found this post useful. I enjoy doing treatments and analysis of species sometimes as well as detailing my forays and expeditions into the Ohio wilderness.

    - Thanks, Mike! It just comes naturally to want to know the history and detail behind the use, natural history and ID of a plant.

    - Out on the Prairie, I'd love to see V. baldwinii. It's one of the Midwest's most common Ironweeds and certainly a pretty plant too!