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.