Ironweeds: The Purple Towers of Ohio Meadows

Happy Monday everybody! This week, we’re featuring Ironweed. Ironweed has been a valuable plant for people living in the Americas for hundreds of years. Even today, research is still being conducted on the uses of chemical properties of certain ironweed species. How cool! As naturalists, however, we might be more excited by ironweed’s value for our local pollinators. Ironweed blooms in Ohio between late July and October, making its flowers a crucial late-season source of nectar for local bees and butterflies. Ironweed is a host plant for the American lady butterfly and many of our other native butterflies feed on the nectar, including Eastern tiger swallowtails, monarchs, skippers, and sulfurs.

Photo credit: Patrick Coin

There are 18 ironweed species throughout the North American content and, according to the USDA PLANTS database, 5 of them can be found in Ohio. In Cuyahoga Valley National Park, however, we’re likely to see just two ironweed species: tall ironweed (Vernonia gigantea, synonymous with Vernonia altissima) and New York ironweed (Vernonia noveboracensis).

First, let’s go over the difference between tall and New York ironweed. Then, we’ll review some unique identifying characteristics of the other ironweeds of Ohio!

Tall ironweed (Vernonia gigantea):

Like Joe-pye weed, tall ironweed is another of Ohio’s wildflower giants (hence the name!). Tall ironweed typically reaches a height of 5-8’, with some reports as large as 10’ tall- Wowza! This giant grows as one skinny main stem with alternate leaves. Each leaf is lance-shaped and finely toothed with hair on the lower leaf surface. At the top of the plant, ironweed boasts its branching, bright purple flowers. The flowers occur on the plant as heads, with each head of the tall ironweed plant holding 13-30 flowers. Each flower head is held together by bracts, or small modified leaves. The bracts of tall ironweed are scale-like with blunt or short-pointed tips.

Photo credits: David D. Taylor and John HIlty, Illinois Wildflowers

New York ironweed (Vernonia noveboracensis):

New York ironweed has nearly the same physical characteristics as tall ironweed and both plants enjoy medium to wet soils in full sun (think: prairies, meadows, woodland openings, pastures, and even streambanks). So, what’s the secret to telling these two apart? It’s their flower bracts. Look closely at the bracts at the base of each flower head. If the tips of those bracts are skinny and thread-like, you’re probably looking at New York ironweed. Additionally, each flower head can have 30-50 flowers, nearly twice as many as tall ironweed flower heads. New York ironweed’s jam-packed flowerheads and thread-like bract tips can give the flower heads a fuzzy-looking appearance.

Photo credits: Esther Westerveld and Susan C. Larkin.

Here are some tips for identifying the other three ironweeds you might see in Ohio!

Missouri ironweed (Vernonia missurica):

Missouri ironweed can grow nearly as large as tall ironweed. However, its stem and the underside of its leaves have dense, white hairs. Additionally, its flowers form a corymb, with flowers of the outer edges of the clusters having longer branches than those near the center. In other ironweeds, the flower branches are all similar lengths. The bracts on Missouri ironweed flower heads are scale-like and resemble those of tall ironweed.

Photo credit: John Hilty, Illinois Wildflowers

Smooth ironweed (Vernonia fasciculata):

Smooth ironweed typically only reaches a height of 4 or 5’. Its leaves are shorter, narrower, and have no hair at all, unlike the three species we’ve mentioned so far. Smooth ironweed’s flowers also form a corymb, but its flower heads are denser and fluffier than those of Missouri ironweed.

Photo credits: Peter M. Dzuik and John Hilty, Illinois Wildflowers

Arkansas ironweed (Vernonia arkansana):

Arkansas ironweed will grow 2-5’ tall and has light green lanceolate leaves. Its leaves can also have short, stiff hairs on both the upper and lower leaf surface. The flower heads on Arkansas ironweed are fluffy and resemble miniature bergamot flower heads with bracts that have prominent thread-like tips.

Photo credits: Karl Gercens III and John Hilty, Illinois Wildflowers

Hopefully, this wildflower feature has been helpful. If you have any tips or things to look for when identifying ironweed, please add them in the comments below! Happy iNatting!

Posted on August 10, 2020 07:23 PM by mklein1216