Curlytop Ironweed (Arkansas Ironweed)

Media
Curlytop ironweed flower cluster viewed from the side
Scientific Name
Vernonia arkansana
Family
Asteraceae (asters, sunflowers, daisies)
Description

Curlytop ironweed, or Arkansas ironweed, is one of Missouri’s five species of ironweeds. It’s easy to identify because of its tapering, curling, threadlike involucral bracts. Also, it is usually a smooth, hairless plant.

Like our other ironweeds, curlytop ironweed is an upright perennial that bears branching, rounded clusters of fuzzy-looking magenta or purple flowerheads in mid to late summer.

Curlytop ironweed is the easiest of our ironweeds to identify.

The leaves are relatively narrow; they are usually sharply toothed but occasionally lack teeth; the margins are often somewhat turned under; the surfaces are glabrous (smooth, hairless) to sparsely hairy and dotted with tiny, impressed resin glands.

The flowerheads are relatively wide, with 50–120 florets per head. The involucral bracts (overlapping structures at the base of the flowerhead) are very distinctive and are the easiest way to identify this species: they are narrowly tapered, threadlike, and curly (except for the outermost/lowest bracts). Blooms July–October.

Learn more about Missouri’s ironweeds on their group page.

Similar species: Five species of ironweeds occur in Missouri. The one most similar to curlytop ironweed is probably prairie, or smooth ironweed (V. fasciculata), which is mostly found in northwestern Missouri, instead of the Ozarks. Although it, too, is a mostly hairless/smooth ironweed with rather narrow leaves that are gland-dotted, its involucral bracts are very different: they are appressed and scalelike (not narrow and threadlike as in curlytop ironweed).

Where ironweeds grow near one another, they often interbreed and produce offspring that share traits of the parent species, making them tricky to identify. Even more confusing, these hybrid plants are usually fertile, creating more unusual combinations. If you find an ironweed whose traits don’t quite fit any single species description, you may have found a hybrid. Look around for its two parent species nearby.

  • Curlytop ironweed often hybridizes with western ironweed (V. baldwinii), especially where dry road and moist stream habitats intersect.
  • Curlytop ironweed rarely hybridizes with prairie ironweed (V. fasciculata), since the two species usually don’t grow near each other.
Common Name Synonyms
Great Ironweed
Size

Height: about 2–5 feet.

Where To Find

Scattered mostly in the Ozark and Ozark border regions; uncommon along the Missouri and Mississippi rivers.

Occurs on banks of streams, margins of sloughs, fens, openings in bottomland forests, moist upland forests, bottomland prairies, rarely glades; also pastures and roadsides.

Curlytop ironweed is the characteristic species of stream banks and gravel bars in the Ozarks, but it can occur in many other habitats.

In the Midwest, ironweeds are a familiar sight in overgrazed pastures, apparently because the plants are unpalatable to cattle. Most species in this genus produce toxic chemical compounds, but the species that occur in Missouri have not yet been implicated directly in livestock or human poisoning.

Native Missouri perennial wildflower.

Ironweeds can be good choices for native wildflower gardening. They are highly attractive to butterflies, and they provide vibrant late-season color, especially when paired with bright yellow goldenrod, which blooms about the same time. If you want to take pictures of butterflies, park yourself next to a bunch of these blooming flowers.

Cattle avoid eating ironweed. On pastures, ironweeds are labeled “increasers,” since their numbers tend to increase over time when a pasture is heavily grazed. If you see a pasture covered with ironweed, it may have experienced heavy grazing. Most ironweeds produce toxic chemical compounds, making them distasteful to livestock, but the species that occur in Missouri have not yet been implicated directly in livestock or human poisoning.

The large, flat-topped clusters of intense purple flowers attract a wide array of bees, beetles, flies, butterflies, skippers, and other insects. These may feed on pollen, nectar, or both.

At least two species of digger bees, Melissodes confusiformis incondita and the eastern ironweed long-horned bee (M. denticulatus), are specialized to collect pollen primarily, or perhaps only, from ironweed species.

Many other kinds of insects eat the leaves or flowers or bore into the stems or roots, or suck the sap. These include midge, moth, and beetle larvae, aphids, stink bugs, grasshoppers, tree crickets, and katydids. Some species of moths use ironweeds as their caterpillar food plants. Leaf miners are insects whose larvae tunnel around within the leaves, eating the fleshy green tissues between upper and lower surfaces, leaving squiggly pale lines visible on the outside.

Crab spiders, orbweavers, assassin bugs, robber flies, and other predatory arthropods hunt the many insects that visit the flowers or feed on the plants.

Though a colony of ironweeds can be the home of nearly an entire food chain’s worth of insects and other small animals, mammals typically don’t eat the bitter plants.

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About Wildflowers, Grasses and Other Nonwoody Plants in Missouri
A very simple way of thinking about the green world is to divide the vascular plants into two groups: woody and nonwoody (or herbaceous). But this is an artificial division; many plant families include some species that are woody and some that are not. The diversity of nonwoody vascular plants is staggering! Think of all the ferns, grasses, sedges, lilies, peas, sunflowers, nightshades, milkweeds, mustards, mints, and mallows — weeds and wildflowers — and many more!