Prairie Ironweed (Smooth Ironweed)

Media
Closeup of prairie ironweed flowerheads in bloom
Scientific Name
Vernonia fasciculata
Family
Asteraceae (asters, sunflowers, daisies)
Description

Prairie, or smooth ironweed is one of Missouri’s five species of ironweeds. Mostly limited to northern and western Missouri, it’s a smooth or hairless plant. Verify your identification by noting characteristics of its flowerhead bracts.

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

The leaves of prairie ironweed are unlike all our other ironweeds (except for curlytop ironweed, V. arkansana): the leaf undersurface is smooth (glabrous; hairless) and dotted with tiny, impressed resin glands (these pits are easiest to see in dried leaves). The blades are relatively narrow. The margins are sharply toothed.

Flowerheads are relatively narrow, with only 10–26 florets per head. As with other ironweeds, the color is magenta or purple. The involucral bracts (overlapping, scalelike structures at the base of the flowerhead) are neither curly and threadlike, nor are they narrowly pointed and curling back. Instead, they are rounded or broadly angled to a bluntly pointed tip, the innermost (top) ones sometimes with a minute, sharp point; appressed (pressed snug against the flowerhead base), glabrous (smooth); the midvein not keeled or only slightly keeled toward the tip. The margins are sometimes minutely hairy. Blooms July–September.

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

Similar species: Most other Missouri ironweeds are hairy plants, but prairie ironweed is usually smooth or hairless, with smooth or hairless, gland-dotted leaf undersides. Curlytop ironweed (V. arkansana) is the only other Missouri ironweed typically lacking hairs, but its flowerheads are usually much wider, with many more florets (50–120) per head, and its involucral bracts are narrow and curly, looking something like tiny noodles (not like appressed scales).

Five species of ironweeds occur in Missouri. Where they 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.

Prairie ironweed can hybridize with all our other ironweeds, though combinations with curlytop ironweed (V. arkansana) are rare, since they usually don’t grow near each other.

Common Name Synonyms
Common Ironweed
Size

Height: about 1½–4½ feet.

Where To Find

Scattered in the northern half of Missouri, especially in the northwest; uncommon elsewhere.

Occurs on bottomland prairies, marshes, fens, lake margins, stream and river banks; also fallow fields, roadsides, railroads. Mostly limited to northern and western Missouri. Look for it growing near prairie cordgrass.

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!