Starting in mid to late summer, ironweeds bear showy, rounded clusters of fuzzy-looking magenta or purple flowerheads at the branching tops of upright stalks. The foliage is often (but not always) grayish due to a covering of fine hairs on the leaves and stalks.
Five species of ironweeds (Vernonia spp.) occur in Missouri. As a group, Missouri’s ironweeds share these characteristics:
- They are perennial, upright, nonwoody plants, lack milky sap, and are not prickly or spiny.
- They branch multiple times near the top.
- The leaves are alternate, simple, unlobed, almost always toothed, with 1 main vein.
- The composite flowerheads are in irregularly branching clusters (often flat-topped) at the tops of the plant stalks.
- The involucral bracts (the green, overlapping scaly or leaflike structures covering the base of each flowerhead) are important for identifying the different species; take note of their shape.
- The flowers are usually pink, magenta, or reddish purple. Rarely, they may be white.
- Bloom time is from midsummer into fall (varies slightly with species)
- After flowering, the seeds are topped with bristles, giving the seedheads a fluffy appearance.
Missouri has five species; each has their own page in this online field guide:
- Curlytop ironweed (Vernonia arkansana)
- Western ironweed (Vernonia baldwinii)
- Prairie ironweed (Vernonia fasciculata)
- Tall ironweed (Vernonia gigantea)
- Missouri ironweed (Vernonia missurica)
Hybrids: Our five species of ironweeds 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.
Similar species: Many plants in the sunflower family are similar to ironweed in some way or another.
- Blazing stars, or gayfeathers (Liatris spp.), have similar purplish flowerheads, but these are positioned along the top portion of an upright, unbranching stalk (not in rounded, branching clusters).
- Thistles (Cirsium spp.; such as tall thistle) also have fuzzy, purplish flowers, but their foliage is spiny.
- The various species of knapweeds and star thistles (Centaurea spp.; such as spotted knapweed and cornflower) usually have solitary (not clusters of) flowerheads whose outer ring of florets have the corollas (the tubular, conjoined petals) enlarged, resembling ray flowers.
- Thoroughworts (Eupatorium spp.) and Joe-Pye weeds (Eutrochium spp.), overall, look something like ironweeds. The flowers of thoroughworts are white, and the leaves are mostly opposite. The flowers of Joe-Pye weeds are pink or pale purple, and their leaves are in whorls on the stem.
- Yellow ironweed (Verbesina alternifolia) is similar in name only. It might have gotten its name because its leaves are somewhat similar. Otherwise, the flowers are very different: yellow ironweed has clusters of yellow flowerheads that look like droopy, ragged sunflowers, unlike the fuzzy magenta flowerheads of true ironweeds.
Height: Prairie ironweed may flower at only 20 inches tall, but tall ironweed can reach nearly 10 feet.
Statewide. The different species have different distribution patterns within the state.
Habitat and Conservation
Ironweeds may be found in nearly any habitat in Missouri, with different species more common in different habitats.
- Most Missouri species prefer lowland, or moist habitats, though our most common species, western ironweed (V. baldwinii), typically occurs in drier, upland sites.
In the Midwest, ironweeds are a familiar sight in overgrazed pastures, apparently because the plants are unpalatable to cattle.
Status
Native Missouri perennial wildflowers.
Human Connections
The name “ironweed” has several possible origins:
- It may have come from the grayish, rather metallic look of some species, which is caused by the dense, pale hairiness of the foliage.
- Others say it’s called ironweed for its tough stalks and overall rugged nature.
- Another possibility is the rusty-red color of the fuzzy pappus bristles that are conspicuous as the seedheads mature.
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.
Some ironweed species grow in damp soils and are good choices for a rain garden: a wildflower garden planted in a low spot that would otherwise be muddy after rains.
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.
Lactones extracted from an African species of Vernonia have been shown to inhibit tumor formation.
Native Americans used ironweeds medicinally, mainly as a pain reliever.
The genus name, Vernonia (pron. vur-NO-nee-uh), honors William Vernon (1666–1711), an English botanist and entomologist who specialized in mosses and butterflies; he collected biological specimens in Maryland in 1698 but otherwise stuck to England. The German naturalist J.C.D. von Schreber named the genus to recognize Vernon’s contributions to biology.
Like the names of many wildflowers, “ironweed” is a compelling, poetic word. It implies tough, scrappy persistence but also denotes the undeniable beauty of these native wildflowers. A Pulitzer prize-winning novel and its spinoff movie bear the name, and so does a Missouri bluegrass band.
Ecosystem Connections
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.
Some species of moths use ironweeds as their caterpillar food plants, including these:
- the ironweed borer moth (Papaipema cerussata), a deep brown, white-splotched moth whose caterpillars bore into the stems
- the parthenice tiger moth (Grammia parthenice), whose adults have black forewings with crisscrossed yellow streaks and bright orange, black-spotted hindwings
- the red groundling (Perigea xanthioides), a handsome rusty-red, brown, and gold-mottled noctuid.
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.
- 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.
The taxonomy of genus Vernonia has been changing in recent years, as botanists use DNA and other data to refine their understanding of the group:
- Some botanists only count about 20 species in the genus, restricting it to plants in North and South America, while older classification systems include 350 or even 500 species in the genus globally.
