Tall Ironweed (Giant Ironweed)

Tall ironweed flowerheads in bloom
Scientific Name
Vernonia gigantea (formerly V. altissima)
Asteraceae (asters, sunflowers, daisies)

Tall ironweed is one of Missouri’s five species of ironweeds. Most common in southeast Missouri but scattered statewide, it can grow up to 10 feet tall. To verify an ID, note leaf and flowerhead characters.

Like our other ironweeds, Missouri ironweed is an upright perennial that bears branching, rounded clusters of fuzzy-looking magenta or purple flowerheads in mid to late summer. The foliage is often grayish due to a covering of fine hairs on the leaves and stalks.

The stems of tall ironweed are minutely hairy, sometimes becoming nearly glabrous toward the base.

Leaves: the upper surface is glabrous (smooth) or somewhat roughened; the undersurface is minutely hairy, especially along the veins; occasionally, sparse longer hairs are also present along the veins; the margins are sharply toothed.

The flowerheads are relatively narrow, with only 13–30 florets per head. As with other ironweeds, the color is magenta or purple, though white-flowering forms occasionally occur. 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) bracts are rarely abruptly tapered to a minute, sharp point, appressed, glabrous (smooth) or sparsely and minutely hairy the midvein not keeled or only slightly keeled toward the tip. The margins are occasionally minutely hairy. Blooms August–October.

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

Similar species: Most similar to Missouri ironweed (V. missurica), but note that Missouri ironweed has 32–60 florets per head and is a hairier plant. Another similar ironweed is western ironweed (V. baldwinii), but its involucral bracts spread or curl backward. Also, keep in mind tall ironweed’s potential for greater height: Missouri ironweed doesn’t exceed 6½ feet, and western ironweed only reaches about 5 feet.

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.


Height: usually about 2½–6½ feet, but sometimes up to 10 feet. Note that although it can be taller than all our other ironweeds, there is considerable overlap among them.

Where To Find

Scattered nearly statewide; primarily in the southeastern quarter of the state; apparently absent from many parts of northern and eastern Missouri.

Occurs on banks of streams, rivers, spring branches, margins of ponds and lakes, bottomland forests, swamps, bottomland prairies, fens; also pastures, ditches, and roadsides.

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!