Plant of the Week

Map of the United States showing states. States are colored green where the species may be found.
Range map of Vernonia gigantea. States are colored green where the species may be found.

Vernonia gigantea
Group of 9-foot tall plants. Photo by David D. Taylor.

Vernonia gigantea
Close up of flowers Photo by David D. Taylor.

Vernonia gigantea
Upper portion of plant. Photo: 2011 David D. Taylor.

Giant Ironweed (Vernonia gigantea (Walter) Trel.)

By David Taylor

Giant ironweed is a member of the Asteraceae, the Sunflower family. In older manuals and guides, this family is called the Compositae because the 'flowers' are a composite of many flowers, often of different types. Many different groups of plants fall into this family based on grouping and type of flowers. All members of the family produce one or more heads (capitulum, the term used in technical keys) of flowers. This plant produces only disk flowers which are shaped like flared tubes. Each tube is 5 petals that are fused together at the bottom and separate at the top. Each flower has a pistil (female part) and stamens (male part). About 17 species of ironweed are recognized in North America, mostly found in the southeast and Puerto Rico.

This ironweed is 1 to 3 meters (3.3 to 9.8 feet) tall. The stem is rigid mostly smooth, sometimes with a few silky white hairs near the leaf bases. Leaves are alternate, 10 to 30 centimeters (4 to 12 inches) long, and 2 to 4 centimeters (0.75 to 1.5 inches) wide. The leaves are smooth on top, usually dark green, and smooth to white pubescent beneath. Heads are generally in clusters of 10 to 20 at the end of branches and multiple branches may loosely overlap to produce flowering masses 10 to 18 centimeters (4 to 7 inches) across. Each head generally has 15 to 25 individual disk flowers in it. The flowers are purple, rarely white, in this species.

Giant ironweed is a species of prairies and other grasslands, old fields, roadsides, savannas and woodlands growing on dry to moist soils. It is especially common in overgrazed pasture. Cattle and other livestock do not eat it and overgrazing provides the conditions needed for germination. It is often considered a bad agricultural weed. This species has one of the largest ranges of the ironweed species. It is found from Iowa and Kansas south to Texas and east to New York south to Florida with the exception New Jersey. It is also known from Michigan and Ontario. It is most common in the Ohio and lower Mississippi valleys.

This species flowers in July to September. It is an excellent nectar plant and is visited by many species of butterflies and bees. It is a strong competitor in the garden and can soon become weedy when cultivated.

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