Ohio Perennial and Biennial Weed Guide

Family:

Sunflower Family (Asteraceae)

Other Names:

Vernonia altissima, giant ironweed.

Origin and Distribution:

Tall ironweed is a native of North America. Its present range is generally confined to the eastern half of the U.S. and includes all of Ohio. The plant prefers to grow in areas such as meadows and pastures where the soil is fertile and conditions are moderately damp.

Plant Description:

Tall ironweed is an upright perennial with a highly visible dark red stem that grows over 7 feet tall and is widely branched at the top. At the ends of branches in loose clusters are saucer-shaped, 1/4-inch-wide flower heads consisting of 30 or fewer purple disk flowers. Attached to the stem are 10-inch-long, lance-shaped, pointed leaves that have short downy hairs on the lower surface. Reproduction is primarily by way of seeds, but new shoots sometimes arise from the large root crown.

• Root System:

  The root system consists of strong fibrous roots and rhizomes.

• Stems:

  The tough erect stems grow between 6 to 10 feet tall and can be hairy. The upper portion is widely branched. Stems turn dark red as the plant matures.

• Leaves:

  Leaves are alternate (1 leaf per node), 6 to 10 inches long, less than 3 inches wide, lance-shaped, pointed at the tip, and toothed along the edge. On the lower surface of each leaf is a distinct white midrib. The upper leaf surface is hairless and there are short, straight, downy, hairs on the lower surface with somewhat longer and denser hairs on the midrib.

• Flowers:

  Large, loose, spreading, flat-topped, saucer-shaped clusters of flowers form at the ends of branches. Flower heads are 1/4 inch wide and made up of fewer than 30 red-purple disk flowers tipped with 5 lobes.

• Fruits and Seeds:

  The single-seeded fruits are cylindrical with 10 prominent ribs running lengthwise. At one end of the seed are short purplish bristles (pappus).

Similar Species:

Western ironweed (Vernonia fasciculata) is rarely found in Ohio. The underside of its leaves lack hairs and its flower clusters are more compact than those of tall ironweed. Lower leaves of broad-leaved ironweed (V. glauca) are broader and more ovate than tall ironweed leaves. Missouri ironweed (V. missurica) is a weed of damp or dry open ground that occurs infrequently in Ohio. Its flower heads are composed of more than 30 disk flowers whereas tall ironweed flower heads have fewer than 30 disk flowers. New York ironweed (V. noveboracensis) flower heads are surrounded at the base by bracts with hairy tips giving flowers a somewhat wooly appearance.

Biology:

Flowers are produced from July to October. In an average year, a single plant can produce between 6000 to 19,000 seeds. Seeds buried in soil lose viability within the first 7 months. Stems tend to persist throughout winter. Research showed that 9 successive years of mowing on two dates during the year caused no significant stand reduction of tall ironweed. Herbicides controlled top growth of tall ironweed in the season of application but had little effect on regrowth from surviving roots.

Toxicity:

None known.

Facts and Folklore:

• The genus name 'Vernonia' was given in honor of William Vernon, an English botanist who worked in North America.

• 'Ironweed' refers to the toughness of the stem and the fact that it is very difficult to dig up this species with a shovel.

• Native Americans used the root to relieve pain.

• Tall ironweed is the third most troublesome pasture weed in Kentucky, and it is becoming increasingly prevalent as a weed in Ohio.