Giant Ironweed
Photo by Matt Whitaker
Scientific Description: The Guide to the Vascular Flora of Tennessee notes that giant ironweed (Vernonia gigantea) is common statewide, usually in mesic pastures, fields, and thickets. It is considered a facultative wetland indicator plant, meaning it is equally likely to show up in wetlands as in non-wetlands. It is most commonly seen in our area on the floodplain of the Tennessee River and creeks, including on Lookout Creek at Reflection Riding. A perennial, giant ironweed is typically 3-7-feet tall and single stemmed until the top of plant at its bloom head. Its 9-inch long and 2.5-inch wide leaves are alternating and densely distributed along the stem and have serrated margins. The stems can be glabrous (smooth) or pubescent (hairy), and the bottom of the leaves also vary between smooth to somewhat hairy.
In late summer through early fall, the 6 to 16-inch flowerheads become covered with deep magenta flowers that open slowly over the blooming season. The flowers have no scent, and the seed are spread by the wind. The plant forms a short thick rhizome with fibrous roots below.
Giant ironweed prefers full sun to light shade, moist to mesic conditions, and a wide variety of soil types (clay to sandy). Like so many of our natives, once established, it is drought resistant but looks best with more moisture, especially if in full sun.
Vernonia gigantea was formerly known as V. altissima.
Habitat Value: The nectar of giant ironweed attracts a wide variety of bees, butterflies, and other pollinators, and the blooms make for great butterfly watching. A number of caterpillars, including Eupatorium moth, ironweed borer moth, and Pyrlid moth, feed on Vernonia spp., especially on the stem pith and roots. A wide variety of other insects use ironweed as a food source.
Landscape Value: Giant ironweed is a dramatic plant that might not be for every gardener because of its size. It is best used as a background plant in a larger naturalized planting or English garden style planting, where the planting is large enough to enjoy layers of plants and it can be a subtle green background plant until the appearance of the intense blooms. Dedicated gardeners can prune it back in the spring to reduce its height and create a denser/multi-stem plant.
As its name implies, ironweeds are exceptionally tough, probably due to their robust root systems. The root system is also likely the reason ironweeds do not transplant easily, especially when full grown. Spent flowerhead should be removed to limit unwanted seedlings.
The bitter foliage makes ironweeds unattractive to deer and cattl\e, which is more and more important here as the deer population increases. Giant ironweed has no serious insect or disease problems.
At our Tennessee Aquarium project, we stockpiled for reuse the floodplain topsoil that was covered in fescue and Bermuda that Baylor School kept mowed. Our planting plan called for Vernonia gigantea, but the planting contractor was unable to locate it. Cleared of the lawn and respread after being piled up for a year in a severe drought, the topsoil had a wealth of native seed that immediately started germinating. The tough giant ironweed was one of the most prolific, and the site today has a beautiful stand of incredible local giant ironweed.
By Matt Whitaker, WMWA Landscape Architects
Matt Whitaker is a landscape architect with twenty years of experience in the profession and an interest in plants that started as a child with his plantsmen father and grandfather. He’s specified, planted, observed, and gardened thousands of plants across hundreds of sites in the Southeast and Mid-Atlantic. While he has an affinity for natives, he has planted an apple or two and a few more of our long-cultivated fruiting plants, in what Jefferson would call a “fruitery.”