vernonia crinita 'mammuth' ironweed perfect among tall, clump-forming grasses because the light-green sward of leaf flatters the vernonia's vivid-purple flowers. 'mammuth' reaches 1.5m (5ft) in uk. after flowering, fluffy white seeds appear and mature to rust-orange, giving this north american perennial the common name of ironweed. members of the daisy family, or asteraceae. herbaceous perennials, the name vernonia honours william vernon, an english botanist who collected large numbers of specie