Iron Butterfly ironweed is a form of Letterman's ironweed which is a perennial plant that is native to Arkansas where it grows in dry rocky sites that may occasionally be inundated. It bears delicate green folaige similar to Amsonia and bright purple one inch flowers in late summer to mid-fall providing a pop of color in the late season. Iron Butterfly grows to a height of around three feet and a width of about four feet in dry, sunny conditions. In an evaluation of ironweeds conducted at the Chicago Botanic Garden, Iron Butterfly ironweed received an excellent rating for growing in local conditions.