Cirsium longistylum, called Long-style thistle , is a North American species of plants in the thistle tribe within the sunflower family. The species is found only in the central part of the State of Montana in the United States, in the portions of the Rocky Mountains called the Big Belt, Castle, Elkhorn, and Little Belt ranges.Cirsium longistylum is a biennial or perennial herb up to 150 cm (60 inches or 5 feet) tall, with a large taproot.