Purpletop is a purple flowered herb with erect stems up to 2m tall which are conspicuously square in cross-section and rough. Leaves are sessile (without a stalk), dull green and also rough. Leaves are narrowly rectangular to pear-shaped and hairy. The base of the leaf is slightly stem-clasping, leaf margin is irregular to regular toothed and apex is tapering to a point. Inflorescence (a group of flowers) occurs in the end of the branch having small purple-mauve flowers on cylindrical flowerheads grouped in threes. Flowers in late Spring and throughout Summer.