An annual or biennial, reproducing by seeds. The stems are erect, branched, and square.
Leaves are opposite, petiolate, and coarsely toothed.
Leaf veins do not extend to the serrated leaf margins. The flowers are dense spikes, bluish-purple in colour, located at the ends of branches or clustered in leaf axils. Stiff spiny bracts at the base of the spike make the head look prickly. The seeds are brownish-black, barely 3 mm long, within a calyx, round on one side and ridged on the other. A calyx contains 4 seeds.