Annual, slender, glabrous, glaucescent herb, 10–20 cm tall, sometimes reaching 30 cm, with a single, erect, simple or branched stem. Leaves are linear, arranged in whorls of three in the lower part of the plant and alternate in the upper parts. Inflorescence consists of dense, capitate racemes that become laxer with age. Flowers are bright yellow, borne on short pedicels, 5 mm or longer. Calyx is glandular-hairy, 3–4 mm long, with linear-oblong lobes. Corolla is bright yellow, with the tube sometimes marked by violet streaks. Flowers have two pairs of unequal-length, included stamens. Fruits are globose capsules, 4–5 mm across, containing discoid, tuberculate seeds.