Scarlet pimpernel has weak sprawling stems growing to about 50 cm (20in.) long, which bear bright green ovate sessile leaves in opposite pairs. The small orange, red or blue flowers are produced in the leaf axils from spring to autumn. The petal margins are somewhat crenate and have small glandular hairs.
The species has been distributed widely by humans, either deliberately as an ornamental flower or accidentally. A. arvensis is now naturalised almost worldwide.
Scarlet pimpernel flowers are open only when the sun shines.