Perennial herb to about 80 cm tall, with the hairless stems coloured distinctively in reddish purple. Leaves in a basal rosette, alternate up the stem, narrowly ovate to linear-oblong, long-pointed and toothed. Flowers in spikes, to 5 cm long, scarlet.
L. splendens Willd. from Texas and Mexico is similar but has more or less linear leaves and flowers arranged mostly on one side of the stem.
L. ×gerardii 'Vedrariensis' has leaves dark green, tinged with red and flowers dark violet; it is a hybrid, L. cardinalis × L. siphilitica.
Source: (2002). Campanulaceae. In: . Horticultural Flora of South-eastern Australia. Volume 4. Flowering plants. Dicotyledons. Part 3. The identification of garden and cultivated plants. University of New South Wales Press.