The reddish green stems of oenothera longissima
are thick and rigid, growing upwards of 9 feet tall in favorable locations, bearing long, lance-shaped, grey-green leaves at alternate intervals of 1 to 2 inches. Leaf edges are somewhat wavy and lined with small teeth, sometimes barely so.
The large yellow flowerheads grow from a narrow green or reddish tube (the hypanthium) which can be up to 5 inches in length - much longer than some similar species; hence the species name longissima. Petals can be over 2 inches in length, often irregularly presented, with crinkled edges. The stamens and four-lobed stigma at the center are also all-yellow. Petals become orange as they start to wither. Underneath the petals are four green sepals, similar in length but narrower.