This flower is so named because its pale yellow petals are the same colour as those of primroses, and the flowers open in the evening. This species is a smaller version of Large-flowered Evening-primrose. The flowers are smaller (4-6cm across), with the style the same length as the stamens; green sepals, and green stems. The two species can interbreed, giving plants with the smaller flowers and red-streaked sepals.
Evening Primrose oil contains GLA (gamma linolenic acid), which helps with calcium absorption, and thus is used in medical products such as the relief of eczema and pre-menstrual symptoms.

Green sepals and stem