|O.perennis are clump-forming perennials with spoon-shaped leaves in green, 1 to 2 inches long. Funnel shaped yellow primroses open in the daytime during the summer months. Best planted in spring among other full sun plants. You should divide the clumps every 3 years to encourage them to flower. Evening Primroses are excellent perennials for weekend gardeners, since they are simple to grow, with almost no pests or problems.