|'Yellow River' blooms in beautiful large canary yellow flowers on red stems. An upright perennial or biennial with lance-shaped green leaves 1 to 4 1/2 inches long. These bright yellow primroses open during the day. 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.