|Upright perennial or biennial with lance-shaped green leaves 1 to 4 1/2 inches long, after a frost leaves turn dull red. Masses of bright colored yellow flowers open during the day from May to July. Best planted in spring among other full sun plants. You should divide the clumps every 3 years to encourage flowering. Evening Primroses are excellent perennials for weekend gardeners, since they are simple to grow, with almost no pests or problems.