Grefsheim Spiraea cinerea 'Grefsheim' - Hardy plant WhiteA mass of white! The Grefsheim (Spiraea cinerea) blooms abundantly with little white flowers on long plumes and arching branches. This ornamental shrub blooms between late March and April. Prune away some of the branches every year. Place the plant in a spot in the sun to encourage it to bloom more abundantly. Watering is only necessary during dry periods. A popular ornamental shrub with lush flowers!