Longwood’s blue-poppies will only flower through the first week in April. Photo by Eileen Tercha. The blue-poppies are blooming at Longwood Gardens, one of the many hallmarks of spring in the Mid-Atlantic. The sky-blue flowers, which are native to the high elevations of the Himalayan Mountains, are forced into bloom by Longwood horticulturists for the annual display. The flowers are …