For 115 years, Longwood Gardens in Kennett Square, Pennsylvania, has stood as one of the world’s leading collections of plants, showcasing botanical splendor and state-of-the-art glasshouses. Spring, when the gardens come alive with blooms and birds, is the perfect time to experience its beauty. Karl Gercens, Longwood’s conservatory manager, leads a specially arranged curator’s tour of the outdoor gardens of this historic du Pont property, where azaleas, poppies, and a host of flowering shrubs greet the season.