House Grounds Gardens Great Tree Dahlia Garden Named in memory of Paul Callahan, a former employee of the arboretum, the Dahlia garden offers several hundred varieties of show quality Dahlias. These showy plants offer a rainbow of colors in mid to late summer into fall. The Dahlia garden is planted and maintained by the Mid Island Dahlia Society. Planting Fields and MIDS are proud to host the National Dahlia Show in September 2001.