In additon to the D. alatum group, which includes 'Black Knight' and the other Pacfic Giant cultivars, there are two other very popular groups: Delphinium belledonna group include hybrid crosses between D. elatum,  D. cheilanthum, and D. grandiflorum. They grow 3 to 4 feet tall and are hardy in zones 3 to 7. Delphinium grandiflorum group, also known as Chinese or Siberian delphiniums, include more compact, 1- to 2-foot plants that have better heat tolerance than other groups. They are hardy in zones 4 to 7.