Culture
Winter hardy to USDA Zones 3-7 where this delphinium is best grown in fertile, humus rich, medium moisture, well-drained soils in full sun. Performs well in alkaline soils. Prefers climates with cool summer temperatures. Appreciates some part afternoon shade in hot summer climates. Plants often struggle in St. Louis summers, and are generally not recommended for growing in hot and humid summer climates south of USDA Zone 7. Plants require protection from strong winds and rain storms via sheltered growing positions and staking. After bloom, promptly cut back spent flower spikes to the basal foliage to encourage an additional late summer and/or fall bloom.
Noteworthy Characteristics
Delphinium is a genus consisting of over 300 species of annual, biennial and perennial flowering plants. Most delphiniums sold in commerce today are complex hybrids. One such hybrid grouping, commonly known as English hybrid types or Elatum Group (often designated as Delphinium x elatum), features a large number of showy cultivars which have been introduced into commerce over time in a number of different series or strains which are considered by many gardeners to contain some of the showiest of the delphiniums available in commerce today.
Genus name comes from the Greek word delphis meaning dolphin in reference to the flower bud shape of some species purportedly resembling a dolphin.
'Casablanca' is a white flowering cultivar in the Belladonna Group, which is made up of several hybrid cultivars resulting from crosses of Delphinium elatum, D. cheilanthum and D. grandiflorum. The members of this group are said to have superior performance in hot, humid climates. An upright, vigorous selection, 'Casablanca' produces numerous flowering stalks up to 3' tall with showy, white blooms. An excellent cut flower. The sturdy stalks may not require staking, especially if placed in a full sun location protected from wind.
Problems
Delphiniums require considerable maintenance (staking, pruning, deadheading, disease control) in order to perform well in the garden. Delphiniums are generally susceptible to powdery mildew, botrytis blight, leaf spots and crown rot. Plants grown in full sun generally show better resistance to powdery mildew. Water plants at the base to avoid wetting the foliage. Crown rot will inevitably develop if plants are grown in poorly drained soils or planted too deep. Slugs and snails can do significant damage. Watch for aphids, leaf miners, stem borers and mites. Taller plants may need staking and appreciate being sited in locations protected from wind. Plants are considered toxic to humans if ingested.
Uses
Superb in cottage gardens. Back of beds and borders. Best in groupings or massed. Plant against a stockade fence for protection from wind. Excellent cut flower. Deer and rabbits usually do not cause problems.