• Zones 1-10,14-24, 32, 34, 36-41
• Full sun
• Regular watering
Delphinium's majestic bloom spikes are a mainstay of many summer gardens. Flower color covers the full range of blues and purples, and you'll find cool pinks, cream, and white as well. Individual blooms are flat, to 2 inches across, borne in narrow, upright spikes. Leaves are dark green and deeply lobed. The tall-growing cultivars of D. elatum are most widely grown; they form 1- to 2-foot-wide clumps and send up 5- to 6-foot flower spikes.
Delphiniums prefer cool, moist summers and chilly but not excessively cold winters. They need rich, porous, nonacid soil; if soil is acid, amend it to neutral before planting. Also work in organic matter and a high-phosphorus fertilizer. When new shoots appear in spring, remove all but the two or three strongest and apply a complete fertilizer. Stake flower stalks early. Cut back stalks after bloom, leaving foliage at the bottom; when new stalks emerge, fertilize again to encourage fall bloom.
Even under ideal conditions, delphiniums are usually short lived; dividing them each year in spring may prolong their lives. If grown outside their preferred climate, they're best treated as annuals. Plant in fall in mild-winter zones, and just after frost danger is past where summers are hot.
Besides cultivars of D. elatum, tall delphiniums include the Pacific Hybrids, with flower spikes up to 7 feet.
Hybrids of D. belladonna, such as 'Bellamosum', are somewhat hardier and less prone to disease than other delphiniums. Don't pinch or disbud these; just let them grow as they will. Their blossom spikes are shorter but more numerous than those of D. elatum.
Chinese delphinium (D. grandiflorum) is somewhat more forgiving of less-than-ideal conditions than D. elatum, though it does have the same soil and climate needs. It's a many-stalked plant that grows just 1 1/2 to 2 feet tall, bearing flowers in blue shades.