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Delphinium is a genus of about 300 species of perennial flowering plants in the buttercup family Ranunculaceae, native throughout the Northern Hemisphere and also on the high mountains of tropical Africa. The common name, Larkspur, usually refers to annual varieties. Many species are cultivated as garden plants and make stunning cut-flower arrangements.
This garden classic bears spikes of intensely colored blossoms in shades of blue, purple, white, and pink. The largest varieties can reach 6 feet tall, with some are dwarf selections. Cut down spent blossoms for another flush of blooms.
PLANTING: Set plants 18 to 24 inches apart. Cool conditions will encourage spikes to attain their maximum size; and choose a location in full sun, with a humus-rich, well-drained but moisture-retentive soil (ideally of neutral to slight alkaline reaction). Good drainage is essential; lighten soil with sand and compost, if necessary. Crowns should be placed 1 to 2 inches below the surface of the soil; deeper planting might lead to crown rot.
MAINTENANCE: Delphiniums should be fed liberally with manure or a recommended fertilizer. After initial blooming, cut back blooming stalks to 6 inches above ground to promote a second bloom. Leaves must not be removed. Taller varieties, especially if exposed to wind, may require staking. We recommend winter mulch. To promote larger flowers and greater numbers, remove all but 5 young shoots, early in the season when they are 4 to 6 inches high. It is important to stake tall growing varieties and this is best done early in the season. Zones 3-7 (but can be treated as a hardy spring annual in Zones 8-10).
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Larkspur is a common name, although this name usually refers to annual varieties. Delphinium flowers may be single or double. Blue is the most common color, however red, pink, white, violet, and yellow flowered species and varieties also are available. Many flowers have white or black centers or 'bees'. Flowers occur in attractive spikes with numerous flowers on each spike. The spikes of most delphiniums make attractive, long-lasting cut flowers. The foliage is palmately lobed and is divided into 3 to 7 parts depending upon the location on the stem. Since plants range in height from 2 to 6 feet, they can be used in a variety of ways. Placing them in the background of a perennial bed is one popular choice. Other gardeners have successfully used delphiniums as a specimen plant or as a small grouping in the garden.
Delphiniums thrive in regions with relatively cool and moist summers, and often struggle in hot, dry summer weather. However, some new varieties are more tolerant of heat. Delphiniums perform best in full sun and moist, well-drained soils. The pH of the soil should be neutral to slightly alkaline. When planting, be sure to set the crown at or above soil level to avoid crown rot. Tall, top-heavy varieties should be placed in an area protected from strong winds to keep from blowing over. Since their stems are hollow, taller varieties of Delphiniums should be staked to prevent damage from wind and rain. These new hybrids tend to be longer lived than their short-lived cousins but may be propagated from basal cuttings.
Delphiniums are heavy feeders and require regular fertilization during the growing season. Well-rotted manure or an all-purpose garden fertilizer, such as 10-10-10, can be placed around each plant as growth begins in spring and again after the first flowering. Overfeeding with nitrogen will shorten the lifespan of the flowers. Water plants during the summer if rainfall is less than 1 inch per week. Soil should never dry out.
Most delphinium hybrids flower from early to midsummer. Immediately after flowering, remove the flower stalk. This will encourage additional flower spikes in late summer or early fall. Delphiniums are short-lived perennials. They often need to be replaced every 2 to 3 years, especially when planted in heavy soils.
Apply a thin layer of compost each spring, followed by a 2-inch layer of mulch to retain moisture and control weeds. Stake tall varieties to prevent hollow flower stalks from snapping in the wind, and deadhead after flowering to encourage rebloom. After the first killing frost, cut stems back to an inch or two above soil line. Divide plants every three to four years as new growth begins in the spring, lifting plants and dividing them into clumps.
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