The stalwart of many perennial borders and quintessentially 'English', delphiniums have broadened their range, both in size and heat tolerance -- good news for gardeners with small gardens or in warm climates.
Best known are the majestic so-called English types, which are blooming now in many areas. These densely clustered blue, pink, purple, red, or white spikes, growing to 6 feet tall, seem to dance along the backs of perennial borders, and they make great cut flowers. There are also versatile dwarf types, bushy plants with shorter spikes of vivid blue, and white flowers that blend well with low-growing perennials in the front of the border. A third type is midsize between these two. Even though there are more than 300 kinds of delphiniums, this article focuses mostly on the tall hybrids developed from D. elatum, and the shorter hybrids of D. belladonna and D. grandiflorum.
In relatively cool and moist summer climates, as in the Pacific Northwest and Northeast, delphiniums flourish, often growing into large clumps that live up to 10 years and can be periodically divided. However, delphiniums don't thrive in the hot-summer areas of the South and West, where they are grown as annuals, if grown at all. By selecting dwarf varieties and trying new, more heat-tolerant tall varieties from New Zealand, even gardeners in warm-summer areas can now grow delphiniums that prosper.
Unlike their relatives the larkspurs (Consolida), delphiniums are true perennials. Most delphinium varieties are listed as hardy to USDA Hardiness Zones 3 through 8, but they thrive best in zones 3 through 6 or in cool-summer areas of zones 7 and 8. The tall hybrids are the most sensitive to heat (days consistently in the 90s), but they can be grown in warm areas if planted in fall and treated as annuals. However, heat causes the flower stems to be shorter. Many of these hybrids were developed from seed, so actual flower colors may vary slightly. Let's look at some popular varieties grouped by their heights.
Tall (4 to 6 feet). English hybrids are probably the most dramatic of all the delphiniums. They come in a range of colors, and produce two or three flower spikes per plant the first year, twice as many in subsequent years.
The best known of the tall hybrids is Pacific Giants (also sold as Round Table series), with dense, heavy spikes of 3-inch-diameter double flowers that require staking. Two of the most stunning varieties are 'King Arthur', with royal violet flowers and a contrasting white bee (a cluster of short petals in the center of each flower); and 'Black Knight', with a dark violet blue flower and a black bee. Although most hybrid delphiniums are blue, pink, violet, or white, 'Beverly Hills' offers scarlet red flowers on 4- to 5-foot-tall plants.
The newest delphinium varieties come from New Zealand, where Dowdeswell's Delphiniums has developed the New Millennium hybrids. These plants compare favorably to Pacific Giants but are hardier and have stronger spikes, more petals per flower, a broader color range, and better disease resistance and heat tolerance. Varieties include pink 'Blushing Brides' and purple 'Royal Aspirations'. These are a good option for gardeners in warm-summer areas who want to grow the tall, large-flowered varieties.
Summer trials in New Zealand showed Pacific Giants dying out after two years from heat and disease, while the New Millennium hybrids were still going strong after four years. The last two years of the trial were conducted during summer temperatures that were consistently in the 80s and 90s.
Medium (2-1/2 to 5 feet). In this group are varieties of D. belladonna, whose 3- to 5-feet-tall loosely clustered flower spikes don't require staking. These bushier plants survive better in summer heat than taller hybrids do, and flower spikes produce six or seven stems the first year, compared to two or three on the taller kinds. 'Belladonna' features sky blue single flowers. 'Casa Blanca' is a white-flowered version of D. belladonna. 'Bellamosum' has deep blue single flowers.
Connecticut Yankee, an older mix, is particularly tolerant of hot summers; its flower spikes are a mix of blue, violet, and white flowers on 30-inch-tall stems. Magic Fountains series, a dwarf version of Pacific Giants, features the same densely clustered, large, double flowers as its parent, but on 2- to 3-foot spikes. Two of the best varieties are 'Sky Blue', with a white bee, and 'Cherry Blossom', with a pink flower and white bee. Clear Springs is a new series whose height falls between Pacific Giants and Magic Fountains; its strong spikes make excellent cut flowers.
Dwarf (2 feet or less). If you really want short delphiniums, choose from varieties that reach no more than 2 feet tall. These don't have the large flower spikes of the tall types, but the plants are better adapted to warm-summer areas.
D. grandiflorum 'Blue Butterfly' has electric blue single flowers, loosely clustered on 15-inch spikes. 'Blue Mirror' has brilliant gentian blue single flowers on 2-foot spikes. Both can bloom all summer in zones 3 through 6.
Two other delphinium species are shorter lived and harder to grow, but feature unusual flower colors for delphiniums. D. zalil features compact plants with deeply cut foliage and single yellow flowers on 2-foot spikes, and D. cardinale, a California native, features red spurred flowers on 2-foot spikes.
Delphiniums look best in clumps of three to five plants planted in well-drained soil with a pH of 6.5 to 7. Sow new seed (delphinium seed loses viability quickly) indoors in January or February for spring planting, or in August or September for fall planting. If slugs and snails are problems in your area, it's better to plant in spring. Plants grown from spring-sown seed may flower the first year, but the flower spikes will be fewer and smaller, and come later in the season than normal.
In hot-summer areas of zones 7 and warmer, set out transplants in October or November in well-drained soil; they'll bloom the following March and April. Dwarf and heat-tolerant varieties produce the best flowers and are most likely to survive summer heat, especially if planted in part shade. Although plants struggle to survive the hot summers, they can take temperatures down into the teens in winter as long as the flower stems haven't formed.
Buying plants allows for better flower production the first year, but in either case, it won't be until the second year that the plants really start producing large flowers and multiple stems, especially for the tall hybrids.
The soil you plant your delphiniums in should be rich and porous, so proper soil preparation is critical. Plants grown in cool, moist soil often rot. If the soil is predominantly clay, it's best to use raised beds. Amend your soil with 3 to 4 inches of compost.
Plant seedlings 1 to 2 feet apart, being careful not to bury the root crown. Cover with a 4- to 6-inch layer of mulch to conserve moisture and keep the roots cool.
Regular fertilizing is important for good flower production. Side-dress the plants with compost or a balanced fertilizer such as 5-10-5 at planting time, when flower stems form, and again after flowering. When weeding, cultivate carefully around the crown, since it is at the surface.
As soon as flowering finishes in zones 3 through 6, cut off flower stems to prevent seed formation. This often stimulates a repeat flowering later in fall.
The flower stems of tall varieties, such as those of the Pacific Giants series, are amazingly pliable and will withstand light wind and rain, but they do need some support for more severe weather. Many methods and products help support plants and flowers, but here is one of the easiest and most effective.
When the plants are a foot tall, place three 4-foot-long bamboo or plastic-coated metal poles in a triangle around a plant or clump of plants. Encircle the poles with thick garden twine about 10 inches above the ground. Once the flower spikes form, make a second loop around the poles at a height just below the flower heads. Usually two loops are enough to support the flowers.
Instead of poles and twine, you can use tomato cages, linking stakes, or flower rings as long as the flower stems are supported just below their heads.
Charlie Nardozzi is the senior horticulturist at National Gardening Association.
Photography by Suzanne DeJohn/National Gardening Association
Article published on June 23, 2008.