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Delphinium Care - Growing the True Blue Larkspur

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Delphinium Care - Growing the True Blue Larkspur

Delphiniums Require Full Sun and Cool Soil

Photo © Angie Muldowney

Let’s state the facts up front: Delphiniums are finicky flowers, and even for gardeners with the perfect climate and soil, they are short-lived perennials. Most plants decline rapidly after the third season of growth.

Now, let’s sweep aside those minor inconveniences and get these blue beauties into our gardens. No other perennial flower offers the range of royal, midnight, and sky blue hues combined with the drama of a six-foot flower spike. Delphiniums also grow in complementary white, pink, and lavender tones, sometimes with contrasting black or white centers.

Latin Name:

Genus Delphinium, Family Ranunculaceae

Common Name:

Larkspur

Zone:

Hardy in zones 3-7, but performs poorly in areas with hot summers.

Size:

2-6 feet, depending on the variety

Exposure:

Full sun, or partial shade with morning sun in hot areas.

Bloom Period:

Early summer

Description:

Delphinium plants may produce single or double flowers, and some blossoms sport black or white centers called bees. Both tall and dwarf varieties produce their flowers on spikes, which last several days in a cut flower arrangement. The foliage is shaped like that of a maple leaf, with three to seven pointed lobes per leaf.

Planting:

Delphiniums demand moist soil, but they will rot in heavy clay with poor drainage. If you have problems with clay soil, it’s better to build a raised garden bed for the delphinium planting site. Even with staking, delphiniums don’t like to be whipped around by high winds, so place them close to the house or a privacy fence for protection.

Delphiniums are one of the perennials that thrive in alkaline soil, so if your pH falls below 6.5, broadcast some lime in the planting area.

Design Tips:

Most gardeners envision their delphiniums soaring above all others at the back of the mixed perennial border, but you can place small varieties in the middle or at the edge of the flowerbed. Pair delphiniums with other perennials that like alkaline soil, such as phlox, alyssum, or iris.

Maintenance:

  • The tallest delphinium varieties require staking to support their hollow stems. Insert the stakes when the plants first emerge in spring to avoid damaging new root growth.
  • Delphiniums are heavy feeders. Feed the plants twice each season with a balanced 10-10-10 fertilizer: Apply in the spring, and after flowering.
  • Because of their stature, spent delphinium flowers look shabby in the garden. Removing them tidies the flower garden and may encourage a second, smaller flush of blooms.

Pests and Diseases:

Like many garden ornamental plants, delphiniums are susceptible to aphids, slugs, powdery mildew, and rust. A bacterial disease called “blacks,” sometimes mistaken as bacterial black spot, is actually a manifestation of cyclamen mites. These mites are invisible to the naked eye and resist treatment, so gardeners should remove affected plants.

Varieties:

  • Black Knight: Midnight blue flowers
  • Blue Bird: Blue with white bee
  • Blue Springs: A dwarf variety averaging 2 feet with blue and lavender blooms
  • Galahad: All white flowers
  • King Arthur: Purple with white bee
  • Pacific Giants Hybrids: A mix of blue, violet, and rose colored flowers
  • Southern Noblemen: A mix of dark blue, light blue, and white flowers on compact plants, averaging 4 feet tall
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