Delphinium are tall perennials with flowers along the length of their spiked stems. They are one of the few flowers that comes in a shade of true blue. Although many gardeners grow delphinium as annuals, they are true perennials and will bloom for several years in your garden before they need to be dug up and divided.
Choose a sunny spot with deep, rich, well-drained soil. Do not plant delphiniums in a spot where water stands after rainfall. Delphiniums need at least eight hours of sun per day in order to bloom.
Improve the soil. Add at least three inches of compost and one inch of well-rotted manure to the surface of the soil. Add one pound of powdered, all-purpose fertilizer per 25 feet of garden bed. Turn over the soil with a shovel to incorporate these amendments. Rake the area smooth.
Plant delphinium transplants 20 to 24 inches apart. Set them into small, individual holes with the crown of the plant even with the surface of the soil. Gently firm the soil around the roots.
Water each individual plant by hand with a watering can. Water at least twice weekly with a watering can until new growth appears. Thereafter, water at soil level with a soaker hose laid on the surface of the soil. Do not water with an overhead sprinkler; excess water on the plants can encourage powdery mildew.
Mulch to control weeds. Do not cultivate around the base of the plants with a garden claw. Delphinium are shallow-rooted, and cultivation can damage the roots. Apply a four-inch layer of straw, hay or buckwheat hulls to mulch the bed.
Stake the plants when they are approximately 24 inches high. Use bamboo stakes that are at least as high as the mature height of the particular variety of delphinium. Depending on the variety, delphinium can grow from two to five feet high.
Feed delphinium monthly with powdered, all-purpose fertilizer. Pull back mulch and apply two to three tablespoons around the base of each plant. Do not scratch into the soil. Replace mulch and apply water with a hand watering can.
Cut off flower stalk just below the lowest flower when the blooms fade. The flowers may bloom a second time, depending on your climate.
Cut down faded foliage after it is killed by frost in autumn. Mulch the roots with at least six inches of hay or fallen autumn leaves. Remove mulch in early spring about seven days prior to the date of your average spring frost.