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Well-grown delphiniums are an impressive sight that inspires onlookers to want them for their own gardens. These are classic perennials upon which much breeding work has been done, producing options for every position in the border. But they are not low-maintenance plants.
There are three basic delphinium groups to choose from:
- the D. elatum hybrids, which have the showiest, tallest flower spikes, 5 to 6 feet tall and packed with bloom. These are great for the back of the border. Zones 4–7.
- the Pacific hybrids, which are similar to the Elatums but usually about 2 feet shorter. A good middle/front border plant. Zones 4–7.
- the Belladonna group, which have branched spikes lined more loosely with individual flowers. Generally 3 to 4 feet tall. Zones 3–8.
Delphiniums require excellent drainage and regular watering, and they prefer full sun. Some midday shade is appreciated in the southern part of their growing range. In poor or heavy soil, add organic matter at planting time to improve drainage. They may need watering several times a week in summer. The soil should stay just barely moist. It should neither dry out nor become soggy.
Delphiniums require some space from their neighbors. They need full sun and good air flow; they don't deal well with the shade and stagnant air that close planting creates. Giving them space will help stave off powdery mildew.
Delphiniums require supports—a good wind or heavy rain can bend them over and snap their hollow stems. Supports should be put in place before new growth reaches 12 inches tall in spring. "Grow-through" supports, often used for peonies, work well for delphiniums, as do cages.
Delphiniums require feeding. They're hungry plants that thrive on rich soil and regular fertilizer. When planting them, add plenty of organic matter to the hole. Top-dress existing plantings in spring with compost. Feed the plants with an all-purpose slow-release granular food, such as Osmocote, in early spring; then treat them to a drink of fish-emulsion fertilizer, like Neptune's Harvest, several times over the summer.
Remove flower stalks immediately after bloom. The plant may flower again in late summer.
Finally, delphiniums are naturally short-lived plants. Expect them to peter out and need replacing after several years. The D. elatum hybrids can be longer lived. Delphiniums are easy to propagate by seed sown indoors in winter or outdoors in spring, or by cuttings taken in spring. Seed sown outdoors will not flower until their second year.