Experts and BHG readers answer.
How can I successfully grow delphiniums here in the USA?
Plant delphiniums in moist, well-drained soil that's rich in organic matter. Add compost, peat moss, or well-rotted manure before planting, especially if you garden in clay. How much sun delphiniums need depends on where you grow them. In areas that experience cool summers-such as the Pacific Northwest or the maritime provinces of Canada-the plants thrive in full sun. In areas that experience warmer summers, the plants prefer a spot with sun in the morning and late afternoon but shade during the hottest part of the day. Most delphiniums are hardy in Zones 3-7. They dislike hot summers and are short-lived in the Deep South, Southern California, and the desert Southwest. In these areas grow delphinium as a cool-season annual. Delphiniums aren't drought-tolerant; they require regular watering in most areas. Spreading mulch over the soil will help keep it cool and slow evaporation. Tall delphiniums require staking, especially when the plants are grown in an exposed area. Stake the plants by inserting a stick or pole into the ground near the plant. Tie the plant stem to the stake using a soft cloth. Use a figure-eight pattern-so the stem of the plant is in one loop and the stake is in the other loop-to prevent excess rubbing of the cloth against the stem. Deadhead delphiniums by removing the faded flower stalks. Deadheading encourages the plants to produce a second flush of blooms. Hundreds of delphiniums are available to gardeners. One of the best is Delphinium grandiflorum. This species grows less than 2 feet tall and produces clusters of true blue flowers over a mound of fine foliage. Like most delphinium species, it's rather short-lived-many gardeners treat it as an annual (or allow it to self-seed). If you prefer the taller, more stately delphiniums, Pacific Giant Hybrids typically grow 2-6 feet tall. 'Galahad' is a variety that bears pure white flowers. 'Black Knight' produces blue blooms with black centers.