Delphiniums are a well-liked choice found often among northern gardens--although some varieties of the plant can survive warmer summer temperatures found elsewhere. Delphinium varies in height. Some varieties grow from 39 to 79 inches tall, while the Chinese delphinium grows 2 to 3 feet tall. The Chinese delphinium flower produces long-lasting spires of blooms in shades of blue. Often treated as a half-hardy annual, delphinium can grow and propagate as a perennial with proper winter care.
Prune the flower stems back after the first freeze. Cut them with the shears either all the way to the ground or leave 1 or 2 inches.
Use a plant marker to indicate the location of the delphiniums, especially if you plan to lift and divide the plants. If you have more than one color of delphinium, note the color on the marker; these plants look best in mass planting of the same color.
Cover the flower bed with a 2-inch layer of mulch or a layer of decaying leaves. This will nourish the soil and keep it warm and moist throughout the winter.
Prepare the soil if the delphiniums are going to be divided and moved to a new planting site. They need full sun and rich soil. A month before dividing the delphinium, dig up the soil 12 to 15 inches deep, remove weeds, rocks and other garden debris and add generous amounts of compost.
Propagate the delphiniums in late fall or early spring, when they are fully dormant. According to the National Gardening Association, “Divide plants every three to four years as new growth begins in the spring, lifting plants and dividing them into clumps.”