Delphinium plants need to be divided every 3 years, in early spring, as soon as there are signs of new growth. This will lead to healthy, vigorous growth and a longer-living plant as well as a crop of new ones. Division should be done when the plant is healthy.
Step 1: Prepare and Water Generously
The day before dividing takes place, the plant should be watered generously because the division process is traumatic to the root system. The feeding and hydration cycle of the delphinium will be interrupted so heavily saturated roots will reduce the shock.
Step 2: Cut Back Excess Foliage
To reduce the strain that the roots are under, excess foliage must be removed by approximately one third. This will reduce any undue stress on the roots.
Step 3: Prepare the New Plant Holes
Using the spade, dig holes for the new divisions. The holes should be slightly larger than the plant but not as large as the original since the new plants will be smaller. The holes should be adequately spaced out to allow for expansion.
Step 4: Dig out the Delphinium
Begin digging at the drip line, since the roots will generally not extend further than that. This will prevent any root damage and permit removal with most of the root system intact. Dig around the root system, severing any dead roots. Dig under the clump, at an angle down, at various points until the plant can be safely lifted from the hole.
Step 5: Keep Roots Moist
Place the plant in a bucket, in the shade, and cover them with newspaper to prevent loss of moisture. Spray the newspaper with water if the roots seem to be drying.
Step 6: Improve Soil Conditions
Add organic matter to the original hole from which the delphinium was removed. This renews the soil and maintains fertility, thus helping the plant to flourish.
Step 7: Divide the Delphinium
Gently pull apart the plant, being careful not to break the roots in the flower dividing process. The new plants should be at most 20 percent of the original since smaller sections will produce longer lasting blooms and also tend to grow more vigorously. Delphiniums multiply quickly, so remember that the plant will at least double in size the following year. If the division is too large, the plant will have to be divided again the following year.
Choose only healthy pieces to replant watching for discoloured stems, eroded crowns and roots.
Step 8: Plant
Place the plants so that the top of the roots are level with the soil surface. Fill in the hole around the roots and gently press on the soil to firm it. Water thoroughly.
Another method to propagate delphiniums is by using root cuttings. Cut 1 to 2 inches from a healthy root and plant in a pot containing moist potting mix with very little soil, placing the top of the root 1/2 inch above the soil line. Place the pot in a large plastic bag to maintain humidity. The plastic bag can be removed after 2 or 3 weeks when the roots begin to adjust to a lower level of humidity. New shoots should emerge within 3 months, but it can often take longer.