Growing Delphiniums from Cuttings
After your delphiniums have finished flowering cut back to ground level and take care not to overwater in the following weeks until the new shoots are well away. This helps prevent crown rot.
The best time to take cuttings is in the early spring when the new shoots are about 2 - 3 inches long but they can be taken right after the first summer flowering. This will have the advantage of thinning out the second flush and give you a little practice for the main event but be warned, you will have to find a really cool place.
When taking cuttings, it is vitally important to get right down to the base of the shoot and include some of the hard brown callus material you find there. You will probably see some tiny crown buds at the base of the shoot. It is a good idea to leave a few of these on the cutting too as it ensures fast formation of secondary growth on the new plant.
Clean and trim (in that order) the base of the cutting and place in a jar with about a half to 1 inch of water or alternatively in an airy rooting medium,(my much prefered method) taking care to keep moist, cool and humid. Be careful with temperature and do not be tempted to give the cuttings bottom heat or they will rot. It is best to keep them as cool as possible, say 50 - 55 deg F.