Delphiniums (Delphinium spp.), also called larkspurs, are prized for their spikes of white, blue, pink or purple flowers. These plants grow as short-lived perennials in U.S. Department of Agriculture plant hardiness zones 2 through 9, although individual species or hybrids tend to offer much narrower potential growing ranges. Proper pruning is necessary to maintain a delphinium's attractive appearance and encourage vigorous growth for as many years as possible. Delphiniums are poisonous, potentially causing a burning of the lips and mouth, throat numbness, vomiting, other serious effects and sometimes even death, so cultivate this plant cautiously and handle its clippings carefully when small children or pets are present.
1 Cut about one in three stems back to just above the soil when the plant is about 6 inches tall, leaving just three to five stems remaining on each plant. Use a sharp knife or bypass-type hand pruners to cleanly make the cuts. This may not be necessary if the delphiniums have never had disease, but it's particularly important if they have been bothered by powdery mildew, botrytis blight or any of the several other diseases that frequently plague delphiniums.
2 Trim off lateral flower spikes that develop, if desired, to encourage a more impressive display on the main flower stalk.
3 Cut spent, faded flower stalks back to just above the nearest lateral flower spike.
4 Cut off flower stalks once the delphinium has finished blooming in the summer and before the plant can go to seed, leaving the plant's basal leaves remaining. This may encourage a second bloom period in the fall in areas with a sufficiently long growing season.
5 Prune off all flower stalks in fall at the end of the growing season once the plant is no longer attractive, cold weather kills the plant back or a second bloom period has finished, and destroy or dispose of the debris. Leave the basal leaves intact. Leaving the dead plant parts on or near the plant can lead to greater disease problems the following growing season.