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I generally cut back and prune my own delphiniums in the autumn once the plant has finished blooming. However, deadheading the blooms throughout the growing season can often result in the second wave of flowers in late summer so I always try my best to promote that second flush of blooms.
Although delphiniums bloom for only a few months, you can extend this period by removing the flowers once they start to die off. I do this periodically through the middle of the flowering season (July and August).
Once the blooms have faded, cut them back to just above where the leaves start on the stalk. This encourages side stalks to start to develop, potentially creating new flowers in time to bloom in late August or September. Plus it keeps the plant looking neat and tidy.
Cutting back and pruning
Cutting back and pruning let you shape the plant so that it stays visually appealing. Plus it’s a task that you can do as part of your winter preparation for delphiniums.
If your flower spikes start to look a bit worn, cut them down to the ground. Leave any side shoots that are growing so that they can produce new flowers. And leave any remaining leaves at the base. Do this carefully as you don’t want to end up with just one or two stems n your delphinium plant.
At the end of the growing season, wait until all the leaves have turned yellow or brown and have started to die. Cut the stalks down to about 3cm to 6cm above the ground, and remove the debris. Once again, leave any side shoots that may be appearing. They’ll die off on their own in the winter.
Remember that when you prune delphiniums, it’s a more aggressive process than deadheading. don’t expect any more flowers that growing season after pruning, as you do with deadheading.