You can do that you know by cutting them back at the right times.
Each time you cut certain perennials back, it will take about 2-3 weeks for them to rebloom if you foliar feed to jump-start them. If you cut them back too early, you won’t see the benefit, but cut them back in July, the plants speed ahead to more bloom.
If you want plants to bloom in September for instance, cut them back late July. If they normally bloom in August, give them a good cut at the end of June. Foliar feed and it will be faster.
Here’s a trick designers use.
Take a mature perennial and cut it back in stages. Leave the back of the plant to grow as nature intends, but take the front-half of the plant and cut it back to half its size in late May or early June. Cutting time depends upon how fast the plant grows and what time during the season a plant will bloom.
Above, the sage cut for rebloom, the gallery below when it was just cut. Click to see the penstemon and sage just cut. You can see how this method becomes easily a benefit to the garden. A few weeks later, the front portion will flower. No staking is required since the bulk of the plant has been cut back. They will not flop in the garden now as the growing front part will now support the back half.
The front of the plant that was cut will begin to bloom weeks later than the back half. As the back half is finished, it can be deadheaded and hidden by the blooming front half. The new bloom hides the back side’s detracting foliage from view. It doubles bloom time. See the gallery above as I cut both Salvia nemorosa and Penstemon digitalis ‘Husker’s Red’.
Delay blooming of one plant if you find it clashing with neighbors. Staggering bloom times can prevent moving a plant to another location.
Some perennials have unattractive bare or brown stems with still blooming flowers on top. Hide what you don’t want to see by this step-cutting method and spread out the bloom time for many more weeks.
I have used this technique on Echinacea, Monarda, Shasta Daisy, Veronica, Asters, Nepeta racemosa ‘Walker’s Low’, Hibiscus, and Campanula grandiflora among others. Try it.
More ideas in Spring for Summer gardening to follow.