December 06, 2021
A plant's biological purpose is to produce flowers that contain the seeds of the next generation, so to induce repeat flowering we must prevent the setting of seed by removing flowers before the seed set.
If we do so, many perennials will reflower, some three times provided you prune them carefully. The key point to understand is the flow of sap from the roots to the stem. If the plant is going from winter dormancy towards flowering, by removing the first flowers after flowering but before the setting of seed heads, most perennials will attempt to flower again.
However, after the autumn equinox (March 20) when perennials prepare for dormancy and sap is retreating back into tissue (say from April to August) then not only will plants not reflower but the wounds of woody perennials may be open to infection.
The backbone of a perennial border are herbaceous plants such as Achillea, Penstemon, Delphinium, Salvia nemerosa, Gaillardia, Shasta Daisy, Phlox, Erigeron and Coneflower, which retreat underground in winter (unlike evergreen Lavender and Agapanthus) and re-emerge in early spring.
Removing the spent flower spikes from the growing crown will induce new flower spikes.
ZINNIAS & DAHLIAS
Dahlias, Cannas and Salvias from subtropical climates need warmer soils to initiate flowering and produce their best flowers in late summer and autumn, so there is with no need prune, other than to remove spent flowers.
Overly vigorous perennials such as Evening Primrose ‘Pink’, Verbena ‘Polaris’ and Catmint revive so rapidly that you can cut all the foliage with hedge clippers right to the base, which will stimulate reflowering at least three times.
Lavender, Rosemary and Santolina will thrive after removing the flowering spikes, so cut them back and bring the perfumed spikes indoors in January, and you'll still enjoy a second flowering.
Romneya, Russian Sage and Verbena bonariensis will not resprout but by dead-heading, i.e. removing stems from the crown, this will prevent seed production and extend flowering until March.
All plants, even annuals, will flower for longer when you prune spent flowers to prevent seed production. Plants like Agastaches, Globe Flowers (Echinops) and Dahlias will repeat flower with the simple removal of the spent flower heads contrasting to Achilleas where the whole flower stem is removed to the ground.
Plants that don't respond to dead-heading include Bearded Iris, Hemerocallis, Gladiolus and Alliums. Succulent Sedum ‘Autumn Joy’ wins the medal for flowering gymnastics with its broccoli-like flowers, needing no attention at all.
COLOUR VARIATIONS IN THE OUTSTANDING CUT FLOWER DAHLIA CAFE AU LAIT