To deadhead plants with multiple blooms on one stem you use a two-stage approach. First remove the individual faded flowers as they die back. Then cut the entire stem once all blooms in the cluster have finished. This method keeps plants looking good while letting buds still opening get their chance.
I learned this technique the hard way with my delphiniums. The first year I cut the whole spike as soon as the bottom flowers faded. But buds at the top hadn't opened yet. Now I pick off spent blooms one by one until the whole spike finishes and I get weeks more color from each stem.
Spike bloomers like lupins salvias and delphiniums open from the bottom up. The flowers at the base fade while buds at the top are still tight. If you cut too early you lose all those upper blooms. Patience pays off with these plants since the show can last two to three weeks per stem.
Plants send energy to their buds that are still growing. When you remove spent flowers that energy goes to buds still opening. The plant finishes its bloom show rather than making seeds on the faded flowers at the bottom of the stem.
RHS guidance on spike-flowering plants points out that timing your final cut matters a lot. Cut the main spike back to strong side shoots or basal foliage once it finishes. This prompts the plant to push out secondary bloom spikes that extend your display into late summer.
Clustered flowers like hydrangeas work a bit different. These plants bloom all at once across the flower head. They don't open from bottom to top like spikes do. Wait until the whole cluster fades and turns papery. Cutting too soon shortens your display for no reason.
My lupins put out three extra spikes each after I cut the main stems back to the base leaves last year. Those secondary flowers were smaller but they kept the color going well into August. I never got that bonus when I left the spent spikes standing.
Learn to spot the difference between fading blooms and buds still developing on flower clusters. Fading flowers lose their color and petals start to wilt or drop. Developing buds stay tight and green or show fresh color at the tips. Only remove what's clearly done.
Cut back to basal foliage once the entire display finishes on any stem. This means cutting down to the low leaves at the plant's base. The plant can then put energy into roots and new growth rather than trying to support a bare flowering stem that won't bloom again.
Read the full article: Deadheading Flowers for Continuous Blooms
