What is deadheading and why does it work?
Deadheading is removing spent flowers before they set seed. It works because it interrupts the plant's reproductive cycle and redirects its energy.
A flowering plant's purpose, biologically, is to produce seed. Once it succeeds, the signal to keep flowering stops — the job is done. By removing faded flowers before seed develops, you keep the plant in a state of trying, and many species respond by producing more flowers to try again.
Seed production is also expensive. Energy that would go into developing seeds instead goes into new growth and new blooms.
How to do it matters more than people assume. Cut back to just above a leaf node, a bud, or a side shoot — not simply pulling the petals off. Removing the petals alone leaves the developing seed pod behind, which defeats the purpose entirely. For plants with many small flowers, shearing the whole plant back by a third after the first flush works well; hardy geraniums and catmint respond dramatically to this.
What benefits: roses, dahlias, cosmos, sweet peas, petunias, geraniums, delphiniums. Sweet peas in particular will stop flowering within days if allowed to set pods.
What not to deadhead. Plants grown for hips or berries — many roses, pyracantha, cotoneaster. Plants with attractive winter seedheads, such as sedum, echinacea and ornamental grasses, which also feed birds. Anything you want to self-seed. And plants that flower only once, where deadheading gains nothing but tidiness.
Do it regularly rather than occasionally. Once seed has set, the signal has already been sent.