Deadheading flowers -- removing them as they fade -- tidies up plants, helps prevent self-seeding and encourages more blooms. Plants usually set seeds and stop flowering after their flowers are pollinated, but removing the fading flowers sends a signal to produce more flowers. You can prune, pinch off or shear old flowers, depending on the plants' growth habits. Remove whole flowers, not just the petals. Before and after deadheading, sterilize pruning and hedge shears by wiping their blades with a cloth soaked with rubbing alcohol.
Plants that produce tall flowering spikes are some of the most confusing to deadhead. Flowering begins at the bottom of a spike and continues upward over time, leaving a long, mostly bare stem with a few blooms at its top. Hollyhock (Alcea rosea) and larkspur (Delphinium spp.) are two plants that produce flowering spikes. In order to deadhead their blooms and similar plants' flowers, pinch off the lower blooms with your thumb and forefinger as they fade, and prune an entire stem to its base when it is about 70 percent bare. Hollyhocks are perennial in U.S. Department of Agriculture plant hardiness zones 2 through 10, and larkspurs are hardy in USDA zones 5 through 9; all parts of larkspurs are poisonous when eaten.
Deadheading bushy plants involves removing part of each flowering stem. Bushy perennials produce flowers at the ends of leafy stems, making it difficult to decide what to remove. Shasta daisy (Leucanthemum x superbum, USDA zones 5 through 9) and columbine (Aquilegia canadensis, USDA zones 3 through 8) are two examples of bushy perennials. Deadhead those and similar plants by pruning each flowering stem at the nearest leaf, leaf bud or flower bud. Because columbines self-seed and can be invasive, deadhead them every week if you want to prevent their spread.
Some plants produce one or more flowers at the top of long stems, and deadheading these plants is a two-step process. Flowers of daylilies (Hemerocallis hybrids, USDA zones 2 through 10) appear at the ends of long stems. Pinch off each bloom as it fades, taking care to remove the developing seed head. When all the flowers on a stem have faded, prune the stem to its base. Find the base of a stem in a dense clump of leaves by feeling down the stem to its bottom with one hand, and prune the stem with your other hand.
Shearing plants that grow in soft mounds can keep them flowering all season. Lobelia (Lobelia erinus, USDA zones 10 through 11), for example, produces dense mounds of soft flowering stems, and removing each of its spent flowers isn't practical. Shear lobelia and similar plants when they begin to look untidy, which is usually from midsummer onward. Shear the mounds with grass shears, removing all the blooms, but leave at least one-half of the foliage to help the plants recover. Bushy perennials can be sheared in the same way to encourage a second flush of blooms.