Deadheading is the term used for the removal of flowers from plants when the flowers are fading or dead. It is done for the dual purposes of maintaining appearance and improving performance.
Appearance
Most flowers lose their attraction as they fade, spoiling the overall appearance of bedding schemes or individual plants - particularly where the display is in a container or extends over several weeks. The flowers of plants like peonies, some camellias and many roses have numerous petals which, if allowed to drop as they fade, may scatter widely.
Performance
Flowers which have been pollinated soon fade, shed their petals and begin to form seedheads, pods or capsules. Energy is channeled into development of usually unwanted seeds, slowing further growth and flower development. Regular deadheading directs energy into stronger growth and improved performance.
Practicalities
Deadheading is often a matter of priorities. A hanging basket display can be spruced up in minutes whereas it can take several hours to deadhead a large rhododendron. Deadheading trees and large shrubs which bear numerous small flowers is usually impractical.
Shrubs
Among the most important are rhododendrons (and azaleas), camellias, lilacs and tree peonies. Use finger and thumb to pick or snap off each dead head where it joins the stem, or secateurs to cut just below the flower head. Avoid damaging buds or developing growths immediately below the flower, particularly when you deadhead rhododendrons.
Roses
Check all types frequently, but stop deadheading in autumn to avoid encouraging further new soft growth which could be damaged by early frosts.
Climbers
Deadhead where practical, particularly Eccremocarpus, which otherwise rapidly produces numerous seed pods.
Herbaceous
Snapping or cutting of dead heads can enhance the flowering performance of many perennials. Do not remove those that have ornamental seeds or fruits, such as stinking iris (Iris foetissima) and bladder cherry (Physalis alkakengi).
Bedding plants
Snap or cut off dead heads where practical, particularly dahlias, calendulas, tagetes, stocks, pelargoniums, polyanthus, snapdragons and fuchsias, and plants growing in containers and hanging baskets for summer display.