It takes quite a bit of effort for flowers to bloom. It takes even more effort and resources for pollinated flowers to produce seed and the fruiting structures that contain the seed. If the seed of certain aggressive plants get dispersed, we need to put even more effort into pulling up the seedlings. It just never seems to end!
Removal of deteriorating flowers, commonly known (even by those of us who missed that generation) as ‘deadheading’, can eliminate so much of this extra work. Not many plants benefit from deadheading; but most that do are really grateful for it. Others that do not care one way or the other simply look better without their deteriorating flowers.
It is of course impossible to deadhead large flowering trees or vast areas of ground cover. Regularly shorn hedges should never need deadheading because they never get the opportunity to bloom or develop fruit. Plants that are appreciated for the ornamental quality of their fruit should of course not be deadheaded.
Most roses get deadheaded as they bloom because the development of their fruiting structures, known as ‘hips’, takes enough resources to compromise subsequent bloom. Removal of these hips therefore promotes bloom. Only the few types of roses that are grown for their showy hips should not get deadheaded. Phlox, daisies, zinias, dianthus and all sorts of plants with long continual bloom seasons likewise benefit from deadheading.
Some types of iris that produce seed perform better with deadheading, not because they will bloom again during the same season, but because they can divert resources to vegetative growth (like rhizomes and foliage) that will sustain bloom during the following year. Most bearded iris (that do not produce seed) and lily-of-the-Nile do not seem to care if they get deadheaded, but are generally more appealing without their finished flower trusses.
Four o’ clocks can not be deadheaded without also removing developing flowers, so can only be allowed to bloom and throw their invasive seed all over the garden. It is easier to pull their seedlings later. We have a bit more control over crocosmia. Even though they do not need to be deadheaded, they are less invasive and more appealing without their scraggly brown stalks and seed capsules.