Deadheading is simply the removal of deteriorating bloom prior to the maturation of seed or fruiting structures. Besides diverting resources, it removes unappealingly deteriorated bloom, as well as unwanted or potentially invasive seed. Deadheading can be delayed if seed from particular flowers is desirable, (although some types are genetically variable).
It was time to deadhead spring bulbs as they finished bloom earlier last spring. Now it is time to deadhead some of the summer bulbs. It eliminates unsightly faded floral stalks of gladiolus, and diverts resources into developing bulbs. It eradicates invasive montbretia seed. For canna, it conserves resources to enhance subsequent bloom through summer.
It is helpful to deadhead some types of annual bedding plants too. Marigold, zinnia, floss flower, pincushion flower and petunia should bloom better with systematic deadheading. Of course, all will continue to bloom without deadheading, but might be slightly subdued, with fading flowers. Modern sterile varieties that produce no viable seed are less reliant.
Fortunately, there is no need to deadhead alyssum, lobelia, nasturtium, moss rose, busy Lizzie or verbena. Their bloom is so abundant that it constantly overwhelms older bloom. Grooming tiny alyssum and lobelia flowers would otherwise be incredibly tedious. Moss rose, alyssum and nasturtium are pleased to self sow, but revert to simpler feral varieties.
Some branched types of sunflowers produce several blooms on several separate stems. Others bloom with only a single flower on top of a tall single stem. If deadheaded prior to the maturation of their seed, the stalks of some single sunflowers generate a few smaller axillary flowers by autumn. This technique inhibits seed production, but prolongs bloom.