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.
Tony Tomeo 
I seem to be constantly dead heading at the moment, especially the roses and sweet pea. It is always useful to see the gardening hints from an expert.
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Deadheading is one of the consequences of good bloom. It can be frustrating for those of us who can not keep up with it all.
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Great article. I deadhead plants I do not want to reproduce and let the others drop their seeds for next years plants.
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We ‘should’ to that with some of the campanula and campion that have naturalized within some of our landscapes, but what I really do is wait for the seed to mature, and then toss it out where I want more of it to grow. I really want to get seed of white campion if it ever appears here.
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I know it needs doing but oh dear, I do hate deadheading.
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It is not so bad on a small scale, but as you know, many of us do not grow flowers on a small scale.
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Mum is too old now to get at her most prolific shrub rose, which is massive. Unfortunately she is not happy unless it has been deadheaded.
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Perhaps we are fortunate that such roses do not perform so well here. Our roses get pruned very aggressively over winter, and (individually) do not get very big through the year. Those that do get big are mostly bramble types that need no deadheading, or that are allowed to develop bright red hips. Those that I see in pictures from other regions are intimidating. I would be inclined to prune them like those that I am familiar with; but then they would not get very far.
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