Julian's Top Tip - Deadhead
Most plants flower to produce seed. So, when you dead-head a plant by removing its spent flowers you make it flower more. In the case of something like a sweet pea constant picking and dead-heading before seed forms results in a flowering period of several months.
If a plant does not re-flower, deadheading saves it the effort of making seed so it grows better.
Don't deadhead plants with ornamental seedheads like alliums.