It’s all about sex, really. The plant’s job is to reproduce itself. If it’s an annual, it does this by flowering, getting pollinated, and setting seed. Once it’s done that, it can rest and let the kids/seeds do the work. Sort of like us. (Right: in what fantasyland does this occur?)
When you snip off the spent blooms, you frustrate the plant’s biological urges. You prevent it from setting seed. So what’s a plant to do? It has to start all over again, bud, bloom, sit around and look gorgeous till some friendly pollinator comes along. Then you nip it again before it's set seed. So it’s just back to work for the plant, no retiring for that puppy.
When you have annuals in containers, it’s usually pretty simple just to pinch off the dead bloom. I use my fingernails for most annual blooms. You can use pruners too, if you don’t have good nails, or if you’re working with a stronger-stemmed plant. I like the wave petunias a lot for containers, but for some reason, the red ones seem to me to need more deadheading. The breeders tout the Wave Petunias as “self-cleaning,” meaning you don’t need to deadhead them to keep them blooming, but they get leggy if you don’t. Other container plants that need deadheading are verbena, snapdragons, and, of course, geraniums. A plant I like for containers, partly because it doesn't need deadheading, is lantana. Another is salvia, especially the one called "Black and Blue."
Many perennials respond well to deadheading too, and I’d encourage you to try it. Sage, for example, has a gorgeous flush of bloom and then produces those long, unattractive seed heads. If you keep them cut off, the sage will bloom for a long time. Same with Veronica. (I’m not so sure about Archie, though.) Most roses respond to deadheading and there’s a plant where you want to use your pruners.
Butterfly bush needs to be deadheaded so it will bloom all season. It generally flowers in threes, with a terminal bloom and one on each side. Make sure you don't clip off the side buds. It flowers and browns out quickly, so you may be deadheading every couple of days.
Some perennials, however, won’t rebloom even if you deadhead till the cows come home. All you’ll get is a yard full of cows and a plant with nothing but foliage. Milkweed is one of these. I have a gorgeous orange milkweed planted next to my Veronica. The Veronica will rebloom if I deadhead it, but not the milkweed.
Deadheading is important too when you have plants that self seed. I like Echinacea (Coneflower), but they self seed like crazy in my yard. I deadhead most of them to forestall massive weeding in the next spring and summer, but I do leave a few seedheads because Goldfinches like them. In fact, the more native plants you put in, the more butterflies and birds you'll attract to your garden. But that's another blog.
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