OK, it’s a stretch to call this pruning, but technically that’s what it is. Perennials have a cycle of growth and flowering that’s generally meant to produce a strong plant that can survive year after year – creating flowers and seeds is something that happens as part of that cycle. The “prime directive”, though, is the survival of the plant.
Annuals, on the other hand, have evolved into machines that produce seeds, and seeds come from flowers. An annual takes a little time to establish root, but very quickly explodes into a flower machine, generally with the ability to give cover the garden in flowers through most of the growing season.
There’s often a trick, though, to keeping that explosion of flowers fresh and abundant. Remember that the real objective of the annual is to produce seeds, and flowers are just a means to that end. Many plants, as they begin to “feel” flowers maturing, will spend energy maturing the seeds from those flowers rather than producing new flowers. If you’re a birder, you want lots of those seeds maturing for the birds, but the gardener in you wants the flowers to continue!
To keep a plant focused on producing new flowers, we “deadhead” the old blooms. It’s a simple process of pinching back the old blooms as soon as they start to fade. The sooner you pinch an old bloom back, the sooner the plant’s energy will go into producing a new bloom for you to enjoy. With highly prolific flowers like marigolds, deadheading is a nearly daily activity if you’re going to stay ahead of it. An approach I like to take is to single out a few plants mixed into a group of annuals, and keep those plants deadheaded and producing new flowers. The other plants I deadhead far less aggressively, allowing many of the old blooms to mature into seeds for the birds to enjoy.
Another trick for many types of annuals is the selective pruning of blooms in order to end up with bigger blooms. Dahlias are a great example of where this is a good idea. By selectively removing buds and small blooms to leave only the ones that are placed the best on the plant, we can end up with much more robust blooms. It’s an energy thing – you’re removing places where the plant’s energy would go, channelling the energy of the plant into the blooms that you leave.
Finally, as the annuals grow throughout the season, we often want to control how big and leggy they get. Remember that a plant that’s a perennial in a mild climate is an annual in a more harsh climate. Many of them will spend energy trying to get bigger and bigger – a bigger plant is able to produce that many more blooms when it does bloom. However, by carefully (and sometimes aggressively) pruning back expansive annuals like annual Salvia, we can channel more of the energy into those blooms that we love to love.