I'm walking along the sidewalk. Or trying to. I get slapped in the face by the drooping branches of a row of crape myrtles. I have to stoop or else walk in the homeowner's flower beds to avoid them. Several houses up, some young street trees thrust branches threateningly toward my chest. Then there's a fence with shrubs growing through it, scraping my arms and legs. Further on, I have to duck beneath trees with branches at head level sticking out over the sidewalk. Leaves grasp at my hair. Limbs assault my body. The sidewalks are an ongoing obstacle course of overhanging vegetation.
Maybe it's the increasing amount of carbon dioxide in the air causing plants to grow faster; reportedly there is scientific evidence of that. Or maybe it's just folks forgetful of how quickly things become overgrown in our rainforest climate. Whatever, it's a jungle out there. People, please prune your trees and shrubs!
Consider this useful image from the guys at Trees Atlanta: trees should grow like they do in the forest, vertically, stretching up toward the canopy to compete for sunlight. Our decorative street and yard trees, removed from their forest habitat, have grown wide and sloppy with an abundance of light. They have too many branches, and too many low-hanging ones. Thin them out! Limb them up! Clear enough space so pedestrians can pass.
It's great that so many new little trees have been planted to replace the dying dogwoods that once lined our streets -- but many of them are looking like pompoms on a stick. Don't let them end up like so many crape myrtles and Bradford pears whose trunks are a tangled mass of densely packed branches that should have been thinned out when they were young. Shape them to fit their space. Cut off excess branches so the tree will direct its energy toward growing up, not out.
There are guidelines for when and how to prune, but I mostly ignore those and just do it whenever a tree or shrub is looking like it needs a haircut. However, there are some useful precepts to keep in mind.
Regarding when, it's usually best to prune after a plant blooms so you can enjoy the flowers and fragrance first. For instance, some sources say to prune hollies in early spring before new growth appears, but I always wait till they've bloomed first, to enjoy their intoxicating fragrance surrounding my front door for several weeks; then cut them back soon afterwards before the new growth goes wild.
Regarding how, selectively cut individual branches back to their point of origin. Except for formal hedges, don't shear all branches to the same length. It's painful to witness a forsythia in spring whose branches, meant to wave gracefully in a wild profusion of arcing yellow shoots, instead have been sheared into a uniform lump, rendered a frustrated dancer with amputated stubs.
I look at pruning as a fun art project. It's natural sculpture, directed by you. You can shape and direct a tree or shrub any way you want. Plus, you get an upper body workout while communing with nature. What's not to love?
First, cut off any dead branches. Then stand back and look at the structure of the main branches. Which ones do you want to keep to maintain balance and beauty? A gracefully arcing branch may need to be preserved and accentuated. Which ones look out of place because they grow downward, horizontally, too close together, too low, or at odd angles to the rest?
Remove the most obviously unwanted branches first, then stand back again and look at your creation from all sides to see what now looks out of place. Each time you remove a few branches, it becomes obvious which ones should go next. Repeat until the shape looks balanced and there are no limbs sticking out where you don't want them, such as into the side of your house, impeding access to cars at the curb, or anywhere people might walk.
Consider the growth of the tree over time; don't leave any branches that will be in the way later. Around sidewalks, cut branches to leave plenty of clearance above head level. Near corners, cut greenery back so oncoming traffic can see each other.
A friend once imparted some simple but wise pruning advice: Leave only branches you want to keep. Those are words to prune by.
Besides, you'll want to clear your sidewalks before spider season.