Pruning plants is like getting a haircut. If you do a good job, it grows back. If you do a bad job... it grows back (except for certain types of evergreens). Relax and pick up the clippers, it's haircut time!
Pruning is done to reduce the size of the plant, to improve it's shape, remove dead or diseased wood or improve flowering and fruiting. It involves removing some parts of the plant such as branches, buds, or roots.
REMEMBER THE RULE OF THIRDS
One of the few rules to keep in mind is the rule of thirds. This rule says that you can remove up to 1/3 of the volume without doing any harm. This could be 1/3 of the total height of the plant OR 1/3 of the total number of branches of the tree or bush. Following this rule gives the plant a chance to regrow in a healthy and normal manner.
Now that we know how much we can cut off, let's look at what parts of the plant we want to cut off.
2 MAIN TYPES OF CUTS
You can either thin out your plants or head them back. Which one you choose depends on what you need to accomplish.
Completely removing some branches where they join larger branches improves the air circulation and light penetration. This cut encourages the plant to rejuvenate itself.
Heading back cuts are made when you take the shears and cut off all the tips on the shrub to make it into a smaller, more defined shape. The plant responds by branching out and getting thicker. There is nothing wrong with this type of cut. However, if this cut is used exclusively for years it creates a dense woody mess and the plant can begin to die out in places. Try alternating or combining thinning and heading back in order to provide the best of both worlds.
WHEN TO PRUNE
There are specific rules for when to prune old wood and new wood bloomers... You don't need to bother remembering them. All you have to remember is this:
Prune plants just after they finish flowering, whenever that is.
Your Lilacs bloom in the spring, so prune them in late spring after they are done. If you have a hydrangea that blooms in late summer, prune it in the fall. If the plant isn't a kind that flowers, or you don't care about the flowers it produces (like shade trees) prune it whenever you feel like it.
You'll never go wrong.
Evergreens can be a little different. Most don't tolerate heavy pruning well. Light, frequent pruning several times a year produce much better results than neglecting the job for years and then lopping off 3 feet. Evergreens that are sheared to exactly the same size and shape for many years begin to get thin and woody. Bare spots appear and eventually won't fill in. To prevent this, allow the plant to grow out a little from time to time and then begin pruning again, but leave some of the new growth behind.
Once evergreens have become excessively large for their spot, they are nearly impossible to shrink back down with pruning. Thinning out is harder to do on evergreens, so it's usually best to just replace them.
When this happens consider using a modern dwarf variety to prevent this problem from happening again.
Just please......avoid mistakes like these.