Students that have taken my class will probably remember how disdainful I was about (well, lots of things, but in this case) books on pruning. They probably still line bookstore walls, but I avert my eyes fastidiously, so don’t quote me. Large tomes on pruning trees and shrubs, each plant type seeming its own chapter. How do I prune a lilac? A maple? A ninebark?
The books are worthless. A little plant biology under your belt, some tools, and we’ll have you all set to go in no time at all.

Timing

George Aiken famously said the best time to prune was “whenever the saw is sharp”. He’s close to correct. There is no bad time to prune, only better times. Most orchards are pruned in the winter. Why? Long winter, nice to get outside. We prune our shade trees on campus during the winter as well, and for the same reason. The only time I would not recommend pruning is early spring, as the buds are swelling and starting to pop open. Trees and shrubs are working so hard this time of year, pushing new growth without their leaves ready to replenish themselves by photosynthesis, so they are using all their stored energy, and could possibly be weaker and less able to recover from pruning. That being said, if that’s the time you’ve got, then so be it.The evil pruning books will also go on about timing, giving large charts about when to prune various flowering trees and shrubs. One rule of thumb will cover all that, though. Prune immediately after flowering. Spring flowering things, like lilacs and crabapples, have set their flower buds last year, so a winter pruning will cut them off before they had a chance to flower. Summer and fall shrubs will probably flower on new wood, so pruning them with the lilacs in June will probably cut those buds off as well. After flowering covers just about everything.

Tools

Anything you got that’s sharp will be just fine. I prefer Felco pruners (http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=felco+pruner) and Silky handsaws (http://www.sherrilltree.com/Professional-Gear/Silky-Saws) but this is what I do, so I’ll spend a little more on something that cuts well and will last. Try and avoid what are called Anvil pruners, the kind with the blade that stops on a flat piece of metal, as opposed to moving past the bottom piece (bypass pruners). The flat piece of metal crushes the live wood around the cut, possibly injuring the branch protection zone.
If you feel like you need rope to climb a tree, or a chainsaw to prune, back away slowly. You’re reading the wrong post. By all means don’t use a chainsaw on a ladder or in a tree. Spend some time on You Tube if you don’t believe me.

Technique

The hardest thing to explain is the necessity of a plan. Why are you pruning this plant? Is it in the way? Funny looking? Too tall? Most plants really don’t need to be pruned, we just think they do. Probably because we planted them in the wrong place, or they have a hazardous condition we would like to remedy. But before hacking away on the poor thing, you owe the plant a couple seconds of your time to stand back and make a plan. And step back often while pruning, checking yourself.
Starting in, remember the 3 D’s, Dead, Diseased, and Dumb. Start by pruning out all the dead wood from the plant. It’s dead, the plant doesn’t need it. (Maybe the ecosystem does, but that’s another blog post.) Also, this prevents you from making a bad mistake a little later on. I’ve pruned much live wood out of a tree to free up a beautiful branch in a direction and location I liked, to later find that branch was actually dead. Once you’ve gotten everything dead, start looking for diseased wood. Little white fishscale fungus, bark peeling away, there are all sorts of little clues that tells you that branch is probably on the way out and will join the ranks of deadwood shortly. Like truly dead wood, no sense to save it, or to count on it when pruning for structure.
2 D’s down, now the fun one. It won’t take a PhD in plant biology to recognize that plants aren’t the most intelligent creatures out there. Sometimes a branch will grow straight down. Cut it. Sometimes a branch will grow right into its neighbor. Cut it. Look for branches that are crossing, rubbing, growing in the wrong direction, growing parallel to one another, or anything else that just plain looks dumb, and get rid of it.
Now that you’ve gotten everything that shouldn’t be there, you can start to prune for structure. That’s a little more than an intro blog post will cover, but here’s a good start. For trees, read http://na.fs.fed.us/spfo/pubs/howtos/ht_prune/htprune-rev-2012-screen.pdf . Pay particular attention to the section on where to make a proper pruning cut. For shrubs, a good basic rule would be to remove any large woody stems-plants bloom and look better with young, vigorous wood. Like lilacs-every year I like to take out a stem or two that are larger than 2”, leaving the younger thinner ones. This has the advantage of keeping the plant a little smaller (so you can reach the blooms to cut them for a vase), a little less gangly, and a little more manageable.
Our department will be pruning trees in the library quad for the next month. Feel free to stop and ask questions.