I like doing it in the fall after the leaves have dropped and when the sap is falling. During the winter is good too, but I try to avoid doing it in the spring when the sap is rising as that causes them to "bleed" more. That's only for major pruning though. Minor pruning to get rid of suckers or new limbs to shape the tree can be done at any time really.
This is a great thread. My wife was telling me to prune a couple of our trees yesterday. I told her that I didn't think it was the best time (she suspected laziness). I meant to do some Googling but this thread might just provide the answers I was looking for. CP rocks!
My gut was that I should wait until fall/dormancy.
All joking aside, I have had the best experience pruning trees in February/early March. If you can find a relatively warm weekend and there isn't too much snow on the ground, you can get two birds stoned at once and take down your Christmas lights and prune your trees at the same time.
Some trees don't apply to the "Monthe ending in 'r' " rule. I have a white birch and it should be done in July. One spring I decided to prune mine and I got one branch cut and it started bleeding sap like a waterfall. My neighbour, who used to have a white birch, saw me and came rushing out to tell me that I'll just kill my tree. They were told that the middle of July is best, as spring and fall is when this trees sap really flows.
It doesn't make too much sense to me, but they are the ultimate green thumb peeps, with a perfectly kept yard, so I decided not to temp fate, and only prune my white birch in July.
Google "branch collar" and try to find a picture of a tree close to your trees species. If you make your cuts to the base of the branch collar and are careful not to cut into the collar the tree will heal quickly. If we aren't talking about fruit trees three main cuts should be 90% of the work. Think big. Decide before you start if you are wanting an umbrella shape to your finished product or a cone shape. The rule of thumb is you can remove up to one third of the tree safely.
What is the benefit to pruning? I have a choke cherry tree in my front yard that is demonstrating fantastic growth year over year - I kinda like it looking big brash and bushy...
So does pruning do anything to improve the health of the tree? Can I just let it grow unchecked?
Pruning dead wood helps keep trees healthy. Beyond that pruning is all about shaping the tree. You can inspire new growth or slow the growth of your tree depending on how and when you prune. You can also control the direction of new growth to fill in holes or prune to avoid the tree growing into an areas you want open. If your tree has the room to grow and you like its look then leave it alone. The truth is any time you prune a tree you are cutting it. Also, trees unlike humans don't heal when wounded. They just try to compartmentalize the wound and grow around it.
Some trees should be pruned for their own good. As in certain apple and fruit trees. Many have quite a horizontal growth pattern. Add on a full load of fruit and the branches start to bend and be prone to breaking. And with the spring snow storms we get, those type of trees are more prone to breaking with the extra weight of the snow.
I have one crab/apple cross with this type of growth pattern, and I prune it quite regularily, in the fall when dormant. On the other hand, I also have a Dolgo crabapple with more of an upward growth and I don't need to prune it quite as severely.
Of course, if I dont pick the fruit in time, Mr. Bear comes along and prunes the fruit trees for me. He has proven to be a terrible horticulturalist.
We have 5 columnar aspens in our backyard. They are in a west to east line. The Western most one is about 30% smaller than the others. They were all bought and planted the summer of 2007. The westerly one has recently drawn the attention of wasps. I am thinking of getting a firepit and swingset. Seriously, there was about 50 wasps crawling all over it but not on the others. Any thoughts?