Here's our thoughts on pruning, based upon our knowledge of pruning techniques in general, our nearly obsessive interest in Japanese maples and Japanese-inspired gardens, and our knowledge of growing these trees. This page is not a course on pruning Japanese maples but some food for thought. Furthermore, this isn't a crash course on pruning techniques and tools either. Those are important, and universal skills, but knowing when or if to use them is a harder thing to learn. And those skills are beyond the scope of this page.
Dead branches can come off. Anytime, always.
If branches are rubbing on one another, one of them can be pruned off. Eventually those branches will be larger and the touching of those branches can do a number of things. It could open the bark to disease, the branches could actually grafts themselves together and, the shape may just look bad.
Learn what a branch collar is and never cut into it. Cut up to it but not into.
Depending on the shape of the tree, branches that grow in towards the centre can be pruned away. But there are exceptions, specifically as it relates to the shape of your tree.
Clean your tools with isopropyl alcohol between each tree you work on. And use sharp tools, the appropriate size for the size of wood you want to work on, as well as, the appropriate tool for the task.
Japanese Maple Specific:
Young trees: A lot of people see a long, willowy branch of new growth coming off their young tree and instantly think it needs to be pruned off because the tree looks unbalanced. Unless you are creating a bonsai, this doesn't apply to you. Young trees should not receive much pruning in the first few years. If you want to see your tree grow up strong and healthy, let the tree grow unless pruning for the right reasons such as, repairing/removing a defect, branches growing towards a barrier, dead wood, or a portion of branch tip that is killed off by winter. There should be minimal to no pruning in the first few years of a Japanese maple's life.
Some Japanese maples have short internodes (space between sets of buds), and when you prune some of those types you may end up with very bushy growth. This applies to many but not all dwarf types. Too much of this can really transform your tree, in the wrong direction.
Japanese maples are known for their beautiful shapes. So why make them look like all the other bushes in the landscape? We've seen people take hedge trimmers and shape their tree like a lollipop! We can appreciate this to some degree but it can look really, really bad. Especially, when it gets trimmed to be square-shaped in the middle of that shopping mall parking lot, where it should never have been planted in the first place! Yes, have actually seen this.
Typically, Japanese maples have two-bud sets. In other words, at every place you see a bud there is a matching bud, hence, a bud set. From each bud there is the opportunity for a whole new branch to grow from that location. Furthermore, the bud set above below or above will grow in a slightly different orientation. This is where you have the opportunity to remove branches that may grow in a direction that eventually may be in conflict with other branches or your garden fence or other.
If you are really on top of things, you can rub out (between finger and thumb) a bud before that even has a chance to grow into a new branch. Again, this should not be done on young trees for the most part.
Weeping forms are kind of like umbrellas. They block the sun from reaching the leaves under the upper canopy. Therefore, as these trees age there'll be a lot of branches under the canopy that naturally die. Prune these dead branches out. Before you shape that weeper, tend to these branches first.
Do not prune more than 1/3 of a tree's canopy away in a single season.
It's okay to prune out one of the competing branches that extend from the same bud set. Often they are growing almost parallel to one another and the joint (bud set) they extend from is not structurally sound in the long term nor is it necessary to have two branches side by side.
Please don't use the pruning technique called pollarding on your Japanese maple. Ever! This may work on many other types of trees but it becomes a royal mess when it's done to a Japanese maple. It ruins the beautiful shapes we associate with being a Japanese maple and it creates all kinds of new growth essentially giving your more and more to prune. It weakens the structure, too. And, opens the tree up to disease every time the loppers or sheers are attacking the tree.
Likely the best time of year to prune, particularly with major structural pruning, is late winter but not right before the trees are leafing out. Trees heal the fastest in late winter. This is before the sap is rising to give us that wonderful spring display. For most climates, this period will be March to April.
Generally, pruning of wood 1/4" or less can be done any time of the year. Pruning is also done in mid-summer around July and August but be aware that thinning/pruning during hot periods will expose leaves and branches that were not previously exposed to the sun. This can scald the branches and scorch the leaves. Look at the weather and plan accordingly.
You don't have to be a professional to prune your tree but you should put some effort into educating yourself on the process before picking up those sharp tools. Once you've cut into the tree, you can't take that back!