Hey guys, I just got a Japanese Maple and it is currently 4 feet tall. I plan on making this a bonsai tree! I was wondering when the best time to do hard pruning is? I want to cut it down to about 30 inches. My question is if now is a bad time to do it. I just got this tree online and its is currently the 23rd of April. Where I live, it is just beginning to get to temperatures in the high mid 50s, lower 60s (Fahrenheit). Should I let it grow out all summer and do this during the fall or could I do it right now before it starts to grow like crazy this summer? I'm in Zone 5 btw. Thanks!
Some shorthand on what you're aiming for with the initial bonsai training of a new unworked tree.
Beginners mostly consider the existing height of the tree as a starting point. Forget that--The height of an untrained tree when starting out bonsai training is mostly useless. What really matters is the bottom third or initial six to ten inches of the trunk up from the ground and the surface roots from the base of the tree. The tree above that level is mostly sacrifice (barring any great characteristics, like dramatic natural taper for instance) The first 6-10 inches of trunk and the roots emerging at soil level (the combination of those is referred to as "nebari"--there really are no equivalent English terms for that combination) are the first pieces of the tree that need to be considered when starting a new bonsai. They are the foundation on which the developing bonsai will rest. They set the initial image and mood of the tree for the viewer. They are the most important visual cues for a good bonsai. They are where the eye begins to view the tree.
A four foot sapling is kind of small if you consider all of the above. It likely has a trunk that is an inch or so at soil level (if it's a lot more than that--congratulations! That's rare and an excellent piece of material to start with...)
That moves us ahead to what you are aiming for in the "final" tree you want. That said, no bonsai is ever finished really though-it will continue to grow and you will continue shape it over time. The initial steps, however, tend to be the most drastic--trunk chops and sometimes radical root work.
Do you want a small shohin sized tree that will fit into a pot that you can hold up with one hand, or do you want a more substantial bonsai with a heftier trunk (bigger trunks induce the eye to see an older tree)? The end result is the product of how you proceed. Getting a larger trunk will entail allowing it to thicken, either planted out in the ground for a number of years, or in a slightly larger growing container. Either in the ground or in a growing container, the tree should mostly be left alone to grow as it will for a few years (three at the minimum to see any progress in thickening.) Growing it until the trunk reaches the diameter you want (along with time and increased diameter also comes a bit of character as bark and wood matures).
If the trunk is the right size for what you want in the relative end of the design, you can chop it--30 inches tall on a thin trunk, however, will make the eye see an immature tree for the most part. The height at 30" on a small diameter trunk will look kind of out of proportion. If you want a smaller tree, you will have to reduce it more substantially, like to 3-6 inches or so. The taller the tree the more substantial the trunk should be in general. That's even true for smaller shohin sized trees. Thicker trunks tend to make bonsai look more like actual trees to the eye.
Chopping now isn't really a problem. Maples backbud on old wood pretty well so you will likely get a crop of new shoots you can turn into branching and a new apex.
The nebari is more problematic. Containerized nursery grown maples typically have poor nebari (surface roots)as well as sometimes awful root masses. All that has to be sorted out. That process is done at repotting time in early spring. The time for aggressive root work has passed for the year if the tree has leaves or the leaf buds have opened. Trying to do substantial root pruning now will severely weaken the tree.
Bottom line, you have to envision what you ultimately want from the tree, then move ahead with a plan to make it so.
Photos of the tree always help us give guidance., as well.
