I have had this healthy, now 7 foot Acer shirasawanum "Aureum" in our garden for the past 4.5 years. As it is now lots bigger than it was when I first got it I muse more about the "Y" trunk. Is there something I can or should do now to reduce the chance of it splitting when it is older and so larger?
I would at least severely shorten, but more likely entirely remove that branch given that you are already having to support it. The grain structure of the wood into a branch was built in when it was a little shoot. I've noted that fruit growers reportedly spray cytokinins to increase branch angles; hence making them stronger. This suggests good nitrogen fertilization might help (certainly it will affect stronger budding). I've also noted that pruning during the season tends to produce narrow angle branching, which suggests only pruning just after leaf drop or just as buds swell (which is probably what you do, since this isn't bonsai).
I knew I should have removed the green 'band' prior to taking the picture. (It is velcro that makes for an easy tie back for nearby clematis i.e. it is not supporting anything on this maple). It is the lowest main "Y" fork that I am thinking about. Can/should I do anything with it as a preemptive measure?
your shirasawnum Aurem is in good healht,is one vigorous cultivar ,if you prune ,Aureum probabily will grown more fast..
I second that, but I'm not sure there's a great risk of branch-splitting unless you live in a place with very strong winds or heavy snow in the winter. I've seen many Shirasawanum that had several low branches developping into "multi-trunk" trees. Maybe the right thing to do would be to prune the top so the lower parts are stronger compared to the overall shape of the tree...
