I had a similar problem in some upper branches and decided it was dieback. The outer layers kept reducing and yet to cut it out would have ruined the shape. I had to just let it run its course. At least the upper branches hid it.

Why don't you contact "Flemings" (Monbulk/Silvan) as I know they wholesale a lot of weeping maples and perhaps you may get some better advice than the local nursery. You need some specialist advice. Good luck. I would be interested to hear any solutions. The last time I posted a message about Jap weeping maples I had only one response. This forum seems a bit slower than some of the others I think.

Is it only the bark that has split, or is the wood underneath splitting as well? Has the bark seperated from the wood, or pulling back from the split?

It could be frost damage.

It will be too late now, but if you wrap the bark with grafting tape, cover with hessian or cloth and keep moist the bark may heel itself. If left too long it peels back on itself, it will eventually stop, but will leave a large scar.

All you can do now is just wait and see.

There are some people you could call.

Raraflora in Berry NSW or Simon Chartres Acer Spades Nursery.

Good luck,

Paul

Thanks for your help, will try the tape and I shall also call Yamina Rare Plants from where I got this tree. Thanks once again.