This situation is not uncommon for any grafted plant. The solution, as Sherwin
described, is to prune off the growth from the rootstock. I would suggest removing
about 1/2 of the A.palmatum immediately. Just pick enough branches to reduce the
leaf area by 1/2. Try to prune branches that may be shading the A.shirwasawanum.
Next spring when you are sure the A.shirwasawanum has survived prune off the
remaining A.palmatum.

I have to disagree with Sherwin about preventing the scion from producing its own
roots. For this type of plant it does not matter. For some things, dwarf fruit
trees for example, the dwarfing is produced by the rootstock. If the scion roots
the dwarfing influence is lost and the tree will develop into a full sided adult
instead of a dwarf. For your plant the rootstock is nothing but a root system. If
the scion also roots you just have more roots, a good thing.

--beeky

sherwindu wrote:

Yes, gradually prune the branches of the root stock, which will encourage the
tree
to send energy to the scion. You could also take some scion from this true
branch
and graft it to the root stock at a suitable height. Once it takes, that can
become the new leader and you can trim off the remaining top growth and the
original scion. You want your graft, or transition point to be high enough off
the ground, so that the base has no opportunity to start growing new branches.
You also don't want your
scion or true branch as you call it, too close to the ground where it might
establish it's
own root system, defeating the purpose of the grafted tree.

Sherwin D.

wrote:

Is there a way to correct a gtaft that allowed root stock to outgrow
the grafted limb?
I recently discovered that a Japanese Maple (Shirasawarum Autumn Moon)
I bought 4 years ago as a young plant in a one gallon container has
only one "true" branch near the base. The rest of the plant is a more
ordinary Acer palmatum, which is now three feet tall, while the Autumn
Moon branch is only about a foot long. Is there a way to salvage this
by pruning the root stock back?