Probably not necessary, but if you want, you could just slit it down one side and let it come off on its own.

I'd advise affixing a stake to the rootstock trunk and loosely tying the main leader to it - a gust of wind or a bird alighting on that rapidly-growing shoot is all it'll take to break that graft right off.

Thanks for the advice, I went ahead and secured it.

Anyone else seen this kind of growth with a mulberry graft? Its now 22 inches long and still growing like crazy, more side shoots too. In a climate where many of the native trees put on only three or four inches of growth all year, to see this single bud jump into action like this was pretty exciting. I did see a reference to IE mulberry as having "extremely vigorous young grafts" in the book Cornucopia.

It also look like success with 8 other mulberry grafts, on in-ground trees, but they were done later and only have grown a few inches. In any event, I'm planning to dig up as many weedy Morus alba saplings as possible next winter, to use as rootstock for more IE grafts!

I got 8’ in one season. IE on a seedling sucker (so good roots pushing nutrients).