Yes, you can repot in fall after the leaves fall. But you are still risking a hard freeze and damage to the roots if left out through winter regardless when you repot. JM roots are extremely vulnerable to cold temps below around 25F........is the container large enough to pose no threat of roots being exposed to those temps (or lower)??

IME, you just run a very big risk of cold damage to any JM's (or their other Asian cousins) grown in containers year round in climates below zone 8. Can you not plant it in the ground?

So bottom line in zone 6, maples in containers should be moved inside (unheated garage etc) during the winter?

Perhaps I will rethink my container choice or else as you say put in the ground.

thanks

Since there is a strong likelihood that winter temperatures could hit 0F or below in your area, yes, the maple will need winter protection. Unless the container is much larger than I am imagining :-)) Even then, I doubt I would risk it.

FWIW, the rule of thumb for permanent container plantings is to select plants that are hardy at least a full 2 zones lower than yours. Since Acer shirasawanum, japonicum and palmatum are listed only to a zone 5 at best, they don't quite make the cut.

Best would be to put it in a conventional container as large as will fit in your planter box, then lift it each fall and over-winter where it's protected. Then, you can repot in spring just as buds start to swell. FWIW - 6" soil temperatures never drop below 25* here in Michigan, no matter how cold it gets - even in the middle of a plowed farm field with no cover crop. Large parts of the heavily forested UP of Michigan haven't seen frost in the ground for hundreds of years. Is the planter in a box attached to the house? How much of the soil in the box is above grade?

Al

thank you both. I decided to follow garden gal's advice and plant the tree in the ground. Too much sun where the planter lives anyway!