If I was attempting this... I would expand that bed...
Attempting to place tree as close as you've drawn... would seem doomed to failure as lucy68 pointed out...
But if you got rid of some more turf... Or better yet, expanded all 3 beds so that that patch of turf in the middle was gone... I think you would have a happier island, whether you planted the maple or not... you would certainly have room for the new tree...
Hmm... Seems like you have a different side in mind... Still, the same advice... expand the bed... all the way around... I think you'll be happier, and the maple would have a better chance out away from that huge hemlock...
What kind of watering setup do you have there?
I'm kind of wondering whether it will even be possible to keep the new plant watered enough... Those island beds where everybody wants to plant... tend to be dry...
