Hey Jimmbo, a fruiting trunk … with production above as well… Interesting Though keep in mind, as the top grows it shades out the trunk, so the shaded fruit may not ‘color up’ well, or continue to form at all. But I can’t think of any harm in trying…

I prune 3 or 4 ‘columnar apple trees’ for friends, and agree, they’re nothing special. But, they are hardy … and definitely consistent in their desire to grow straight up - and not out And, they’re covered with fruit spurs.

I, too, have had space, thus no reason to restrict the width of a tree. But, in a tight setting (with adequate sun..), columnar’s have likely earned a place. As far as using them as an interstim, from the vigor I’ve witnessed, they’d likely do fine in passing nutrients to whatever’s above, while producing along ‘the trunk’ for awhile at least.

If you try it - please keep us posted.

...one consideration may be any current plant patents on a specific columnar variety. Not an attorney, I can’t give advice, so proceed at your discretion.