I'm just getting started with slender spindle training myself (I planted most this spring) so I don't have a solid answer for you. I don't think that rootstock much larger than M26 are recommended for high density plantings.

But, one thing that would keep the size down would be branch bending to horizontal. If you bend the branch down when the branch is short, it will tend to stay short, so you can control how wide you allow the tree this way.

On some of my trees, I waited a bit long to bend the branch down and they are now 3-4 feet long, which is non-ideal for a 5' spacing. Luckily I was able to bend most of those longer branches outwards, rather than at their neighbors. Even so, I'll be more vigilant next year and try to keep them to about 2'.

I do have one tree on a full-size rootstock (planted in spring 2010, when I really didn't know much). It was advertised as "semi-dwarf", but from everything I've read Antonovka is pretty much fill size. I've been branch bending and have managed to keep it reasonably narrow. We'll see how long I can maintain it that way.

What is "the renewal pruning system". What is the variety or varieties of apple? How much space do you have specifically?

All pruning on a bearing tree that has filled its allotted space revolves mainly around spur renewal pruning. If you can bring any tree into bearing in a give space, I believe you can keep it there.

one of my Liberty apple trees. I planted it in a space that may be a little small for semi dwarf. I can make the space bigger but would rather prune it back. Renewal may be the wrong pruning word. Similiar to the tall spindle pruning. Never allow the branches to get more than a half the size of the main stem of the tree. Cut the branch off but leave enough bark for a weaker more productive branch to grow.

Yes, I get it now. By half the size you mean half the diameter, which does keep a tree fruitful in a smaller space. For free standing trees on M7 and up, the usual ratio to accomplish this is 1/3. Remove all branches more than a third the diameter as the trunk at point of attachment. You can also choose a leader for each scaffold and treat secondary branches with the same math.

Liberty is only moderately vigorous, so that helps. I find it to be somewhat reluctant to provide adequate secondary and tertiary branches however. Doing fine pruning around petal fall can be helpful for this. Secondary branches that start too thick can be subdued by stub cutting at this time and turned into useful pieces.

Are the trees already fruiting? There's lots of tricks to keep trees small and get them fruiting early.

All three of my liberty gave me apples the second summer in the ground. The one I am speaking of gave me over twenty apples. I had to cut back five on the top of the tree because it was sagging over. lesson learned! Don't be greedy the first few years. I think as a home gardeners tree it is good but won't be replacing macs or the like. To me it seems like a crazy grower with lots of growth and branches.

They are crazy growers but only moderately vigorous. I get the chance to watch them side by side with many different varieties. By crazy, I mean that they don't aspire to architectural beauty as branches zig-zag and scaffold don't put out enough preportional secondary wood.

exactly what i'm seeing. I'm glad i joined this site today.They are already the ugliest tree with crazy growth. That's why i thought renewal pruning would work well with this tree it seems to want to grow alot of branches.