columnar apples

I'm low on space. I also have some issues with buried utilities that make planting trees dicey. I've been looking at columnar apples. They're pretty spendy and everything I've seen in print requires two different cultivars for pollination. That said. Most providers have recommendations about pairings. But most only carry a few and sometimes cultivars are not available in the same season. There doesn't seem to be anything that looks at columnar apples in aggregate in order to be able to make decisions between providers. For instance, Supplier A might carry varieties 1 & 2 said to be suitable pollination companions and Supplier B carries variety 3 & 4 nothing same for theirs. I haven't seen anything that says 1 and 4 might be suitable for each other.

On another note, I wonder if the old adage that roots spread to roughly the drip line of the tree holds true with columnar. Does anyone have experience digging established columnar apples up?

Hello Echo,
My parents had a row of cordon fruit trees - apples and pears - as a dividing screen in our garden. I don't think that the pollination requirements will be different for a columnar trained apple as opposed to a bush or free trained tree form. Normally you need another tree with close flowering times, although some apples will set some fruit without a partner, you normally get more and bigger fruit if the flowers have been pollinated. If you have other apple trees in your vicinity (either cultivated or crab apples) that may be enough to pollinate your apple. The closer the other apple tree is in time and space, the more likely they are to pollinate each other. here is a table for pollination groups for UK apple varieties. You should be able to find something similar for your region of the US (I suspect flowering timing may vary in different climate areas). I personally wouldn't buy trees from someone who couldn't advise on basics of cultivation like that.

As regards the root form, I'm guessing a bit, but the rootstock is what makes the tree less vigorous (which is what is required for a cordon or columnar form) therefore the roots will be stunted compared to a more vigorous tree. They will still radiate in a circle unless constrained by a wall or similar. That used to be how orchard trees were restricted in size before rootstocks were standardised - the trees were planted on a layer of rubble to restrict their roots and stunt their growth, thus favouring early and heavier fruiting.

THANKS!

Due to differing flowering times, several of the suppliers have specific pollination partners. I guess what I'm trying to get at is it may be advantageous to plan on some diversity in partners but there doesn't seem to be a source looking at the field of columnar apples as a whole... Rather, the small info one can get is within each supplier and other than early, late, etc. it's not specific enough to plan on a wider selection which might be compatible. A couple of weeks for flowering would make all the difference.