Thanks everyone for your kind advices.
My apple tree is the Scrumptious variety, from Ken Muir nursery. I chose this one because is resitent to the cold weather and ideal for Northern areas. Althought it's self-fertile, I also bought a Laura Crab Apple to be its pollination partner. According to the nursery this crab apple could be planted in a big planter so that's what I did.
Both trees arrived on the worst time possible, right after the snow blizzards, early Feb. I kept them wrapped up and checked the roots everyday, soaking it for half an hour and packing it up again.
When the snow melted I could resume digging the hole again. I did cut that unwanted root as far as I could and I made a much bigger, deeper hole. Then I hit a layer of rocks...
Almost broke my back removing them all. I added loads of organic compost (couldn't find manure in my local garden center...), some water crystals, planted the maiden tree and mulched.
Ta-daa:
The stem union is still visible, can you see it? The rocks around the tree are few of the ones I had to dig out from the ground. I kept them to remind me of the hard work.
It's still very very close to the leylandii (which is approx. 1.8m high) but as I said before, my garden is small and that was the only south facing, free spot I could plant the tree...
And here's the Laura Crab Apple:
What should I expect now? I mean, how can I know if the tree is OK of if it's dying...? Would I only know in the spring? Since I planted it we had really strong gales here and the tree was super fine. Yesterday I cover it to protect from frost. Anything else I could do for a new planted tree?
Thanks again everyone for taking time to read my post and reply.
Marcia