I agree. It would be a never-ending battle to keep your plants alive if you plant roses in the root zone of a thirsty tree. Large, bottomless containers would be a better bet in such a small area, but they'd still need full sun to flower well.
I do use empty jugs in the bottom quarter of my large containers -not packed full, but with areas between them for soil to make a conduit for roots that do want to progress downward into native soil.
Even just a few inches of soil piled over your tree's entire root zone can kill it. A few tall and narrow containers that cover smaller sections are probably okay, at least I find that to be the case here.
This is one that we built about five years ago. It's a long one, but it's roughly three feet tall by twenty-four inches wide. Something similar to this might be really nice in between bench sections.
