And old thread but I was thinking about this the other day. Since I work around a lot of clay type soils. I learned some interesting things here, I didn't think that the roots grow out more than down...I thought they just push their way equally but run into competition growing latterly. I don't think wide pots are a wise idea indoors, since square is the way to go and that width is usually about plant numbers...but some grow bushes indoors with large containers.
I have these holes outside, that look so stable from the clay type soil. It's made me think about what it is going to be like, digging out root mass. I haven't put enough effort in out door experiments to really see fact verses fiction. The current holes should be perfect to see where the roots get to.
The holes are about as deep as my fist to my elbow, and about as wide in a circle. Large enough to put both feet in and have it at the very base of my knees, some holes are larger. I'm glad I read about the 'drip zone' so I know how far to place top dressings. That is very helpful since top dressings also help keep away tender weeds from invading.
I think I will get a root ball down to the bottom of the hole and some past that, but it gets pretty thick with clay type soil...it's not pure clay, but it's not draining well. If your roots aren't growing deep, it may not be draining well down there....that is a thought.