plants have shallow, wide rooting areas because they

a) like aeration?

b) like first dibs at rain and nutrition?

c) they just are like that?

The reason I ask is that I really like blueberries but I have a container garden in New Orleans; Top Hat is a coldweather cultivar and I find the local type blueberries only use the top eight inches in the containers. If I made a wide flat container that was permeable (like a big hardware cloth envelope) and lined it with burlap and filled it with dirt could I expect the blueberry to spread its roots through it? Or will it just try and fill the top eight inches again?

I guess I'm asking, can I espalier the roots? It would give me more room on my patio if I could.