What variety of bamboo are you planting? Do you know the specific name?

Kt

Yes, I have settled on Phyllostachys Aureosulcata and Phyllostachys Bissetii

I would think that in your zone, that 14 inches would be enough to contain Aureosulcata and Bissetii.

Kudzu9 can tell you more about trench containment, since I know he does rhizome pruning and he would know more about the depth.

Here, my Henon needs no more than 16 inches. All others need less in my area, and all of the phyllostachys can run wild here.

Good Luck

Kt

I've only grown several varieties of Phyllostachys and for only 5 years, and in Zone 6. I've used trenching for the past year, and I've never seen a rhizome try and cross an open trench deeper than about 4 inches below ground level. My deepest trench is about 8 inches deep and 6 inches wide. In areas I had not yet trenched, it was easy to locate where an 'errant' rhizome crossed it's allocated but imaginary boundary by following the new clums, and the rhizomes were usually very shallow. I leave the trenches open. Barriers have to be much deeper, as rhizomes often try to grow under them. They don't seem to want to grow under a trench.
Hansel

Hansel, that is correct. A rhizome will reach a 'dead-end' and try to go around it. Rhizomes have a tendency to grow downward before upward unless it grows against something that curves its growth upward. Most rhizomes will try and grow down, then underneath a barrier although I have had some try and go over the top.

A barrier will make rhizomes grow deeper than they would without one.

Kt