I live in San Francisco and want to plant bamboo in a 3 foot wide by 15 foot long area that is between an 18 inch high retaining wall and the foundation of a building. To be clear, the soil in which I am planting the bamboo, is at the BOTTOM of the retaining wall. At the top of the retaining wall are the flagstones that comprise my patio.

If I do not put in a barrier, is it possible that rhizomes will go under the retaining wall and come up through the flagstones? That would be bad!

The foundation of the retaining wall is not deep, so that is not going to keep the rhizomes out. The retaining wall is 12 inches wide. So to reach the surface, the rhizomes would have to travel 12 inches horizontally and then 18 inches vertically. Can they do that? Or do they "give up" when they grow vertically and don't find the surface of the ground within a few inches.

Thanks to anyone that has knowledge or experience ....or both!

Conrad