I think the main problem is that, whether you put in barrier or not, at some point the rhizome mass will start pushing against the concrete blocks causing the wall to start splitting at the mortar joints. In addition, at some point the planting will get rootbound and it will be nearly impossible to remove divisions to give the bamboo room to continue growing. It's a very nice planter, but I think the only solution to keep it looking good is to put the bamboo in utility pots that are as large as you can fit in the planter. That way, when it starts expanding too much and needs dividing, you can simply lift the pot out, do the division, and put back the repotted divisions. It may not look quite the way you intended, but it will save you a lot of work and heartbreak in the long run.
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Rebecca- It’s never a good idea to post your email address on a public forum, especially one this big. Instead, you should go into your account settings and make sure you’ve enabled the messaging system for these forums so that other members can message you privately and securely.
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