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Hi Elizabeth, Your choices for a physical barrier are: concrete, brick, gravel or a special plastic that prevents rhizomes from pushing their way through (I used this out in our front garden which is a shared bed with a neighbour who has perennial weeds - i.e. bindweed).It's not going to be an easy job.
1 - First thing you need to do is dig a trench about a metre deep - dig at least .75m but 1m is best - around the entire area where the bamboo currently is growing (& you want to keep it there). Now choose one of these 4 options: 2 a - Then, you can pour concrete into the trench. The concrete should be at least 8cm thick & should go about 5cm above ground level (to prevent any runners bolting out above ground). It may freeze & crack over the years, so that may be an additional maintenance burden.
2 b - Stack engineering bricks all around & in the trench, preferably alternating them horizontally (with frog facing down) & vertically (soldier) in different rows so that the bamboo rhizomes can't get through. Like the concrete option, the bricks should go about 5cm above ground level (to prevent any runners bolting out above ground). The bricks may settle or get pushed around by earth so if you want to prevent movement, you would have to bed them anyway.
2 c - Gravel can be dumped into the trench. This may seem the easiest option, but this will require annual maintenance - you'll have to cut into the trench to chop off any rhizomes that try to get into the gravel.
2 d - Buy a plastic barrier, basically a pond liner (30 mil polyethylene which is .76mm thick) & place it into the trench with the shiny side towards the bamboo you're keeping. (It's kinda like when you line a pond, if you've ever done that before.) With the barrier & separate pieces make sure they overlap by about 10cm & seal them with heavy duty tape. And, as above, have the plastic barrier go about 5cm above ground level (to prevent any runners bolting out above ground). I found 2 d (the plastic barrier) the best option for dealing with perennial weeds, but it's still not an easy job to do. Just digging the trench is a lot of work. You might want to look into hiring a mini digger depending upon how big of an area you are talking about. (BTW if you're wondering why I know about all 4 of these options, it's because I took a City & Guilds Hard Landscaping course.) Good luck!
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