if you get in touch with a bamboo specialist they see a bamboo membrane thats aprox 30" wide and as long as you need sold bye the metre . if you cut the runners of that are as you say 20` away so your left with the mane plant you put the membrane round that . the bamboo only throws these shoots up in about the top 6" from the top and sometimes above . id measure round with your garden hose about a foot out or youl hit roots . add a good overlap of about a metre then its the hard bit but not as hard as digging it up . you have to bury the membrane down nearly all the 30" leaving half an inch hanging out the top . when i did it i slightly sloped the top of the membrane out and the bit buried the deepest slightly in helping any shoots to come up rather than come down . you will have to just keep an eye out for any shoots going over the top and cut them of otherwise it will be fine . i hope this helps and theres know easier way im afraid but still far better than digging the whole plant up and you get to keep what is an atractive plant so its not all bad .

many thanks.

your more than welcome .