So, you're getting plants that are fairly well established as they're 3.5-4' high, the growth from them will happen over another year or so and their ultimate height will be 4-6 metres !

If plants are happy where they are though it's not unusual for them to just keep getting bigger.

A warning - this one spreads via runners, so be sure to contain the roots in some way, either by using root rhizome barrier, or using a smaller dustbin with the bottom cut out and inserted vertically into the soil, in which you then plant your bamboo - one for each. Needs to be a barrier 18 inches deep minimum, or it'll take over your garden after 5 years.

I have 2 clumps of each that I planted 7 1/2 yrs ago. All the root balls were in about 12" wide. They have settled well and are forming lovely clumps. Each clump is now about 2' wide in diameter. I have never found them to be very invasive, They behave themselves well, but it could be that my location doesn't allow them to go nuts. So saying that, I did have a customer who moved into a house where 7 bamboo plants had been put along a 12' fence, (you couldn't see the fence) which was on a bank, with a waterfall that leaked. Her bamboo 'aurea' were walking down to her pond, and they were a real bugger to remove, especially under tough membrane that was on the bank.

I'm in the South 2ndhand - planted 2 of these in a rectangular bed set in paving in a garden 5 years ago. I used thicker weed membrane inserted vertically - what a waste of time, the bamboo's punched right through that and is now popping up all over and six feet away in the gaps in the paving... nightmare...

I agree bamboo, the cust. i had was 300' above sea, but up here at a 1000' it seems to behave itself.