Summary of below: Don't fear using bamboo with moderate spread, it’s important to give you coverage. Controlling the spread is simple and effective. But if planting in a confined space or right up against a fence or you simply want a maintenance free solution consider using a root barrier.
Spread on the taller varieties is a reality, all but the Narihira have some spread. The Royal Horticultural Society states that the Phyllostachys varieties (most of our taller bamboo are Phyllostachys) 'are classified as spreading bamboo but in cooler climates such as the UK they behave like clumping bamboo.'
But spread shouldn't be thought of as undesirable infact it's really important if you are looking to create coverage. If you take our advice and plant at one bamboo per metre, you will need some spread from your bamboo to completely fill the spaces in between.
If you choose for the best behaved most clumping, least spreading bamboo, it will take quite some years to fill your spacing. But if you choose one of our Tall Bamboo' with moderate spread you will have a bamboo that will fill your spacing in in a much more reasonable time frame.
It's hard to be specific in terms of time as it depends so much on weather, ground and growing conditions. But to give you some idea, some of our taller bamboo are so bushy that even planted at a one metre spacing they will straight away be touching their neighbour as it fans out further up the plant. Using these bamboo and you would expect full coverage by the end of the following season.
Spread in unwanted areas is easily controllable by pruning. This is usually a twice yearly job, in early summer and again if necessary early Autumn. You can use secateurs or even a spade to cut back/chop of unwanted spread.
If planting right up against a fence, next to a patio or against a shed or even a pond we would recommend using our root barrier. In these spaces access for pruning becomes more and more restricted as the bamboo grows. So ideally try and plant a reasonable distance (70cm) away from such things. If you can’t, really do consider using root barrier.
So if you are planting where access may become a problem over time or you simply want a maintenance free solution use the root barrier. Yes it costs a little more to start with and there is a bit more digging involved but it really is an effective long term solution to preventing any unwanted spread.