It isn't a clumper. To use the proper terms, there is absolutely no doubt that Semiarundinaria is leptomorphic, not pachymorphic. A large number of bamboo sellers in Britain say that plants from genuses such as Phyllostachys are "clumpers", because the conditions in Britain mean that these species are not as rampant as in places with conditions more suited. But grow them in Cornwall and things might be rather different. For example P. aurea is usually very well behaved in the average British garden, but it is a rampant noxious weed that has taken over entire hillsides in the Azores.

At least one seller in Britain markets S. fastuosa as an ideal hedging bamboo precisely because it is a runner, and precisely because if you contain it on two sides it tends to run off in a straight line in the uncontained directions.

In botanic gardens, it is common for the bamboos to be well contained with underground rhizome barriers. Perhaps this one has managed to escape its containment.