It will spread on its own. Yes keeping it on an island will slow it down but my understanding is that it will spread thru means other than simply splitting. That said, You can keep it many people do. Where they make the mistake is they often see a new piece elsewhere in the display and leave it be because it’s kind of interesting that it did that. But if you remove the strays you can successfully keep it in check. However if you aren’t diligent it can take over.