Does anyone know what this is? It has appeared in my garden over the last couple of days and seems to be growing at a quick rate
You can query "bamboo from neighbour's yard" and read lots of advice. I have a friend who breaks off every single shoot whenever she sees it. She's been doing that for years, but there are very few coming up now. She catches them all at the size you've shown.
I believe that as long as the parent plant is there, and no barrier is in place, this problem will persist indefinitely.
