They are called black flexible water pipes.
I like them ,because they are precut and I can put them in my car without looking like an idiot driving through town with PVC pipes sticking out of my back window.
Although, I still buy the PVC pipes sometimes - seems like I can't avoid looking like an idiot - at least occasionally.
OK continuing with the tutorial!
I also used a few branches, cut into 8 inch lengths, to hold the pipe in the soil, but I think I may end up using some kind of bracket instead, because the hoops tend to fall over when it rains and the soil gets soft.