Cortaderia selloana....

.......or pampas grass.

This Andean native looks so out of place in a suburban garden, but is truly astonishing against a big blue sky.

Just like it gets in the high Andes.

(Later edit)
Now I've got a bit more time, let me tell you about this particular clump.
It is very ancient, and has separated into six distinct "sub-clumps". It is a variety called "Sunningdale Silver" and the flower-heads end up being around 2.5m tall. Every other year we cut it back really hard and burn the stumps to clear out all the papery dead stuff.
It just keeps on coming!