What happens to pampas grass if it's not cut back annually?
Pruning Pampas Grass
Certified GKH Gardening Expert
You can see some die back from the grass if it is not pruned.
It can also become invasive if left to reseed on it's own.
I do know several gardeners that only prune back the Pampas Grass every other year with great success.
Here is a link with more information.