Q.What Happens If You Don’t Like The Plumes On Zebra Grass And You Cut Them Down Early, As They Begin?
I don’t want my zebra grass to grow any larger than it is now. Thought if I cut off the plumes that would help slow it down?

Certified GKH Gardening Expert
Are you cutting it back to about 6 inches in early spring and dividing it if needed? If you don't want to accelerate any growth, don't fertilize or add compost. But if it is really too tall where you have it, you could relocate it and get something that won't grow as tall.
You also could try cutting it back halfway midseason but that is painstaking work if the clump is large. And that would ruin its graceful look.
https://www.gardeningknowhow.com/ornamental/foliage/maiden-grass/care-for-zebra-grass.htm