Planted A Very Small White Pampas Grass Last Year, It Grew Up To 4ft, But Didn’t Bloom ,was It Not Mature Enough?

Question

The plant when planted was only about six inches tall.

Answer 1

It likely was not mature enough as you say. It will probably bloom this year, if it comes back. You are right on the edge of its hardiness zone. However, some perennials take three years to bloom.

https://www.gardeningknowhow.com/ornamental/foliage/pampas-grass/growing-pampas-grass.htm