Q.Planting Zone
I am planting in a Zone 5a and the plant Feathered Reed Grass is zoned for a 4. I would think I’m more than safe in planting this variety up north at my cottage, am I correct? It will be under an evergreen that has lost it’s branches to about 6ft. I thought these would make a nice filler. Any other suggestions? Thanks so much for your expert help!

Certified GKH Gardening Expert
Is your evergreen alive and well? If so, I wouldn't put the grasses right under it. Trees have roots within the top 6 inches of soil that you don't want to disturb with digging. I always recommend adding mulch under a tree rather than plants that also compete for nutrients and moisture.
However, a curved row of grasses not too close to the tree would look nice, and hide some of the barren ground.