purple fountain grass
The foliage has turned brown; plumes are sparse. Is this a seasonal thing or are the plants in distress?
Your Fountain Grass is reaching the end of it's growing cycle and is starting to go dormant.
You can leave the plant as is and allow it to die down naturally.
You can trim back the plant now or when you start to see new growth.
It will began to grow again after it has rested.
Most regions grow the Purple Fountain Grass as an annual but it is a perennial in your area.
Here is a few links for you.