It'll grow back, some, on it's own. I don't think ferts will either increase or accelerate it's growth, and if it does, it'll flop as soon as it gets tall enough, especially if you load it up with that much N.

How mature a clump is it? I'd be more concerned with fertilizing it for root growth, so it will be well prepared for winter, especially if it's not mature.

Just planted this spring, about 3 or 4 weeks ago. Landlord was on vacation when I planted it and this was the first time he'd been out to the place since I'd planted it. He thought it was the weed that HAD been growing in there before I pulled it out and replaced it with the Pampas Grass.

Sojourner

I'd give it a light dose of N, a good charge of P and a moderate K now, and water it religiously until fall. Then, in September give it another dose at about the same levels, heavy on P, moderate on K and light on N. Mulch it well if you don't have reliable snow cover during the coldest months of winter. Set some pine boughs (your Christmas tree will work) over the clump, and fill the boughs with more pine needles or whole fall leaves.