I don't think your plant will reach full size in the remaining time. It sounds like the two 'Fireworks' were small starts which were stuck into trade gallon pots by the nursery, and probably not properly grown on.

I have 3 PFGs which I bought from Santa Rosa's clearance sale and planted out in July. They were in 4" pots. I installed one in a planter and two in the ground. They are all full size now and heavily plumed. Our weather has been unusually hot, so they've been growing like weeds.

Thanks Donn.
I saw over 50,three foot size at the local store last month but they were all brown. Was that last years growth from a hothouse? It was $13.00 in a 5 gal. pot. How does it grow? I thought it still would need time to give new colour and plumes so I passed.

I can't answer about the brown grasses. Typically, PFGs which go into the market in northern areas start as plugs..small plants which are grown on by nurseries to retail sizes ranging from 4" pots to trade gallons. In your (and mine) zone, they are used as annuals; planted out after last frost for a nice season of growth and display.

Here's a current example:

Suppose I have a greenhouse (I don't..yet). I could buy a flat of 21 Pennisetum setaceum 'Rubrum,' a typical PFG, for $1.60 each plus shipping. 21-size is about a 1.75" diameter plant. I pot them up in 4" pots and over-winter them in my greenhouse. By next spring, they will be ready to pot on to trade gallons, and be ready to sell at retail. My customers would plant them in the ground and in containers in May. By September, they will be at mature height and in full bloom. In late fall, when the weather turns cold, they will go dormant and ultimately die.