Hi Patricia, Kimberly here from landscape solutions for you. Thank you for asking the question about your Pampas grass.
Pampas grass should be pruned back in mid to late winter. This is the standard time to prune this specie. Some people will prune in the fall, but the recommendation is to prune way back in the mid to late winter.
My crew and I have trimmed several of these species and make sure you wear long sleeves and gloves. The ends of Pampas grass are very sharp. Make sure you have good pruners, the larger size, to prune the Pampas grass.
Pruning down the Pampas in mid to late winter will promote growth for the entire plant including the Plumes next summer.
I hope I have answered your question and thanks again for asking the question here at landscape solutions for you. Feel free to ask anytime and have a great day!
Kimberly