Based on your region's climate, plan on cutting back or burning pampas grass in mid to late winter. Do it early enough so there are no sprouts of new growth already coming up from the roots through the dried thatch in the grass clump.
Burn the dried grass stems and sheaths back to height between 6 and 12 inches. You may find that the pampas grass doesn't burn evenly or perfectly. The high silica content in this grass, especially when moisture remains in stem bases, retards burning.
Pampas grass clumps become quite large. Think twice about starting a fire on your property since the flames on a large grass clump will tower above the plant and potentially scald nearby plants or even catch lawn, debris or mulch on fire. Cut back upper foliage so the fire doesn't have as much fuel to grow large and hot.