I realize this is not the answer you might have hoped for but...
if it's Pampas grass and has viable seeds I would not plant them - period!
Pampas grass is lovely but I learned the hard way that the sharp edges of the plant made it impossible for me to do any work near the plants - arms and legs get sliced badly. It took me 3 years to find someone willing to remove the plants from my yard.
Sorry I could not give you a more positive answer. Hope you have more luck with yours.
