Hi, Pampas grass is a normally arching plant, and when it flowers the flower stalks go upright, but are stout enough to not need any staking, these have a large solid rootball, and get fairly large, so I hope your pots are REALLY BIG, Derek.

They don't need supports, they stand up perfectly well on their own, but unless you've bought the dwarf form of pampas,they won't do well in pots for longer than 6 months or so.

What shape are the pots. The roots could break a pot which is other than straight or sloping out at the top. I agree with the comments above.