I guess it's a lack of warmth, that makes them struggle, Charlie.
To my experience, they can be grown in full ground, too on a moist spot.
Did anyonye notice, how cruel Thalia is? Wasps always get stuck in their flowers and are starved to death eventually!
Generally, T. dealbata is an unfussy, completely easy & hardy plant. No pests or diseases, nor invasive behaviour or feeble growth, just a bunch of nice leaves.