How about a large container such as a half whisky barrel? Plant them in a completely organic material that stays very moist to even wet. It sounds like your plants last year were starved for moisture and probably nutrients. When in growth they are heavy feeders! Also, give them plenty of sun too, provided the roots remain moist. Hotter soil will give you better growth. Another option is to buy a small hard plastic kiddy pool and sink it in the ground. Put a few holes half way up the sides (four is enough) but not in the bottom. Fill with top soil or the like. This mini-bog will suit them well.

In Florida these grow wild along canal banks and can get HUGE. In the south I would protect them from full on sun, but in your zone I'd give them much more. PF