The two yellow brugmansias I have in 35cm diameter by 27cm (26 litre) plastic pots are just beginning to flower, but are very pot-bound. They need watering at least once a day, and up to three times a day in hot weather. I rang the RHS advice service to ask about root pruning. The advisor recommended sawing off a couple of inches of roots from around the side and the bottom of the plants when they are coming into growth in the spring. If I am feeling cautious, I could do half the plant one year and the other half the next. This root pruning should be done every two or three years, but some growers do it every year. The plants will still need frequent watering.