When the rains finally arrived, this bougainvillea grew and grew, so vigorously that the roots clogged the drain hole in the bottom of the pot. Standing water in the pot was going to kill the plant, so what to do? We tipped over the container, drilled out the root from the single drainage hole, then created several more in the bottom. It took cement bits and lots of muscle!
Next, I drilled holes around the outside edge of the container just 2 or 3 inches from the bottom.
A day later, the plant is happy. Hope this has solved the problem.