My personal opinion is that you should get them in the ground no later than March so that they can form a good root ball before the heat hits. They would also need some water, not much though, until established, the second or third year.

Also, be aware that the root systems do not like to be handled, so I just dig a hole, put the pot in with the plant still in it, tighten the soil around it, then pop out the pot, carefully remove the plant and pop it in the hole. It makes a hole exactly the size of the root ball with very little handling.

There are a lot of zone nine that frost bougainvillea to the ground every year. I would wait until after the last danger of frost before planting a plant that has probably come right from the greenhouse. Usually the pot is cut off by slicing down the side. If your soil is well drained you should not have to worry about too much water. In the right location most are such rampant growers that pruning to keep them in bounds can be a job. On Chestnut street in Santa Cruz there is one thirty feet up a conifer. Al