There isn't much you can do to ease the transition. Water faithfully once each week, thoroughly soaking the entire root mass. Healthy plants should recover from transplant shock. Bougainvillea really resents root disturbance and the tops can even die to the ground - but new shoots should develop from the crown of the plant. If you must replace your bougainvillea, or if you intend to plant another, here's a way to avoid extensive transplant shock: Cut the bottom away from the container, run a knife blade around the inside of the container so the root ball can slip out easily, then place the container and plant in the planting hole. Slowly slip the container up and over the foliage while a helper puts soil around the exposed roots. This will help keep them together in one mass. You may damage some of the stems and leaves as you pull the container up and over the top of the plant, but these are easily pruned away. Best of luck with your bougainvillea!
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