Answer: You didn't say if you had transplanted it into the ground or not. If you did, it's probably just transplant shock. If it's in a container, it may need fresh soil, more room for its roots and/or fertilizer. Bougainvillea take full sun, and don't like to be overwatered. Soil that's too wet can be another reason for yellow leaves dropping.
Bougainvillea (Nyctaginaceae) is an evergreen shrubby vine in tropical climates. They will endure a light frost but not consistent freezing weather. Plant in full sun, in rich, well-drained soil. Feed the plant in spring and again in summer and provide a trellis or other support for the vining branches. Bougainvillea respond to heavy pruning by sending up an abundance of new flowering stems. The vibrant color of the bougainvillea comes from three large bracts that surround the tiny flowers.
Bougainvilleas winter over very nicely indoors if they kept in a cool (57-68 degrees F) and bright location. Reduce watering, and when the foliage drops, do not water at all. Cut the plant back in March and repot it.
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