There are fertilizers formulated for bougainvillea. I've never used any of them because my bougainvilleas have always flowered consistently and been healthy. Here in Tucson, I apply a 3-4" layer of compost around the base of my bougainvilleas in late winter/early spring. This is a more extreme climate, and the soil isn't as rich, so my bougainvilleas appreciate the extra boost. Plus, the compost helps to hold in some of the soil moisture once the temps creep above 90F.