According to TLC Garden Centers, you can use Osmocote plant food for your Bougainvilleas and it makes for an excellent fertilizer. Time-released, it can keep your Bougainvilleas happy and healthy with a steady feed of nutrients that will last for about 6 months.  The NPK for this fertilizer is 15-9-12, so make sure that it’s compatible with your current soil nutrient levels so that you aren’t overdoing it with your nitrogen – as this can lead to more leaves, but fewer blooms!