The delicate beauty of bougainvillea can be deceiving, because this plant needs rather harsh conditions to bloom well. In addition to a site that gets at least a half day of baking sun, bougainvillea should rarely receive water beyond the month after it is transplanted. Fertilize your plants twice a year, using a high-potassium, low-nitrogen fertilizer (the first number on the fertilizer package is nitrogen; the third number is potassium). When bougainvillea fails to bloom, it is usually due to too little sun, too much water, or too much nitrogen fertilizer. The bougainvillea bloom season stretches from December to September, depending on the cultivar.