Oleanders bloom in summer. It is a good idea to prune them in early spring. This is to remove all the cold damaged areas of overgrowth. If you have planted oleanders, you will get the best results if you let them expose to the full sun during their growth. If you live a comparatively cooler place, some reflective heat from a wall towards the west or east will also do some good. Partial shade is alright to get the flowers by, but can give it an awkward and open shape. Make sure the soil is properly drained, even though; oleanders are very tolerant to any soil type. To encourage longer bloom it is important that you get rid of lifeless cluster of flowers.