Oleander is often grown as a bush, but can be trained to grow like a tree, similar to rose tree, if desired. If you are growing your oleander as a bush, a yearly pruning -- at least for the first few years -- will help it grow full and lush, even near the base of the plant. Knowing when to prune an oleander is important for optimum growth.
Prune oleander bushes back after they are finished blooming. In the early fall works well, but don't wait too long so your plant has time to recover before winter. At this time, you are pruning to shape the oleander bush to encourage fuller growth and more blooms in the next growing season.
Prune just above a three-leaf node. In the spring, three new branches will be forced to from this point and at the end of these three new branches will bloom a cluster of flowers. Cut as far down the bush as you want. In general though, you should trim the plant back closer to the ground in the first couple years so the oleander bush fills out on the bottom nicely. As the plant matures and fills out, you will not need to cut it back as far.
If you notice a diseased or damaged branch, cut it back just above a leaf node at anytime to get rid of the afflicted branch. However, if your entire bush is diseased, such as from oleander blight which affects the whole bush, cut the bush back hard near to the ground in the winter and then trim it back lightly after blooming in the fall. According to Texas A&M University Extension, this type of pruning may save your bush.
Choose a oleander variety that is rated hardy in your climate. Consider how much sunlight the planting site will receive and choose accordingly. In general, most varieties need lots of sun to flower well. Supplement rainfall when it is scarce and during the summer, you may need to apply about 1 inch once or twice a week to keep your oleander thriving. Oleander in general does not need fertilising, but to improve growth, you could apply a balanced fertiliser in the early spring and early fall, applying amounts specified on the labels.
The sap in oleanders is highly toxic. Wear long sleeves when trimming an oleander and wash your hands when the task is complete. Never burn oleander trimmings, as the smoke they produce is toxic as well.