Hello Anonymous, I am so jealous of where you live. Especially since in the summer you get at least one quick rain shower a day which is great for poinsettias.
In May you want to cut no more then 3 inches from the end of your poinsettia tree. Some will tell you to cut 4 to 5 inches but I have found problems when cutting so much off. You do not have to cut the max, judge the plant and the leaves if your poinsettia is fairing well cut only an inch off. Any cutting will send energy to the root. Please, make sure that you leave a few healthy leaves on the stem that you are cutting.
I used a cactus fertilizer every two weeks in the summer months. That is the only time that I water. Otherwise I leave them ALONE. They are adaptable trees, and they seem to survive its elements.
IN HIS SERVICE