| Where in SoCalifornia? Give a town, please. Big difference between Zone 9 USDA or Zone 9 Sunset. Can you plant ON the slope, or is it outside your property? How far is it between the house and the slope? You don't want a tree that eats the house just to hide the slope. Cottonwood is a definite hazard - they get big, then fall over or send huge limbs flying in storms. Consider a mix of medium size vertical-growing shrubs with small-to medium vertical-growing trees along the back of the yard. If I knew where you lived I could be more specific. | 
