I'm building a trellis/clothesline and using 4x4s as the end-poles. The load on these will be pretty small, and mostly bending moments (almost all) and not axial stress. I live in southern California — very little annual rainfall and no frost/ice/snow.
I'm planning for between 4-5' of height above ground and do not want to set the posts in concrete (only backfill, and will install something like this: http://youtube.com/watch?v=hkhz1RY0cY8&t=6m12s). To what depth should I set the posts under ground? I couldn't find rules of thumbs for this scenario (no concrete). I have planned for between 1.5-2' deep holes, which is just about sufficient for posts set in concrete. Should I dig them deeper or will these suffice?