In fact, I find a dense overhead cover worse than none at all. Your ceiling may be a canopy of spreading branches. It also may be an overhead structure covering a patio or terrace to create a weatherproof unit or one that is partially open for filtered light and shade. For a small garden, I feel that walls and fences have greater value than planting screens because they require little ground space and achieve the purpose faster. To get the same effect with plants, you would have to give up at least 4 to 6 feet of surface area and wait three to five years.
Fences are easy to construct from pre-formed units or from scratch.
I encourage homeowners to think hard about the location for a fence - there's no need to completely surround the area. Instead, you should identify critical areas where you would like to have screening and privacy. Then use sections or panels between your garden area and what you want to screen from view. Screening panels need not be located on your property line. They can overlap to create density, and they can be placed at angles to each other to create patterns.