Answer: In addition to thinking about planting a tree in the location, you might want to consider a narrow, tall growing evergreen such as Thuja (American arborvitae). These plants have a columnar growth pattern; 25-30' high and only 2-3' wide, which will fit perfectly in the planter space you describe. You can plant several of them 4 to 5 feet apart and allow them to grow into an attractive privacy screen.
Trees will be slower growing than the Thuja, but here are a few suggestions: Acacia, Citrus, Cypress, Eucalyptus, Olive, or Pyrus (evergreen pear).
Best wishes with your landscape!
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