I need to block out a single window, or single unsightly object.
A tidy, even screen formed by a line of repeating evergreen trees of one variety is an effective and space-saving solution. Classic, economical solutions include Photinia x fraserii ‘Red Robin’ and Ligustrum lucidum ‘Excelsum Superbum’.For something a little different but still relatively economical, try Viburnum tinus 'Lucidum'. For a highly ornamental finish, try the highly fragrant Osmanthus x fortunei , a tree form Camellia japonica, or the jolly holly Ilex ‘Nellie R. Stevens’.
For larger spaces, a screen of repeating evergreens can be an effective backdrop for ornamental deciduous trees, or, for a natural, informal finish, try a mixed screen of evergreens and deciduous. See suggestions for Problem #1.
If the budget can accommodate, pleached trees can provide a highly ornamental, containable, and dense screening solution. An increasing number of evergreen options are becoming available, including pleached Magnolia grandiflora, Photinia or Quercus ilex. Panel or box-pleached hornbeam or panel-pleached beech can also be excellent solutions. Although deciduous, they tend to retain their dry leaves through the winter, increasingly so with maturity as the branches become denser, and offer the benefit of seasonal interest.
In cases where it is really only three-season garden privacy that is required (For example when a neighbour’s windows outlook onto one’s garden or patio but not into the house), then glorious options such as Pyrus x call. Chanticleer, Liquidambar styraciflua, and Parrotia persica can be considered. As deciduous trees, they provide seasonal interest, but these species retain their magnificent autumn colours well into December. Pleached and trained versions of many decidious treescanalso be useful, as their branches become quite dense over time, eventually providing a degree of winter privacy too after many years (Pyrus cal. 'Chanticleer' (Pleached), Acer platanoides 'Globosum'). Read more on panel-pleach options under solutions to Problem #5.