Ask Mr. Smarty Plants

It is hard to determine exactly what you are looking for because of the wording of your question.  Is it correct to assume that it could be perceived as un-neighborly to plant a solid wall of green along your back property line?

In that case you can plant groupings of evergreen trees and small deciduous flowering trees that are strategically placed to screen parts of the yard.  Determine which sight lines you would like to block (for example your patio and their deck or their kitchen window or their upstairs window).  Those lines will help you determine which type of tree you need to plant, and where.  Keep in mind that an evergreen tree is generally broader at the base than at the top and a decidous tree is the opposite (and doesn't offer much screen in the wintertime).  Plants that are placed at the property line need to be much larger than those planted closer to your seating area in order to provide screening.  You might want to ask a designer for some help with this (many of the larger nurseries in your area will provide this service if you are buying plants from them).

You live in a part of the country where the possibilities are almost limitless but here are a few suggestions:

Large evergreens:

Chamaecyparis thyoides (Atlantic white cedar)

Ilex opaca (American holly)

Pinus strobus (Eastern white pine)

Thuja occidentalis (Arborvitae)

Ornamental decidous trees:

Amelanchier laevis (Allegheny service-berry)

Cercis canadensis (Eastern redbud)

Magnolia virginiana (Sweetbay)

Oxydendrum arboreum (Sourwood)

Smaller evergreens:

Ilex glabra (Inkberry)

Kalmia latifolia (Mountain laurel)

Morella pensylvanica (Northern bayberry)

And if you are not concerned about screening in the wintertime, the possibilities using deciduous flowering shrubs are almost endless!  If you visit our Native Plant Database and do a Combination Search selecting Pennsylvania, the plant type (tree, shrub and so on) and your conditions, it will generate a list of suitable plants native to your area.  Each plant is linked to a detailed information page.

Chamaecyparis thyoides

Ilex opaca

Pinus strobus

Thuja occidentalis

Amelanchier laevis

Cercis canadensis

Magnolia virginiana

Oxydendrum arboreum

Ilex glabra

Kalmia latifolia

Morella pensylvanica