Here are some things you need to consider, given your space requirements.

Open-grown trees often have wider root systems than trees closely planted together, but even if you planted a group of trees, your width requirements can make planting a conifer problematic.  Conifers are known as surface rooters, which means that their roots will start to spread out one to two feet below the surface.  Unfortunately, those roots can spread out to one and one-half to three times the tree's height or even more.  As with many tree species, even the smaller roots can penetrate obstacles they encounter, like the walls of an in-ground pool, water or sewage pipes, or the cement floor of a building many yards away.  Conifers will always try to find water.

It sounds as though you want full-size trees.  It might be possible to contain the roots of a dwarf specimen to 12 feet, but that would necessitate a very small tree.  Eastern or Canadian Hemlocks (Tsuga canadensis), for instance, usually grow to 50-70 feet high, but they can attain heights of 100 feet in the right circumstances.  Deodar cedars (Cedrus deodara) can grow to 150 feet tall, although in these latitudes they usually don't get much taller than 70 feet.  Weeping Alaska-cedars (Chamaecyparis nootkatensis) typically grow 60-100 feet tall.  Multiply those numbers by two or three, and you're looking at a tree that can reach out for the water on your neighbor's property, as well.

In general, conifers do best when planted in late winter or early spring, when the ground is soft enough to dig, so you have a bit of time to make your choice.  The New York Botanical Garden is in Zone 7A, so this might be a good time to visit both our Benenson Ornamental Conifer Collection and our Ross Conifer Arboretum that stretches from the Visitor's Center to the Home Gardening Center.   While we have many interesting conifers throughout the NYBG, those two areas are especially dense with beautiful ones and it may be possible for you to find a dwarf variety that pleases you.  When you see a specimen you particularly like, take a picture of it with your cell phone and also get a picture of the brown plant information sign that will be on a stake in the ground in front of the tree or attached to its trunk.  You should probably also consider other small trees besides conifers.

When you have these pictures, you can contact the nearby Cooperative Extension Service.  They will be more familiar with your area and can either discuss your options in more detail or can refer you to a professional arborist in your area who can help with the best selection.  Your photographs will help them advise you.   

We applaud your foresight and wish you well in your search. 

Courtesy of NYBG Plant Information Service