You'll get the densest hedge if the plants will get a lot of sun as well as limited root competition from trees. So keep that in mind. You don't say how much sun this area gets. That's KEY.

I would remove all those trees right next to the fence. They are leggy and will compete with anything you'd plant. Just my opinion. But I'd start out with a clean slate.

Plants to consider would be photinia (Fraser's etc), Skip laurel (Prunus laurocerasus schipkaensis) hollies (Oakland, Needlepoint, Burford, Yaupon, etc), osmanthus (Fortune's), Eunonymous (japonicus). Carolina Cherry Laurel 'Bright and Tight' or other compact variety.

You'll have to consult a local nurseryman to see how those do in your area. Everything I've listed will need at least some regular pruning.

I think a variety of junipers would be your best best for good screening but they do require a lot of sun to be effective and dense. As Dave pointed out, it looks like sunlight competition will be pretty intense the way things are now. Starting fresh with you screen is required IMO

Go to Tony's tree plantation and get some local advice as well as an excellent selection of plants. It's in Midwest City on S 29th and Post I think. Huge place. I could spend a whole day there.

Edit: actually Nellie R Stevens holly may be better screen since the shade seems like it will be a large factor. Your soil ph and heavy clay may slow it down somewhat from its optimal growth rate

This post was edited by j0nd03 on Thu, Oct 10, 13 at 16:40

Thanks for the suggestions. I will check out Tony's as well. Sunlight is an issue. The branches overhead are from a very large pecan tree right in the middle of the yard. I'm not sure what can be done about that...

Do you know what the trees are which are already there? Would they be amenable to pleaching?

amyokie - My dwarf italian cypresses (cupressus sempervirens 'Monshel') do great in zone 7. They are gorgeous, unique trees.