Fast growing trees and hedges for zone 7a?

[Major Nikon]

3. A couple of recommendations

Lacebark Elm, aka Chinese Elm

Plant the smallest specimen you can find and as long as it has a decent location it will make a very nice shade tree in short order. After about 10 years it should be about 40' tall and about 25' wide. Fully mature it will be around 60' tall and 40' wide, so plan accordingly. They have a good resistance to Dutch Elm Disease which plagues other elm varietals, and generally have good longevity compared to most other fast growing trees. The leaves are quite small, so they are easy to rake in the fall. It does produce seed pods in the spring, but they don't produce enough mess to really worry about like some other trees. You'll need to prune it each spring for the first few years.

Texas Ash

There are a few different ash varietals. The one I have is a Texas Ash which I planted about 15 years ago and it's about 35' tall and almost that wide. It's a little slower growing than the Lacebark, but still a pretty fast growing tree that makes a very nice shade tree as it matures and also can expected to be disease free and long lived. Like the Lacebark the leaves are small and there's virtually no seed mess to worry about. Fully grown they can get to about 80' tall, so make sure you have a big place for it to mature completely.

Natchez Crepe Myrtle

If you want something a bit faster than those two which doesn't get quite as big, this one is a good choice. This one is actually a hybrid between the Japanese crepe myrtle and the standard western crepe myrtle. It takes a more tree like form and has showy bark like the Japanese varietals, but produces larger and more prevalent flower clusters like the western varietals. I planted one of these about 10 years ago and it's about 25' tall and wide today. I limited it to three trunks. Occasionally you get black mites on the trunks that tend to turn them black, but otherwise don't really affect the health of the tree. They are fairly easy to eradicate, but they don't tend to persist past the winter unless you get them again.