Kind of thinking out loud (or with my keyboard)...How will you keep the weeds (invasive trees, poison ivy vines, etc, etc) out of the barrier? If you let the barrier deteriorate from these invasions, won't that defeat it's usefulness? What if you need access through the barrier (fallen tree, picking up litter, chasing a run-away pet, etc)?

Thanks for thinking about this. I've had those same thoughts.

For the unkept areas of the woods (the woods along the street), I was thinking that there might be some sort of natural, inhospitable thorn-bush that would stand up to the competition.

It wouldn't need to be an incredible wall of hell, but just enough that strolling leisurely onto my property would be a big hassle.

There are some thorny type, stiff "vines" that shoot out of the ground in the woods. If could transplant them, perhaps they will do swimmingly. They might be some sort of wild rose, but I have not lived here long enough to witness the blooming season, so I can't tell.

Knock Out roses can get tall, dense and wide. Once established like that, I would hope that they would defeat most of the stuff seeking to compete.

Along the fence line in the open area... the area in view of neighbors....I do worry about a maintenance hassle. The sheer amount of mulch alone could get expensive! Perhaps I need to find a less formal, rough and cheap type mulch for that area.

Just wondering, why not use a good, strong barbed wire fence? Properly installed, it could be a very good deterrent to entry, and maintenance would be minimal. Locked gates could provide alternate access points when needed. You could also use some type of hedge along the cleared property edges, but the fence along the wooded portions.

Another down-side to using thorny things along the road would be litter. Removing plastic bags, bottles, and other junk could be a real pain with a thorny border.

You don't want a rose fence in Tennessee because it will get Rose Rosette and it will spread along the fence line rapidly and lethally.

For visible parts, Pyracanthas look especially good in winter and if your part of Tennessee is mild enough, the pyracantha fruit make a good jelly.
The bitter orange / crown of thorns woody shrub is a true thorny border.
Blackberry hedges do have their merits.

Pyracanthus looks great but can grow somewhat tall, good choice if you can maintain it regularly. Mine need cutting back every year even though they are not a fence.
Blackberries mostly need to be staked, don't they? Nice but not low-maintenance.
Barberry is a beautiful thorny bush, berries can be used for seasoning pilaf, and varieties with purplish leaves exist, and it stays a medium size but it is prohibited in some areas, you'll want to check.
Gooseberry is a beautiful thorny edible! Can be trained into shape and is very resilient.
As to the spiny vines sticking out in the woods - you could mean smilax or wild sarsaparilla.These are completely unmanageable. I wouldn't bother growing them; lots of people desperately try to get rid of them. The tender shoot tips are edible and really good though.

As with any gardening project, stay away from significantly invasive species. Berberis thunbergii (Japanese Barberry), for instance, is probably the most commonly grown barberry species, but is a class 2 (pretty darn bad) invasive according to the TN-EPPC.

Here is a link that might be useful: TN-EPPC Invasive Plant List

I'm rather fond of Leatherleaf mahonia, mahonia japonica bealei, which I see is on the invasive plants list. I had a large stand of the stuff on the north side of my Maryland house, shaded by a large cherry. I suffered from the hard sharp leaves while digging out a volunteer cherry. Initially I found it ugly, but fell in love with the pale yellow sprays of flowers blooming in late January through March. Against a dark background, the sprays floated mid-air when picked out by the porch light when arriving home from work during the early winter darkness. Birds love the gray/green/black berries during the spring. I've seen mature plantings of it in home landscapes here in Knoxville, so I know it grows here, apparently invasively.

We have what my dad calls "Devil's Walking Sticks" Which looks like a dwarf version of an African Thorn Tree. It has needles about 2-3" long. It would make a great barrier and deterrent! I don't know what it looks like in the spring or summer, but if you want it you are welcome to get some. We have spent several weeks trying to get rid of it as we have kids who like to play in the woods.

Gapeach, bring some to the ETPS!!! If you can get some with a decent amount of root, I'd love to have it!

I'll try brandon7 - I'm telling you this thing is murderous!!

I want to thank you all for this post. I was wondering the same about a line on my lawn (can't afford a fence right now)and you all have convinced me not to use the thorny bushes. It sounds as if they would be difficult to get rid of and to maintain. Thanks again

Black Locust tree is a smallish tree with pretty pink flowers, and mesquite looking seed pods. With the most angry looking spikes of thorns that are about two inches long. A native here in S.E. TN. I would love to have one or two growing in my yard. Birds love them also.