First of all, a few questions. What kind of light does the area in question get? What direction does it face? Also, do you know what the soil's like? Sand, clay, depth, pH, all that. Thinking about those things will help you choose something that will thrive!

When you say a low border, do you mean 1-2' or 3-4'?

I see you're in zone 8; whereabouts are you, specifically? Do you want something formal, like a square hedge, or something more fountain-y? Are you interested in flowers and color, or fall color, or feeding birds? Or a berry/vegetable producing area?

With such a big area, there are lots of possibilities!

We are inthe Conroe/Montgomery Texas area. The area gets some mid-day sun but otherwise high shade to shade most of the day. The soil is light sandy/silty. PH is 5.5 to 6.

Definetly an informal look.

AS for how high, that is part of the question. It needs to be, I would say 3 to 4 feet high to block the lower part of the area from view. This is the area that is the "messiest". Or maybe there is another way of handling it ?

We are very much into the birds, that is part of the reason I don't want to clear it out. Most of the area around us is developed. This area serves as an oasis for birds.

Thanks

I think I would just put a line of rocks at the division to make a visual separation of the two areas. The rocks don't have to be much over 4 to 6 inches high to mark the separation between wilderness and yard. Limestone is easy to come by and fairly common around my area. The white or gray stone make a nice mark to divide the areas and the stone is easy to mow/edge one side and ignore the other.

Then again you said pine trees, which means you are a little east of my location and limestone may not be as easy to come by as it is here.

My wife and I settled on this solution once before in a similar situation. Before we put in the rock division line, she though it looked like the wild area was part of the yard that we did not get to when we mowed. She liked the rock border separating her yard from the wild area.

Since you like birds, you may also consider planting Turk's cap and other shade loving, flowering shrubs that the birds can enjoy.

Consider these shrubs for attracting birds:

Barberry
Mahonia
Nandina (also has beautiful red winter color)
Pyracantha
Holly (also provides shelter for birds)
There's a much bigger list on page 3 of the article I linked to below.

You could also put naturalizing bulbs or perennials on your side of the shrub line, to add some color. You could even mix it up some, put a variety of things, for more of a cottage look.

Here is a link that might be useful: Bushes for Birds

a short fence,rocks(I second that rocks looks nice) and some 3/5/7 low shrub repeats. (plant groupings in 3,5 or 7 look natural) PJ

If you are trying to please those who think that lawn is 'good' and natural is 'messy', then I would not plant a mixed hedge. That might also be interpreted as 'messy'.

Try a line of azalea bushes, with a clean edge between the lawn and the mulched azaleas.
The azaleas wouldn't be too tall to see over (assuming you choose short varieties), they wouldn't need to ever be trimmed, yet would still not look 'messy'.

A row of nandina might also fit the bill, and have berries for the birds, but might look more scraggly.

Maybe begin a hint of paths to make it look like the beginning of nature trails. Add a stump or a large tree branch that looks like one could sit down.(after a hike).
When I had "woods" next to me, I cleaned out the first few feet, to smooth the transition.
I wanted to make it a positive, rather than try to hide a negative.

Thanks to everyone that offerred suggestions. I still haven't finally decided but the combo of rocks and shrubs sounds like it will do the trick.