Aside from converting it to a container, installing a vertical barriar beneath the soil like those that prevent roots going beneath home foundations will keep roots at bay.

Yes that was what I was thinking and trying not to think it.
But I suppose I could make it deeper? Is it worth the trouble? And what is it that would prevent roots from going into the soil, landscape fabric?

I'm also debating as an alternative adding blueberry bushes where I would put that vegetable bed. I wish it would warm up a little so I could walk around the yard at a leisurely pace and think about where else I might fit in another bed.

I believe most of us wish it would warm up so we could take leisurely walks around our yards.
Back to the barriar,it is made of material other than landscape cloth. Fiberglass,pvc or similar structrial sheet material. Google tree roots invading home foundation for several approchs to the issue.

Well, it depends how far the maple trees are from your garden spot. But there is a simple solution to this.

Say that the side "A" of your bed is on the maple trees side,
Now dig a trench along that side and cut off all and any roots that are coming under the bed. Once the roots of maple trees are cut off from the mother, they will rot and become composted.

In addition, you can also till the bed site and remove any/all the roots that you find. This, depending on how big an area we are talking about will require some work. Either you do it with machinery (Bobcat trencher, tiller) or manually.

This past fall I did a similar thing in a smaller scale. A western cedar had invade a whole bed (just about under its canopy). I disconnected all the roots and dug out a wheel barrow full of fine root from the bed. That should take care of it for another 10 years.

The trench works, although not perfectly, and specifically against maples. I have used it many times to control roots in my previous garden, which was ten feet from the woods. I still had roots, just one instead of twenty.You dig a one spade deep trench, then you thrust your spade in the dirt for another spade length. Just trusting the spade will chop anything below 0.5 inches. You will feel it when you cut one.

If you encounter a root too big to cut with a spade, expose it and chop it with your axe, but in moist soft soil if it is deep you can also saw through the soil (you can feel through the saw where it is, and when you break through it). This will kill every root up to a depth of 18 inches.

But some roots will come up in your beds from the lower depths. Ideally, you could get a trencher and trench down to four feet, and fill the trench with wood chips or other cheap organic matter. In the trench the soil will be soft enough that a handsaw will cut down to 2 ft+ without digging, and 3 ft if you dig some. I do think that 3 ft would completely eliminate the problem. You need to do maple roots every two years.

I had the same problem with red maple tree roots invading my compost pile ; a sheet of plastic under compost pile = problem solved.
A fairly deep trench, (I would guess at least two feet deep) lined with Tex-R Fabric may solve the garden problem. Tex R Fabric comes in rolls 4' X100' and larger. It's not cheap and hard to find.

Yeah, maples especially will grow right through chicken wire. The plastic will work, until the roots poke a hole in it out of sheer orneryness. :-]

Also, turn your compost now and then, and use it before it's a year old. I use mine in about 6-9 months from when the pile is first started, so tree roots don't have much time to really grow.

Imagine a rectangle that is 100ft by 50ft and that is my back yard. The house represents the East side of this rectangle. Solid fencing on the South side and the North side and a 4ft post and rail on the 100ft West side. We have only one tree in that rectangle and it is a NOID Maple in the far Northwest corner just about on the lot line. It’s 20 yrs old and about 25ft tall. I have two neighbors to the West of this rectangle. The back of their property meets in the middle point of my yard. The neighbor to the Northwest has three mature Silver Maples that start about 10ft on the other side of my post and rail fence. The neighbor on the Southwest corner, has a pool and one mature Silver Maple that is less than 5 ft on the other side of my post and rail fence.

Then there is the South side where that neighbor has a 60yr old Silver Maple that is double the size of the others. And that is less than 10ft away from my solid wood fence in the Southwest corner.

If that isn’t bad enough, I have a neighbor to the north that has 6 mature Spruce trees and a mature London Plane tree all within 5ft of the solid fence on my north boundary all squeezed in to a 45 ft length.

I do appreciate the privacy and I use the leaves. One neighbor even throws all his leaves into my yard, at my request. So I do benefit from them all, but …trying to grow a garden here is just a little bit of a challenge.

I do seem to be able to do okay with the raised vegetable beds but it’s been awhile since I dug in any of them. I am going to dig them all this spring when I am replacing the frames and see if any of those tree roots are finding their way into my beds.

The beds are positioned in the North side of the rectangle with no obstructions to the south of them. The west side of the vegetable space is about 15ft away from our Maple. Or maybe it is more, because it is not under the drip line. So the trio of silver maples is another 10ft beyond that. The North side of the vegetable space is about 20ft away from the neighbor’s spruces and LP tree.

There’s just lawn to the south of it and a single car garage is to the east of it. I wanted to add more beds to the west side of the vegetable space, which would make that one bed about 5ft closer to the 3 silver maples and my maple.

I have been mulling over the solutions suggested and I’ve been reading more in other threads and on google. The idea of digging the roots out in a trench although more work, at first sounds like a good solution, but getting 10 years of no roots is just not going to happen with a silver maple. I think I would be lucky if I got 2 years out of it. And that is not the worst of it. What would concern me the most, is that cutting roots of the trees, could cause the roots to react like they’ve been pruned and they grow back with a vengeance.

The idea of putting down barriers under the beds with the layers of plastic and possibly hardware cloth is a reasonable solution. I was going to put hardware cloth on the bottom of the bed anyway, but I do garden organically and using that plastic at the bottom of my beds, just doesn’t sit well with me. And I feel like I would be creating a ‘container’ instead of a raised bed that has access to the subsoil.

I think those are the only two solutions too. So, I am seriously considering adding blueberry bushes instead of another vegetable bed on that west side of the vegetable space. And to expand with more beds, I can go to the south and take up a little more lawn and I might be able to squeeze another bed to the east closer to the garage.

This weekend is supposed to be in the 40s, I can get out there and take some measurements and see if I can do what I’m thinking I might be able to do. And I'll check the distance from my Maple to the west side of the vegetable space because I am guessing.

toxcrusadr, thanks for the suggestion of turning my compost pile more often. I have had a ‘no turn’ pile and I do leave it there a long time. That could work for me.

All of your input has made the situation a lot clearer to me. Thanks! :-)

The TEX-R fabric should provide more protection than plastic. Sown below is some info that came a roll of TEX-R that I ordered a few months ago:
{{gwi:261812}}

Thanks squire john for posting a photo of the label that comes with that material. I'll have to look into that a little more. It is just copper coated, right? There's nothing about copper that would be a problem to have in your soil, right?

Well, it's gorgeous here today, after all the single digit temperatures, it has to be about 50 degrees out there and sunny! Had a chance to get out and walk around the yard. Took the measuring tape. The west side of my vegetable beds are about 20ft to the drip line and 37ft to the trunk of my Maple tree. That's better than I thought.
And the North side of that area is only 18ft away from the fence with the trunks of all those spruces and the LP tree less than 5 ft on the other side of it. I have a feeling that those tree roots may not travel as far as the Maples though. At least the gardening I've done near them has not resulted in a lot of roots getting in the way that I've noticed.

And I measured if there is room on the south and the east side of the vegetable area and there is enough to add more on those sides. I won't be sure about the East side until spring, when I can see where the garage casts shadows.

Raise the beds off the ground a little bit with bricks or wood legs. Put a bottom of hardware cloth attached to the bottom of the 4 x 4 bed and support the hardware cloth by a few lengths of wood. Some people use plywood on the bottom but unless it is marine grade plywood it won't last too long. The gap between the ground and the bottom of your box will stop the roots.

That's an idea too, Yolos, thanks. Now if spring would just get here!