I wonder if just deep rototilling a barrier strip between your garden bed and your trees would do the job. But maybe that's what you're talking about.

no room for a barrier strip without reducing the already tiny space, 8x4. But these roots were deep into the garden within about 4 weeks I'd guess.

I have to grow in large pots with a saucer underneath in the bed 20 feet from the big hackberry trees.

I'm a little surprised that with a garden bed so close to a tree trunk you don't have room for a barrier strip, that you actually have some sun. If you're thinking about boxes, might there be a better place for them?

I wish there was a better place. My only thought is on top of our recently reinforced shed roof.

But, in any case, I'm thinking it's time for boxes as Dave said. I'd be happy with half a dozen tomato plants, a couple tomatillos, a couple Ichis.

Herbs I'd prolly put in bags or boxes on the ground.

Warning, the roots will grow into the raised bed boxes. Ask me how I know. Unless you can build the box deep enough to basically be a huge pot with the barrier at the ground level, they grew into my raised beds.

I'd hope that 2 foot off the ground, I could at least hack invaders.

Well yeah, if there is an air gap. LOL! The two places I have to grow any tomatoes, one has the tree roots and the other is full of nut grass.

Rather than put the pond liner underneath the garden (which would cause all sorts of problems) you could try digging a small trench a couple feet deep or so and put the pond liner down one side of the trench vertically as a barrier before backfilling. It would only need to be on the side(s) closest to the tree. Not sure how long it would last but it would be worth a shot.

Rodney

He said he didn't have room for a barrier.

Certainly, in a raised bed, you need to line the bottom with root-proofing material. Basically a big container, sitting on the ground, or in the ground.

The OP said they didn't have room to rototill a barrier strip. What I am suggesting is not even close to that. Think of the plastic lawn edging used in landcaping except one that extends downward a couple feet. Takes up zero garden space but will help prevent the roots from getting into the garden for a couple years or so (as long as it doesn't get chewed up when digging in the garden). If the pond liner is folded over a few times so as to increase the thickness it would work better.

Rodney

A couple of feet deep? Tree roots will easily come up from below that. They will search out water and nutrients, and head straight for that.

The majority of a trees feeder roots are in the top 18 inches of soil. You don't think 2 feet will work, then go down three feet or even four. It will be a hell of a lot less expensive than building raised beds because if the beds themselves aren't raised up off the ground the tree roots will simply invade the raised bed soil. And if you are suggesting using a root barrier underneath a bed that is on top of the existing soil it will be no different than if the OP dug their soil out, put a liner under, and backfilled like they had already thought about. Not to mention that it would just be adding cost to an already costly project.

Rodney

nah, done digging and playing. elevated boxes next year. I'll try a small one first and perhaps a couple on the shed roof.

this year I planted in fabric planter bags and small plastic, resin and concrete planters. The maters did very well.
But I think next year I'll roto the old garden, reclaim the dirt as best I can.
Put a dozen or so plants in pots.
Unfortunately garden is very small, heavily tree shaded and facing west. The pots on the driveway facing south did better.
I also need to protect my basil against unseen critters. Guess I have to leave the row cover on the whole season.