I once put the smallest chickenwire at the bottom of a bed, actually putting the wire under the block/wood and across. This seemed to work for a few years anyway AND kept out the gophers that ravaged anything with a bulb or tuber. I think I would use the plastic kind if I were to do it again because it wouldn't eventually disintegrate like the thin metal.

Good luck on your project.

Hardware cloth is another possibility.

Have you considered using two layers to form the barrier?

Hardware cloth, hmm. If I use fabric I planned on multiple layers, at least two, but which type of fabric? Not too concerned about impeding drainage since the block allows drainage through the sides and the beds will be quite high/deep.

The fabric will disintegrate in a year or two.

They make some seriously thick and heavy weed barriers, almost like a blanket. I'm sure those types would remain for a long time, but I have no idea how impervious they are to root penetration.

Forget chicken wire. It'll rust out in a few years! Hardware cloth is a very small (1/4-1/8th) wire mesh that will keep out the gophers, but I'm not sure about roots! Contact your local extension office or a reputable nursery (NOT a big box!) to ask.

For a long-lasting solution, any of the poly's will work well, as in polyethylene, polyproplene or polystyrene. Those plastics are not biodegradable. Most of the commercial weed barriers and landscape fabrics are made of those. For those, you worry about UV degradation, but not under the soil! If you're serious, I can't imagine considering any metal or cloth.

45 - 60 mil EPDM (pond liner)?

Hardware cloth! It's 1/4-1/8 VERY strong wire! cHICKEN WIRE WILL ROT!

Like Dan said, wire isn't going to do anything for tree roots. Instead of a horizontal barrier along the bottom of the bed, which would impede drainage and possible root growth of your vegetable plants, I think your best, most long term option would be to use a method similar to how people keep running bamboo from spreading. A deep, vertical barrier of pond liner or rigid plastic in the area between your trees and vegetable garden should keep the tree roots from spreading into the garden for several years at least.

Rodney

I like pond liner. Nice idea. 40-50 mil EPDM rubber. It's also a roofing product, and it has been shown resistant to root penetration in green rooftops. It lasts a few decades on rooftops and even there is degraded mainly by UV, which won't be an issue underground. It's also non-toxic and pretty environmentally friendly.

I have some of my raised beds next to a maple tree. I took a different approach. I put in a trench at the perimeter of the raised beds, chopped off all the tree roots I encountered, and put HDPE bamboo barrier in the trench to prevent a re-invasion.