I did a quick internet search and it appears that people do this with potatoes (I'm looking to do it with sweet potatoes).

If untreated plastic disintegrates, why do people make such a fuss about plastic bags/wrap/packaging in landfills?

They do not disappear, they just rip because of the sun exposure... I use clear plastic bags from 4 mil plastic to solarize compost for seed starting. I fill them with compost and place flat in sunny location. When 2 month passed, you can't lift them, you can poke it with your finger easily. I have to rip them open and scoop compost out of what left of them.

garbage bags might be treated with insecticides. If they have been, they aren't recommended for food use.

Thank you for your input. I looked around and am getting some commercial fabric grow bags but will also try out some plastic 32 gallon trash cans with holes drilled for drainage.

I would think the bag (if it survived in one piece), if just laying on the ground, would slowly spread out. The advantage of a pot with solid sides is that it protects and supports the roots. It keeps the soil from shifting around, which would likely break roots. That is, a pot is a support structure for roots. A bag doesn't do that as well. I know people use hanging bags, but in those cases the soil doesn't really shift around much. For on-the-ground gardening with large containers, the trash can idea seems a much better one than bags.

I think that commercially sold grow bags would not spread out like an ordinary trash bag. But I'll see, I've ordered some 50 gallon fabric grow bags, they will be here next week.

The OP was talking about large heavy-duty plastic trash bags. My understanding is that the fabric grow bags you buy actually have some sort of side support. In that they're kind of like a pot with fabric sides. That is, they stand up when nothing is in them. Is that not the case?

Look on you tube for videos about growing in bags. There's a guy who grows tomatoes in reusable shopping bags like you can get a Walmart. They have to be woven and made out of a certain material that holds up through the summer but lets air through.

Growing tomatoes in bags