We have lawn grass invading into some adjacent gardening and landscaped beds. I have dug a trench about 10" deep and have placed a plastic barrier to block incoming grass roots.

I also dug up the dirt where the grass invaded and have turned it over and broken it up, but there still are many small grass root fragments in the dirt. Will these roots fragments grow into new grass? Would it help if I just left the dirt in a pile before replacing it? Is there anything else I can do to prevent grass from growing there or do I just have to take the dirt away?

Normally, most of the roots will regrow. My suggestion is to wait until you see actual green new growth, then hand pull. Being that you have turned the area containing the unwanted grass, it should not be a hard manual task. However, it will be a weekly task for 1-2 months where you pull a few grass sprouts each week.

Thanks for the information.

Quack grass can go down over a foot.