Converting shady grass parkway into native flower garden, how to deal with the grass?

I am working with a community center to improve the parkway (e.g. the space between sidewalk and street) next to the building. It is a 20 ft by 10 ft space that has a mature ash tree surrounded by scraggly lawn grass. There is already a fence on the sidewalk side, so it does not get walked on too much. Our plan is to make a bed of native woodland plants, edged with brick or wood to raise it just about 4 inches or above the sidewalk.

I am wondering what the best way to deal with the existing grass is. Usually I would want to dig it and flip it over "sod-buster" style, however, I think this might be harmful to the tree roots.

Would a layer of newspaper under about 4 inches of soil/compost be enough to suppress the grass?

Any other non-toxic ideas?

That's about how we make new beds or walkways between. I'd use a few layers of newspaper. We usually use cardboard, but if you'd rather avoid the glue, newspaper does work. We find this works well for a few months, then it all breaks down and grass / weeds come back. If your work area is surrounded by concrete, that will probably slow that down. You'll want to watch for spreading by seeds more than by rhizome. If you fill the "vacuum" with plants you want, or at least mulch, and stay on top of it, I think you'll succeed.