Drainage is the only thing that would concern me. I have raised beds that i put on top of bermuda and basically all i did was dig out what i could w/ renting a sod cutter and then threw down cardboard boxes and newspaper.

Still fought bermuda a little the first year but kept it manageable.

- Brad

Sorry, I did NOT rent a sod cutter.

I have recently used sod cutter to remove Bermuda grass to establish 14 new raised beds. You can see pics and more info on another thread link below -Chandra

Here is a link that might be useful: Raised Beds

if you can wait until June, glyphosate will permanently get rid if the bermudagrass and it is very cheap.

At 18 inches the roots will not have to go further down, but I still hate putting plastic down there for drainage reasons.

Drainage, mold, fungi would be my concerns.

I use cardboard. It's free, it smothers the grass, and in a year or so it breaks down and enriches the soil.

I also use cardboard covered with straw for my paths. That way my husband doesn't have to mow between my beds. I ran out of cardboard so I put an ad on Freecycle. Lots of people responded wanting to get rid of cardboard!

Jo