18" is deep enough to smother pretty much anything I could think of. On some quick beds I have dug... I just flip the sod over and add a fraction of what you will be adding... no cardboard or paper used... and no problems here with anything growing through except at the edges where it tapers down to the level of the surrounding lawn.

What kind of grass ... if it's Bermuda of any kind, don't do it. I've seen it come up from about 3 feet down where a palm tree was removed.

Other grasses aren't as able to resurrect themselves.

I have no idea what kind of grass... We just moved in here in July and the lawn was already here. Is there a way I can tell?

If it helps, here is a pic of the yard from above. You can see the lawn and the dug-out area plus the pile of sod under the tree.

Look at how the grass grows ... Bermuda sends out runners with a specific leafy pattern.

http://lazygardens.blogspot.com/2009/04/buffalo-grass-vs-bermuda-grass-what-to.html

Many people have done just that, buried sod at the bottom of a raised bed and have found there is a tendency that most invasive grass species, Bermuda, Quack Grass, Johnson Grass, will eventually grow up and, if allowed, take over the bed. Others have observed that these invasive grasses invade these beds anyway, whether sod is buried or not.

At a depth of 18 inches cardboard or newspaper would not make any difference since the primary purpose of those items is to deprive unwanted plants with access to the sunlight they need to grow and the depth of the soil will do that quite well.

kimmq is kimmsr

lazy gardens, that link does not work for me. It leads to the blog, but the message is Sorry, the page you were looking for in this blog does not exist.

ARRGH Houzz's link maker id FUBAR. It sticks extra characters at the end.

http://lazygardens.blogspot.com/2009/04/buffalo-grass-vs-bermuda-grass-what-to.html 

Bermuda has a network of runners like piano wires above the ground, you can pick it out right away. I would point out that if it was Bermuda - and it does not really look like it from your pic - but if it was, flipping it over and burying it even under 18" of stuff would not stop the surrounding grass from growing right into the raised bed. But if you have what looks like conventional little grass plants and no net of those runners going everywhere, you're fine. I don't think you do because it would be in all those mulched areas too. It can grow under borders even several inches deep.

Thanks everyone, this has been helpful. I have looked closely all over the lawn and haven't seen any runners, especially none right on top as Toxcrusadr just described. Looks like we are good to go. :)

I appreciate all the help!

Hey fujiapple - I was wondering how this turned out after a few years? Did you end up with grass growing up through the beds after using sod in the bottom?

I’m currently wrestling with what to do with my excavated sod. My husband scraped up the grass in a 40’ x 20’ area using his tractor bucket. It’s too much and too heavy to discard with my yard waste pickup; and I don’t need it elsewhere. I thought about composting it, but after putting my pile in a marsh area at the back of my property, pesky beavers took those slabs of sod, and used it to block the storm water drainage in that area. Ugh! So, I’ve decided to flip the sod over (roots up), and use it as my raised bed base. I’ll be putting 12” of topsoil and compost on top of it. The area is quite large, so enough cardboard/newspaper is not accessible to barrier future grass and weeds. Will I regret not taking this step? More concerned about grass growth than weeds. Help!

As above, should be fine.