if the bed high enough( at least 12 inches) for roots you may put weed cloth on the grass - it will not get through as it wouldn't have access to light. But if it is lower, I am afraid the digging out grass to make bed deeper is only option. Also, keep in mind that weed cloth retain more water when it rains, and also work as barrier between cooler and moist deep soil and your bed mix, so it will dry faster on hot days. So eventually (may be next year) you will need to take soil out of the bed and remove the cloth. By the time grass will be killed and you can put soil back.

This is a frequent question here and the search will pull previous discussions for you with more details. But basically, if you aren't going to dig out the grass then the usual suggestion is to put down several layers of cardboard sheets and set the bed on them. The cardboard will decompose over time and the grass will be killed in the process. When that happens the plant roots will have access to the soil below.

However digging out the grass first is by far the better option and most find they regret not doing it.

Landscape cloth is not usually recommended because it causes drainage problems, water pooling. How much of a problem that may be all depends on how deep the beds are.

Dave

Just did this yesterday to add an additional bed. I placed newspaper down, a section a time, and then placed less decomposed garden material on top of it. Next I placed compost like soil and sprinkled coffee grinds on top and the mass was completed. Covered all of that with yard fabric temporarily so weeds do not take hold. Straw will replace the fabric over the next few days. I soaked the new bed with lots of water at the end. It's about 2.5 feet tall right now. I know I'll need to add extra grass clippings/leaves in the fall to keep the height up and next spring I'll till in to the soil below it. I chose this way to go because I do not own a sod kicker and manually digging up the grass was going to be a major chore.

One additional note, I plan on adding an addition layer of garden type soil on top right before I plant in to it this season as the bed is not a solid mass of planting space.

Sorry, I did search, but didn't find the old threads. I'll look again. The beds are 18" deep. It sounds like if I place the weed cloth over the grass, the main issue would be drainage. I'm in Southern California, where we get virtually no rain during the growing season, so the amount of water that the garden gets is within my control. Does that make a difference? Thanks for the responses. Much appreciated.

I've had excellent results placing 12"-18" deep raised beds over grass. I did nothing other than fill my beds with good compost and topsoil. I did prepare the beds in the beginning of March for primarily mid-May and onwards planting, so the grass never really had much time to perk up from winter before being covered. Plants were vigorous and I had excellent yields.

This may not be a problem in your climate, but I'll just copy a little warning I made in another thread.

Cutworms may be prevalent where there was sod. If you plant tomatoes, peppers, or brassica plants you can put a little collar of aluminum foil around the stem about an inch above and below the surface.
Unfortunately, you may may still wake up one morning to find a row of beans mowed down. If this happens, try to find the critter just under the soil at the last plant eaten, and send him to the Elysian Fields.

I think they type of grass matters a bit too. In WA we had nice "friendly" grass. I could put a nice thick layer of newspaper down or even sheet compost on top of it "Lasagna" garden style and plant right on top no problem. Here in AZ Bermuda grass is a nightmare. The suggestions I have gotten are to try to dig it out as much as possible AND put a layer of cardboard or newspaper. Grubs might be a problem in grassy areas too. Beneficial nematodes can help with that. I am no garden expert but, so many people have been so willing to help me on here that I thought I would try to contribute by adding my 2 cents as it were. :) Good luck with your garden!
Karoleana

I've built two raised beds with nothing more than cardboard lining the bottom. So far I haven't had any issues with the grass below. I agree with karoleana, the kind of grass matters. Good luck!

I strongly advise against putting down a cloth weed barrier. Your plants will do much better if they have access the the soil below. With a barrier, you're just making a giant container.

I've been spending the last 5 years covering about 5,000 sq ft of weedy lawn with garden beds. I cut the grass short with a lawn mower, lay down cardboard on top of it, cover with 3-4 inches of soil and mulch. I get very few weeds popping through. Definitely with 18" of soil over the top of your lawn, the grass won't survive. Line the bottom with cardboard or paper grocery bags if you'd like, but be sure it's something that will decompose. Dump your dirt on top. Many vegetable plants, such as tomatoes, have deep roots, and that decomposed grass will give you a good bounty later in the summer.

"I strongly advise against putting down a cloth weed barrier. Your plants will do much better if they have access the the soil below. With a barrier, you're just making a giant container."

Agree. I just spent a whole bunch of time pulling up weed barrier from beds around my house (that the previous owner put in). The soil is just destroyed underneath...no nutrients, compacted, anoxic. Gonna be a lot of work fixing.

I put in a raised bed (12") over Bermuda without digging it up or killing it and that stuff has no problem coming up in the raised bed, especially along the inside edges. Next time I'll certainly kill it first and use the cardboard method described above.

Good luck.

I put in a raised bed (12") over Bermuda without digging it up or killing it and that stuff has no problem coming up in the raised bed, especially along the inside edges. Next time I'll certainly kill it first and use the cardboard method described above.

Good luck.