Answer: You could cover the area with a heavy mulch to smother the grass. Newspaper about 5 sheets thick may be practical if the area is small, but a sheet of black plastic laid over the area for about two weeks would also be useful. The grass would need to be growing (not dormant) for this to be really effective.
Another option would be to use the herbicide glyphosate (an example product is Roundup). This herbicide will kill most green things it is sprayed on and breaks down quickly in the environment. Give it a week to move in and begin to kill the weeds. Then, the area can be plowed or rototilled to prepare for the garden. A week after rototilling, you can plant seeds. This product works best on actively growing weeds, not stressed or dormant ones.
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