better to kill it off first with a systemic weed-killer like roundup. it will also get rid of some perennial weeds if there are any before putting the membrane down. You will still get plants growing in the stones but they will be easier to remove.

If you simply cover the grass with membrane the grass will keep on growing and make your new, stoned, area all bumpy and uneven looking. You have to remove the grass and any weeds, such as dandylion, from the area and level it.