Lay down a thick layer of CARDBOARD in your raised garden beds to kill the grass. It is perfectly safe to use and will fully decompose, but not before killing any grass below it. They’ll also provide compost and food for worms. @ Home Improvement Ideas
Author says she has never spent more than $8 to build a 4'x4' raised bed or $35 to build a 4'x20' one. They buy two 1x8"cedar boards, which are naturally rot-resistant, & cut in half, so that it is 4-feet long. Then use a 3" length of a 1x1" pine stake, cut into four pieces, to nail the cedar boards to at corners. Each box costs less than $10.