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.
Nice looking, low cost flower bed or raised garden. 3 x 5 x 8 Pressure Treated Landscape Timber only cost $ 1.97 at Home Depot. I'll use this basic design for inspiration of a raise flowerbed in my front yard. I like the capped top and for a final touch I'll stain the wood.