Your raised bed may help keep your plants' roots warmer, but if you haven't added a barrier between the bed's soil and the ground underneath, eventually you may find that those hospitable conditions make the area attractive for grasses and weeds. When that's the case, you could strain your back as you pull out each blade of grass one by one, or you could create a barrier between your precious garden soil and the ground below. The process might take a little time, but it's an inexpensive way to keep grass out of your raised bed.
Measure the width and length of the raised bed with a measuring tape, and then obtain enough flat cardboard to cover the area of the raised bed twice over.
Dig a trowel or spade several inches deep around any existing plants you have in your raised bed, and then carefully pull out the plants and set them in a bucket or garden pot. Take care to dig a circle wide enough around each plant so as not to disturb the plant's roots.
Shovel out the dirt in your raised bed and place it on a tarp or other non-porous surface, where you can use it again. If the raised bed is totally overrun with grass however, you may opt to replace the existing garden soil with new soil.
Mow or weed whack the area around the raised bed, as well as the area inside the raised bed to get the existing grass as low as possible.
Insert a pitchfork into the grass around the inside perimeter of the raised bed. Press on the pitchfork firmly with your foot and then turn the top layer of grass over. Pull the top grassy layer out with your hands, leaving just the soil underneath behind in the raised bed.
Lay one layer of cardboard inside the raised bed, completely covering the surface inside the perimeter. Overlap each piece of cardboard by about 3 to 4 inches. If you can, slide the cardboard under the bottom edges of the raised bed.
Lay a second layer of cardboard over the first, putting the middle of one piece over the seams of two pieces on the lower layer. At the edges of the raised bed, flip the cardboard up slightly so that it rests against the inside walls of the raised bed.
Spray water on the cardboard with a garden hose until it is soaked thoroughly.
Replace the soil you removed, or add new garden soil and manure to the bed, and then replant any plants you removed. Generally, you should have about 1 foot of organic material or soil on top of the cardboard layer.