Once you've planted the plants in your raised bed garden, chances are you have lots of "black space" in between the plants. Those areas of exposed soil, not being used by your precious vegetables, are like a welcome mat for weeds. To keep weed seeds from settling on your soil -- and to prevent grasses and other weeds from popping up from down below -- you need some type of barrier that blocks the sun that weeds need for growing. One option: a layer of pea gravel spread over the raised bed.
Plant your garden plants and allow them to grow to a few inches tall, keeping each plant's space requirements in mind. For example, tomatoes require spacing of 24 to 36 inches between plants, while lettuce only needs about 8 to 12 inches in between plants.
Add compost or any soil amendments you want to use for your growing plants. Once you add your pea gravel to the beds it will be more difficult to add solid compost or powdered or granulated fertilizer around the plants; you may instead have to use a liquid fertilizer or compost tea.
Lay sheets of cardboard, coffee bags or landscape fabric around the plants, giving each plant 2 to 3 inches of open space around its stalk. Completely cover all areas of the raised bed, leaving no gaps or cracks in coverage except for the area directly around the plants.
Lay 2 to 3 inches of pea gravel over the cardboard, coffee bags or fabric, taking care not to lay any gravel in the area around the plant stalks. To cover the area with 2 inches of gravel, you'll need about .01 cubic yards per square foot. The idea is to cover the area that is not being used by the plants, making it more difficult for weeds to take root.