A raised garden bed is a freestanding structure usually constructed of wood sides about 1 to 2 feet above the ground and can vary greatly in area. Regardless of size, multiple raised gardens should be spaced at least 18 inches apart to make it easier to walk in between beds; however, if grass is growing in between, leave enough room for your mower--usually about 24 inches will suffice. Raised gardens are excellent to raise vegetables or herbs in, but are also good for growing flowers and other plants, especially in areas where rocky and clay soils are an issue.
Calculate how much soil you need to fill your raised garden. Topsoil, mulch and similar products are sold in cubic feet or yards. Multiply the length and width in feet to figure out the area of your raised garden. Then, multiply that number (the area) and the depth in feet to calculate the amount of soil in cubic feet you need to fill your garden. Divide the total by 27 to convert it to cubic yards.
Purchase the soil medium to fill your raised garden. Typically, most plants grow well in a mixture that contains between one-fourth and one-half organic matter, such as compost, peat moss or composted manure, and the rest top soil. Vegetables grow well in equal parts topsoil, peat moss and coarse sand. Commercial topsoil, rather than your own soil, is ideal, since it will be loamy and rich and not contain any weeds or disease.
Lay 1 to 2 inches of small stones or pea gravel at the bottom of the raised bed to improve drainage. Evenly spread the stones or gravel with your hands, garden rake or hoe. Place a leveling tool in the middle and near the sides to get it as level as possible. Continue to spread the gravel or stones around until the level indicates the gravel or stones are level. For example, classic levels have a bubble which floats in the middle of a small tube filled with liquid when it is level, and some newer levels have electronic displays that indicate when it is level.
Put down a piece of landscape cloth to stop the weeds from growing over top of the gravel. If you have issues with moles or other burrowing pests, lay chicken wire or a piece of hardware cloth over top of the landscape cloth.
Lay 3 to 4 inches inches of each kind of medium on top of one another (e.g., 3 inches of compost, then 3 inches of topsoil). Then, mix the layers all together with a hoe or garden rake. Then continue to add another layer of each kind of medium and mix them together until your soil is about 4 to 5 inches from the top rim.
Grade the soil so it gently slopes from the middle of the raised bed to the outer edges. Push the soil with your hands, garden rake or hoe so the center is about 1 to 2 inches higher than the sides. The slope should be gradual and not easily noticeable. This will help improve drainage.
Plant your plants, and then add 2 to 3 inches of mulch, such as bark mulch, around the base of the plants.