A raised garden bed is a contained planting area elevated above the native soil, offering a controlled environment for growing. The success of this structure begins with preparing the ground beneath it. The base layer serves multiple purposes, including preventing unwanted plant growth, deterring burrowing pests, and regulating moisture movement. A properly prepared base ensures a long-term, productive garden.
Establishing the Foundation and Barriers
The initial step in preparing a raised bed is establishing a dual barrier system to control weeds and pests entering from the ground below. Weed suppression is achieved by smothering existing turf and weed seeds with organic, biodegradable materials. A single, overlapping layer of plain corrugated cardboard works efficiently by blocking light, preventing photosynthesis and killing underlying vegetation.
Remove all plastic tape and glossy labels from the cardboard before laying it down, as these materials do not break down. Alternatively, use several sheets of black and white newspaper, which functions similarly to starve weeds of light. Both cardboard and newspaper decompose over time, adding carbon material to the soil structure.
To protect against burrowing pests like gophers, voles, and moles, use a physical barrier of galvanized hardware cloth. This metal mesh should be secured directly to the bottom frame of the raised bed using staples or screws, ensuring no gaps. A 1/4-inch opening is the most effective choice to reliably exclude small rodents without impeding drainage.
Utilizing Organic Fillers for Volume and Nutrition
For deeper raised beds, typically 18 inches or more in height, a layering technique known as “lasagna gardening” can save on the cost of purchased soil. This method uses common organic materials to fill the bulk of the bed volume while providing a slow-release source of nutrients as they decompose.
The base of this organic filling should consist of “brown” or high-carbon materials, such as untreated wood chips, small logs, dried leaves, or straw. These materials break down slowly, helping to maintain aeration and structure. These layers are alternated with “green” or high-nitrogen materials, which fuel decomposition.
Appropriate green materials include fresh grass clippings, coffee grounds, and kitchen scraps like fruit and vegetable peels. These nitrogen-rich layers should be thinner than the carbon layers to maintain a balanced composting ratio. This organic filler must be topped with a sufficient layer of finished growing medium, as plants require soil to establish roots and access immediate nutrients.
The Role of Proper Drainage and Soil Depth
A common misconception is that adding a layer of coarse material like gravel or broken pottery to the bottom of a raised bed improves drainage. In reality, this practice can be counterproductive due to the concept of the “perched water table.” Water moving through a fine-textured growing medium will only drain out when the entire column of soil above the coarse layer is saturated.
This phenomenon creates a saturated zone directly above the gravel layer, which is where the plant roots reside, promoting root rot. The water does not move down into the gravel until the capillary tension of the finer soil is overcome. Therefore, the quality of the growing medium controls drainage, not a separate drainage layer.
For most common vegetables, a minimum soil depth of 12 inches is necessary for proper root development and nutrient uptake. The ideal growing medium should be a custom blend, typically a mix of balanced topsoil, aged compost, and a coarse aeration material like perlite or vermiculite. This composition ensures the necessary pore space for air and water, allowing excess moisture to drain naturally.
Materials That Must Be Excluded
Certain materials should never be placed in a raised bed, especially one intended for edible plants, due to the risk of chemical contamination. Lumber treated with older chemical compounds, such as chromated copper arsenate (CCA) or creosote found in old railroad ties, can leach harmful substances into the soil. These chemicals pose a significant risk and must be avoided.
Also exclude any diseased plant matter or weeds that have gone to seed, as these introduce pathogens or persistent weeds into the clean soil. Do not use glossy, colored paper or cardboard that may contain heavy metal-based inks or dyes. Finally, human or pet waste should never be used as a fertilizer or filler due to the potential for introducing harmful pathogens like E. coli into the food production system.