A raised garden bed is a self-contained planting structure built above the native ground, offering gardeners control over soil composition and depth. Preparing the bottom layer is a foundational step that influences the long-term health and success of the garden. A properly prepared base prevents issues from the native soil, such as aggressive weeds, and saves money by reducing the volume of expensive topsoil needed. The initial layers establish drainage, deter pests, and ensure a thriving environment for plant roots.
Blocking Weeds and Existing Grass
Stopping unwanted plant life from growing up into the cultivated soil is a primary consideration for a raised bed placed directly on the ground. Organic materials like cardboard and newspaper are favored barriers because they are temporary and naturally decompose. A thick layer of overlapping corrugated cardboard, with all plastic tape removed, will smother existing grass and weeds by blocking sunlight. This barrier eventually breaks down, allowing earthworms and plant roots to move between the native ground and the raised bed soil.
Newspaper acts similarly but requires a significantly thicker application, typically four to eight sheets deep, to be effective. As these paper-based layers decompose, they add small amounts of carbon material to the soil structure. This process is preferred by organic growers because it avoids creating a permanent separation between the bed and the earth beneath it.
An alternative is landscape fabric, which provides a more durable and permanent separation while still permitting water to drain through. This woven synthetic material is effective at weed suppression and is less susceptible to tearing than cardboard. However, its permanence can restrict the beneficial movement of earthworms and hinder the natural blending of soil layers over many seasons. For a productive vegetable garden requiring annual digging and soil amendment, temporary organic barriers often prove more practical.
Protecting the Beds from Burrowing Pests
In areas with burrowing mammals, such as gophers, voles, or moles, a physical barrier is necessary to protect plant roots. The most effective material is hardware cloth, a sturdy, welded wire mesh. This material is often mistakenly called “chicken wire,” but hardware cloth is significantly stronger and has a much smaller, square grid pattern that pests cannot squeeze through.
The recommended mesh size is typically 1/2-inch or 1/4-inch. A 1/2-inch mesh is usually sufficient to exclude gophers and allows more space for plant roots to grow through if they reach the bottom layer. Using galvanized or stainless steel hardware cloth is recommended because these materials resist rust and corrosion, ensuring the barrier lasts for many years.
To install, the hardware cloth should be laid across the entire bottom opening of the raised bed and secured to the inside frame. Bending the edges of the mesh up the interior sides of the wooden frame by a few inches creates a vertical barrier. This prevents pests from tunneling up the side walls and into the bed. Securing the mesh with staples or screws along the bottom perimeter ensures a tight, continuous seal.
Layering Organic Material for Volume and Drainage
Filling a deep raised bed entirely with premium bagged soil can be costly, making the strategy of layering organic material an economical and ecologically beneficial choice. This approach, often called “lasagna gardening” or similar to the German method of Hugelkultur, uses bulky organic matter to fill the bottom half of the bed. As this material slowly decomposes, it creates air pockets, improves soil structure, and acts as a long-term, slow-release source of nutrients.
Starting with the coarsest materials provides the most volume and takes the longest to break down. This base can include logs, thick branches, and untreated wood chips. These materials are covered with progressively finer layers, such as straw, leaves, grass clippings, and compostable kitchen scraps. This layering process significantly enhances the bed’s capacity to retain moisture and improve drainage.
A common misconception is that adding a layer of gravel or rocks to the bottom of the bed will improve drainage. In a containerized environment like a raised bed, this practice is actually counterproductive. Water does not easily move from a fine-textured soil into a coarse-textured material like gravel until the soil above is completely saturated, a phenomenon known as the “perched water table.”
The presence of the gravel layer effectively raises the water table, causing water to pool just above the rocks and significantly reducing the usable depth of well-drained soil. This can lead to perpetually soggy soil at the bottom, which encourages root rot and hinders the growth of deep-rooted plants. Instead of rocks, relying on the organic layering technique and a well-structured soil mix provides the most reliable and beneficial drainage.