A raised garden bed provides control over soil quality and drainage for growing vegetables and flowers. Preparing the base layer correctly is crucial, as it prevents long-term problems like invasive weeds, burrowing pests, and poor water management. This foundational step ensures a healthier and more productive garden.
Materials for Weed Suppression
The primary goal of a base layer is to block existing weeds and turf from growing up into the soil. An effective organic approach uses plain corrugated cardboard or newspaper as a biodegradable barrier. Lay a single layer of thick, non-glossy cardboard or several overlapping sheets of newspaper directly on the ground after removing any tape or staples. This material smothers vegetation by depriving it of sunlight, yet still allows water and earthworms to pass through freely.
As the carbon-rich cardboard decomposes, it attracts beneficial organisms and slowly adds organic matter to the native soil below. This temporary barrier lasts about one or two growing seasons, by which time the weed seeds are usually exhausted. For installation, generously overlap the sheets at the seams to prevent light from filtering through, and then completely saturate them with water before adding soil.
Inorganic options, like landscape fabric, offer a more durable solution but have drawbacks. While landscape fabric prevents upward growth and is permeable to water, it is not biodegradable and can interfere with the soil ecosystem. Over time, organic matter settles on top, allowing weeds to germinate and root directly into the fabric. This makes the fabric difficult to remove without tearing, often creating more maintenance issues than it solves.
Physical Barriers Against Burrowing Pests
In areas near open fields or wooded lots, the base layer must exclude burrowing pests like voles, gophers, and moles. These pests enter the bed from below and consume root vegetables and plant roots, causing significant damage. A simple weed barrier is ineffective against these animals, requiring a dedicated physical barrier.
The recommended material is galvanized steel hardware cloth, specifically a mesh with openings of 1/2 inch or 1/4 inch. This size is small enough to stop juvenile gophers and voles, yet large enough to permit water drainage and allow beneficial insects and earthworms to move through. Standard chicken wire is inadequate because its larger openings and thinner metal can be easily chewed through or distorted.
To install this barrier, roll the hardware cloth across the entire base of the bed and bend it up the inside walls for several inches, creating a protective bowl shape. Secure the edges of the mesh to the inside of the bed frame with heavy-duty staples or screws. This prevents pests from tunneling up the sides and ensures a continuous, impenetrable barrier that protects root crops for many years.
Addressing Drainage and Base Fillers
A widely held misconception is that a layer of coarse material, such as gravel, rocks, or broken pottery, must be placed at the bottom of a raised bed to improve drainage. Scientific principles of soil physics demonstrate that this practice is actually counterproductive when the bed is placed directly on the native ground. Water does not readily move from a fine-textured soil (like garden mix) into a coarser material (like gravel) until the finer soil is completely saturated.
This phenomenon is known as the “perched water table effect.” The water accumulates just above the layer of gravel, artificially raising the saturation level higher into the root zone. This can lead to root rot and deprive plants of oxygen, which is the exact opposite of the intended effect. Therefore, for beds placed on soil, the best practice is to fill the entire depth with a high-quality, well-draining garden soil mix.
Organic Base Fillers for Deep Beds
For exceptionally deep raised beds, often exceeding two feet, or for gardeners seeking to reduce the cost of imported soil, organic base fillers can be used. Following the principles of Hugelkultur, coarse organic matter like partially rotted logs, thick branches, and wood chips can be placed in the bottom third of the bed. This woody material slowly decomposes, which helps to retain moisture like a sponge over time and gradually releases nutrients into the soil. As this material breaks down, it will cause the soil level to settle, requiring a top-up of fresh compost and soil after a year or two.