A raised bed is a large container placed directly on the ground or an elevated surface and filled with a prepared soil mixture. The primary function is to provide better control over the growing medium, offering superior drainage and improved soil quality compared to native earth. The decision to leave the bottom open or install a barrier impacts the bed’s function, determining how roots grow, water drains, and pests are managed. This choice depends on the specific location and the gardener’s goals.
Advantages of Leaving the Bottom Open to Native Soil
The most common approach for a raised bed placed directly on the ground is to leave the bottom completely open. This design choice provides significant benefits to plant health and growth. The open bottom ensures superior water drainage, preventing the soil within the bed from becoming waterlogged. Excess water moves freely into the underlying native soil, which helps prevent anaerobic conditions that lead to root rot and fungal diseases.
Allowing direct contact between the raised bed soil and the ground below maximizes the available rooting volume for plants. Deep-rooted crops, such as tomatoes, carrots, or squash, can extend their root systems past the confines of the bed and into the subsoil. This unrestricted growth provides access to a much larger reservoir of moisture and nutrients. The open design also facilitates the exchange of beneficial soil organisms, such as earthworms, which move between the native soil and the prepared medium, improving aeration and nutrient cycling.
The interaction between the two soil types helps stabilize the moisture level within the bed. The native soil acts as a wicking layer, pulling moisture down during heavy rain or drawing moisture up during dry weather. When a raised bed is built over poor or compacted native soil, the introduction of loose, well-amended soil encourages the gradual improvement of the underlying earth. This improves the overall health of the garden area.
When a Solid Base or Impermeable Liner is Essential
While an open bottom is preferred for beds placed on a lawn or garden area, a solid or impermeable base is necessary in specific situations. A closed bottom is required when a raised bed is situated on a non-soil surface, such as a patio, deck, rooftop, or concrete slab. In these cases, the bed functions more like a large container, and a solid base is necessary to contain the soil and manage drainage.
For beds on impervious surfaces, the bottom must be completely sealed with an impermeable material, such as thick plastic sheeting or a solid wood base. The base must then be equipped with drainage holes along the bottom or sides. This setup is necessary because excess water must be directed out without soaking the surface below. Without proper drainage holes, the soil will quickly become saturated, leading to plant death.
Another reason for using an impermeable liner is when gardening over contaminated ground. If the native soil contains harmful substances, such as heavy metals or industrial runoff, a continuous, durable barrier is necessary. This barrier prevents contact between the contaminated earth and the clean soil within the raised bed. The liner must extend up the sides to the soil line to ensure complete separation, providing a safe environment for growing edible crops.
Using Permeable Barriers for Pest and Weed Control
A third option involves using a permeable barrier that addresses specific garden problems without sacrificing the benefits of ground contact. This method is a compromise solution, primarily employed for pest and weed management. Permeable materials allow the passage of water and air but block larger, unwanted elements.
For controlling burrowing pests like gophers, voles, or moles, a layer of galvanized hardware cloth is effective. This wire mesh, typically with quarter-inch or half-inch openings, is secured to the bottom frame before filling the bed with soil. The mesh provides a physical barrier strong enough to deter rodents from digging upward. It still allows water to drain freely and fine plant roots to grow down into the native soil.
Landscape fabric or cardboard can be used to suppress existing weeds and turf grass from growing up into the bed. These materials act as a barrier to light and growth, smothering unwanted vegetation. Unlike impermeable plastic, most landscape fabrics allow water and air to pass through, maintaining drainage while weed seeds are deprived of light. However, very dense fabric or cardboard may restrict the downward growth of plant roots, making it more suitable for shallow-rooted crops or temporary weed suppression.