A raised garden bed is a frame, often made of wood, stone, or metal, that sits directly on the ground and is filled with an imported growing medium. These structures offer significant advantages over traditional in-ground planting, such as improved soil temperature regulation and better drainage, which prevents root rot. They also eliminate soil compaction since gardeners never step on the growing area. The primary benefit is the ability to completely customize the soil composition, requiring careful planning of the fill materials. This guide addresses what materials to put inside the frame to create an optimal, long-lasting environment for your plants.
Preparing the Ground Before Filling
Before adding any growing material, the ground beneath the raised bed requires preparation to suppress weeds and exclude burrowing pests. For weed suppression, lay down a thick layer of plain cardboard or several sheets of newspaper directly on the native soil. This material acts as a barrier, smothering existing weeds by blocking sunlight, and will naturally decompose over time, adding organic matter to the soil below.
To exclude subterranean pests, a physical barrier is necessary. Laying down a sheet of galvanized hardware cloth is an effective way to prevent voles, gophers, and moles from entering the bed from below. For the best protection, a 1/4-inch gauge mesh is recommended to block even the smallest pests. This mesh should be secured to the bottom edge of the frame to create a sealed perimeter that allows water to drain freely.
Prior to placing the cardboard and hardware cloth, lightly loosening the native soil underneath with a garden fork can improve the connection between the raised bed soil and the ground. This action helps to break up any compacted subsoil, which encourages improved drainage and allows plant roots to eventually grow deeper than the confines of the bed itself.
Creating the Ideal Growing Medium
The top 10 to 12 inches of a raised bed must contain a high-quality, balanced growing medium tailored for production. This layer requires a blend of components to achieve the necessary structure, fertility, and drainage, distinguishing it from dense topsoil or lightweight potting mix. A reliable formulation involves three main ingredients: screened topsoil, cured compost, and aeration amendments.
A common and effective ratio includes roughly 50% high-quality screened topsoil or garden soil by volume. This component provides essential mineral content, structure, and a reservoir for water retention. Integrating about 30% fully cured compost is necessary to supply organic matter, a diverse microbial population, and a steady release of nutrients. Using a blend of different compost types, such as leaf mold, worm castings, and manure, can enhance the overall fertility and biological activity.
The remaining 20% of the mix should consist of aeration and moisture-retaining amendments to keep the soil light and well-drained. Materials like peat moss, coconut coir, coarse vermiculite, or perlite prevent the soil from becoming heavy and compacted. This combination ensures that the plant roots have access to necessary oxygen while preventing waterlogging. Organic slow-release fertilizers or rock dust can be incorporated at the time of filling to boost micronutrient levels and provide a foundational feed for the first season.
Strategies for Filling Deep Beds
For raised beds deeper than 12 inches, filling the entire volume with optimized growing medium is often unnecessary and costly. Instead, use a layering technique, sometimes called “lasagna gardening,” to fill the lower half of the bed with inexpensive, bulky organic materials. This strategy reduces the required volume of topsoil and compost, offering significant cost savings.
This layering process utilizes the principles of Hugelkultur, where large organic materials decompose slowly over time. Acceptable bulk fillers for the bottom layer include untreated wood chips, small logs, thick layers of straw, dried leaves, and grass clippings. These materials break down gradually, improving the bed’s long-term water retention capacity and slowly releasing nutrients to the root zone.
Since the decomposition of high-carbon materials consumes nitrogen, the bulky material should be capped with a layer of partially decomposed compost or manure. This mitigates the effect before the final growing medium is added. The high-quality soil mix should always occupy the top 10 to 12 inches of the bed, ensuring primary root systems have immediate access to optimal nutrients and structure.