A raised garden bed with legs is a large container garden elevated off the ground, offering comfortable working height and improved drainage. Preparing the base is a multi-step process unique to container gardening. The materials placed at the bottom must prevent the fine potting mix from falling out, block climbing pests, and regulate moisture for healthy root development.
Essential Liners for Soil Containment
The immediate concern for any raised bed, especially one with slats or gaps, is preventing the soil from washing out. A liner is necessary to keep the potting mix contained while still allowing excess water to drain freely. Choosing a permeable material is important to avoid a waterlogged environment, which can lead to root rot and fungal diseases.
Landscape fabric, often called weed barrier cloth, is an ideal choice. This woven material is water-permeable, acting as a sieve that holds back solid soil particles but lets water pass through. Burlap or natural fiber sacking can also serve as a temporary, biodegradable option, though it breaks down quickly.
The fabric should be laid inside the bed and stretched up the interior sides, then fastened with a staple gun to the frame just above the base. This provides a clean pocket to hold the soil. Plastic sheeting or other non-permeable materials should be avoided entirely, as they trap water and suffocate plant roots.
Physical Barriers for Pest Exclusion
While a fabric liner manages soil and water, it offers no defense against small mammals that can climb the legs or access the bed from below. Elevated beds are susceptible to voles, mice, and squirrels, which can chew through fabric liners to reach roots and seeds. A durable, physical barrier is required to fully pest-proof the growing space.
Hardware cloth, a galvanized or stainless steel wire mesh, forms the most effective barrier. It is a rigid material that pests cannot chew through or push past. The ideal mesh size for pest exclusion is one-quarter inch (1/4″) or one-half inch (1/2″).
The smaller one-quarter inch mesh provides maximum protection against tiny rodents like field mice and voles, which squeeze through larger openings. This wire mesh should be installed first, fastened securely to the wooden frame of the base, creating a solid metal floor before any fabric liner or soil is added. This forms a permanent shield, allowing water to drain while keeping climbing or burrowing creatures out.
Strategies for Reducing Soil Volume
Filling a deep raised bed completely with commercial potting mix can be expensive. Gardeners often use filler materials to occupy the lower sections. This technique involves layering organic, bulky materials that decompose over time. This approach saves money on initial soil costs and adds slow-release nutrients as the filler breaks down.
Acceptable filler materials for the bottom third or half of a deep bed include clean, untreated wood scraps, logs, thick branches, and straw. These materials create air pockets, promoting aeration and improving drainage capacity. Logs and thicker branches are preferred because they decompose slowly, meaning the soil level will settle less dramatically over a single growing season.
Avoid using materials that will not break down or that will hinder water movement. Gravel or coarse rocks, for instance, do not improve drainage and can actually create a “perched water table” effect. This phenomenon occurs when water saturates the soil above the layer of rock, making the soil wetter and potentially drowning roots.
Cardboard is another useful filler, as it is a carbon-rich material that breaks down quickly and attracts beneficial earthworms. Flattened cardboard boxes, with all tape and labels removed, can be laid over the bulky wood layer. This layer acts as a temporary barrier to prevent the finer potting soil from sifting down into the large gaps.
For the final layer, ensure at least 10 to 12 inches of high-quality potting mix is added on top of the filler materials. This provides sufficient depth for most vegetable roots to develop fully. As the organic materials decompose, plan to top off the bed with fresh compost and potting mix annually to maintain the soil level.