Composite decking is manufactured from a mixture of reclaimed wood fibers and plastic resins, typically polyethylene, polypropylene, or PVC. This material offers an attractive, low-maintenance alternative to traditional lumber for horizontal surfaces. Repurposing this material for vertical structures, such as a raised garden bed, requires careful consideration of the material’s specific chemical composition and its altered structural performance when constrained by soil.
Safety and Composition for Edible Gardens
Composite decking is a wood-plastic composite designed to resist rot and insect damage. However, using this material for growing food raises concerns about the transfer of chemicals from the boards into the surrounding soil.
The primary concern revolves around chemical additives compounded with the plastic resins during manufacturing, such as UV inhibitors and color stabilizers. When boards are in constant contact with moist soil, there is a theoretical pathway for these constituents to migrate through leaching.
Most modern, reputable composite brands are manufactured to be relatively inert and non-toxic once installed. The plastics used, particularly HDPE and polypropylene, are the same polymers commonly found in food-grade containers. This adherence to strict standards results in a product that poses a low risk of measurable chemical transfer to the garden environment.
Gardeners should verify that the selected composite material is free from any older, potentially harmful chemical treatments sometimes associated with earlier generations of wood composites. It is advisable to choose products specifically certified for ground contact or those that explicitly state their suitability for non-leaching applications. Focusing on the composition label and choosing a product with minimal additives provides the greatest assurance of safety for cultivating edible plants.
Structural Performance When Used as Retaining Walls
Composite decking is engineered primarily for spanning horizontal distances and supporting vertical loads. When repurposed as a retaining wall for a raised garden bed, the material faces different forces it was not designed to manage. The most immediate structural challenge is the considerable pressure exerted by saturated soil against the vertical walls.
This force is compounded by the material’s high coefficient of thermal expansion, common to plastic-based products. Composite boards can expand and contract significantly—up to one-eighth of an inch per foot—with changes in ambient temperature. When constrained by cold soil on one side and exposed to sun on the other, this differential movement is exacerbated, leading to substantial bowing or warping if not adequately reinforced.
While composite is highly resistant to rot, it is not entirely impervious to moisture absorption. The wood fiber component can still absorb a small percentage of water, causing minor dimensional swelling, especially in conditions where the boards are perpetually wet or partially submerged in waterlogged soil. This swelling further contributes to the stress on the fasteners and the overall structural integrity of the box shape.
To counteract these forces, a simple four-sided box construction secured only at the corners is insufficient for beds taller than 10 to 12 inches. The boards require robust internal bracing, often involving dedicated vertical posts placed every three to four feet along the length of the wall. These reinforcements must be securely anchored to prevent the long spans of decking from deforming inward under the persistent load of the contained earth.
Economic and Construction Requirements
The financial aspect of using composite decking is a major factor, as the material typically costs significantly more than alternatives like cedar or redwood lumber. A premium composite board can cost two to three times the price of pressure-treated wood. This high initial investment is offset by the material’s expected lifespan, which often exceeds 25 years without requiring staining or sealing.
The assembly process for composite differs from working with traditional wood and requires specific techniques to ensure longevity. Because the material is denser and less forgiving, all fastening points must be pre-drilled to prevent splitting or cracking when screws are driven in. Using specialized, corrosion-resistant fasteners designed for composite material is also advised to accommodate the material’s movement.
Even when used as a retaining wall, the boards should not be fastened tightly end-to-end. A small expansion gap must still be incorporated at the seams to allow for the material’s expected thermal movement. Failure to account for this expansion can lead to buckling, particularly during the hottest periods of the year.
The non-porous nature of the plastic composite means that water cannot permeate the walls, unlike wood. Therefore, adequate drainage must be proactively engineered into the garden bed design. Drilling several one-half to three-quarter-inch drainage holes near the bottom of the boards ensures that excess water can escape and prevents the soil from becoming waterlogged.