A raised garden bed is a contained structure, often made of wood, metal, or stone, that sits above the surrounding native soil. A common question for anyone building one is whether to use a liner to separate the soil from the bed’s walls and base. The decision to line a raised bed is driven by practical concerns, but the material chosen significantly impacts plant health. This analysis determines if standard impermeable plastic is the right choice by examining the science behind common gardening motivations.
Understanding the Core Motivations for Lining
Gardeners consider using a liner for two primary reasons. The first is extending the longevity of the raised bed’s frame, especially when using lumber that is not naturally rot-resistant, like untreated pine. Wood constantly exposed to moist soil decays much faster than dry wood. A physical moisture barrier slows the degradation process of the wood.
Another element is preventing fine soil particles from escaping through the gaps in the structure. Repeated watering can cause fine soil components to wash out through seams or cracks in the side walls. This loss of material leads to soil level depletion and reduces the volume available for root growth. A liner acts as a containment mechanism, keeping the planting medium inside the defined structure.
Drawbacks Associated with Impermeable Plastic
The use of standard, impermeable plastic sheeting, such as thick poly sheeting or trash bags, has several biological and chemical drawbacks. Since plastic is non-porous, it creates a waterproof barrier that impedes the movement of water and air. This lack of permeability often leads to waterlogging, trapping excess moisture and creating an anaerobic environment. Anaerobic conditions suffocate plant roots, promote pathogens, and quickly lead to root rot and plant death.
Another significant problem, particularly in regions with high sun exposure, is excessive heat buildup. Dark-colored plastics absorb solar radiation strongly. When placed directly against the soil, this can cause the temperature in the root zone to rise to unhealthy levels. This thermal stress damages roots and inhibits the biological activity of beneficial soil microbes.
Furthermore, the chemical composition of certain plastics presents a risk of leaching into the edible soil, which is a major concern for growing food. Plastics like polyvinyl chloride (PVC) contain chemicals such as phthalates and Bisphenol A (BPA), which are known endocrine disruptors. These compounds can migrate from the plastic into the soil, potentially contaminating the crops. Choosing non-food-grade plastic introduces a chemical risk that should be avoided in a food-producing environment.
Superior Alternatives for Raised Bed Containment
Instead of using impermeable plastic, gardeners should opt for materials that maintain essential soil aeration and drainage while addressing wood protection and soil containment. The most effective alternative for lining the sides of a raised bed is breathable, woven or non-woven landscape fabric. This material is water-permeable, allowing excess water to drain freely and preventing waterlogging. Landscape fabric also prevents fine soil particles from washing out through the wall gaps, successfully addressing the containment motivation.
When installing landscape fabric, staple it to the interior sides of the wooden frame, stopping just below the soil line. This placement creates the necessary barrier to protect the wood from direct contact with damp soil, extending the structure’s lifespan. Biodegradable burlap or natural coir liners are also viable, non-toxic alternatives. These materials break down over several seasons, enriching the soil rather than contributing to plastic waste.
For beds placed directly onto native soil, a bottom liner is usually unnecessary unless the goal is to deter burrowing pests like gophers or voles. In this case, the superior choice is galvanized hardware cloth with a small mesh size, typically one-quarter inch. This metal mesh creates a durable, long-term physical barrier that pests cannot penetrate. It allows unrestricted water drainage and movement of beneficial organisms between the native soil and the raised bed. Hardware cloth, unlike plastic, is entirely inert and provides a highly effective, specialized solution.