Lining a raised garden bed involves installing a barrier material against the inner walls or beneath the structure to separate the soil from the frame or the ground below. Lining is not a universal requirement; it is unnecessary for beds built with naturally rot-resistant wood like cedar or inert materials like galvanized steel. However, lining becomes important when the goal is to prevent contaminants from leaching into the soil or to extend the lifespan of a wooden frame.
Primary Functions of Raised Bed Liners
A primary function of any liner is to create a moisture barrier between the damp soil and the wooden frame. This separation prevents the wood from absorbing excessive moisture, which causes decay and rot, extending the structural life of the raised bed. For untreated or less durable woods, like pine, a liner can double the number of years the frame remains intact.
Lining also serves as a chemical barrier, which is important if the bed is constructed from materials that could leach toxins into the growing medium. This barrier prevents direct contact between the soil and chemically treated wood, such as older railroad ties or pressure-treated lumber containing compounds like chromated copper arsenate (CCA). By blocking this contact, the liner ensures the soil remains safe for growing edible plants.
When placed on the base of a raised bed, a liner also provides effective root and pest control. Aggressive weeds and grasses from the ground below can be blocked from growing up into the garden soil. For beds placed directly on soil or grass, a metal barrier is effective at preventing burrowing rodents, such as gophers and moles, from invading and damaging plant roots.
Recommended Lining Materials for Longevity and Safety
For creating a moisture and chemical barrier on the interior sides of a raised bed, High-Density Polyethylene (HDPE) plastic sheeting is highly recommended due to its inert nature and durability. HDPE is a stable, non-leaching plastic (identified by recycling code #2) that is safe for food and water contact and will not introduce harmful chemicals into the soil. A heavy-duty thickness, such as 40 mil (0.040 inches), provides excellent tear resistance and a lifespan that can exceed the wood frame.
An alternative for lining the sides is non-woven landscape fabric, a permeable option that allows for the exchange of air and water vapor. This breathability prevents moisture from becoming trapped between the soil and the wood, which can accelerate rot when using non-porous plastics. Landscape fabric prevents soil from washing out through gaps in the wood but offers less chemical barrier protection and a shorter lifespan than thick HDPE.
For preventing animal intrusion from below, the best material for the base of a raised bed is hardware cloth, a galvanized or vinyl-coated wire mesh. Unlike landscape fabric, the rigid mesh forms an impenetrable barrier to rodents like gophers and voles. A mesh size of 1/2-inch or less is recommended to block pests while still allowing for excellent drainage and the movement of beneficial organisms like earthworms.
Materials to Avoid and Common Failures
A number of materials should be avoided as raised bed liners because they pose a risk of chemical leaching or lead to poor performance. Highly toxic materials such as old carpet, roofing shingles, or certain vinyl plastics (like PVC, marked with recycling code #3) can leach chemicals, including volatile organic compounds (VOCs) or plasticizers. Using these materials introduces a risk of contaminating the soil where food is grown.
Older pressure-treated lumber, particularly wood treated with chromated copper arsenate (CCA) before 2003, contains arsenic and copper that can leach into the soil. While modern treatments like Alkaline Copper Quaternary (ACQ) are safer, using a robust liner is still prudent if the wood is treated or its composition is unknown. Railroad ties treated with creosote must be avoided entirely, as the oily, tar-like substance is carcinogenic and will continuously leach into the soil.
A common performance failure occurs when gardeners use impermeable plastic sheeting that fully wraps the bottom of the bed without adequate drainage holes. This approach creates a bathtub effect, leading to waterlogged soil, root rot, and a lack of oxygen for plant roots. Furthermore, using thin materials like cardboard or newspaper to line the sides for wood preservation is ineffective, as these organic materials break down rapidly, requiring the bed to be emptied and re-lined quickly.
Installation Techniques for a Successful Liner
When installing a liner on the sides of a raised bed, secure the material tautly against the interior walls to prevent the soil from pushing it out of place. Use a staple gun with galvanized staples or short, non-corroding tacks to affix the liner to the top edge of the lumber. Placing fasteners every six to twelve inches provides sufficient hold to manage the pressure of wet soil.
To prevent water from wicking out of the soil and accelerating decay on the top edge of the wood, the liner should be trimmed to stop just short of the top rim. The material should end about one to two inches below the frame’s top edge, which also keeps the liner out of sight. This small gap allows the top of the wood to dry out, while the liner remains protected from direct sunlight that could accelerate its degradation.
For base liners, such as hardware cloth, the mesh must be secured to the bottom frame of the bed before filling it with soil. The hardware cloth should be bent and fastened up the interior side walls by several inches to create a continuous barrier, ensuring no gaps exist for pests. If the bed is placed on a hard surface like concrete and an impermeable liner is used, several small drainage holes must be pierced through the material to provide an escape route for excess water.