Raised bed gardening is a widely adopted method for cultivating vegetables and flowers, offering advantages like improved soil control and reduced back strain. A raised bed liner acts as a barrier placed between the growing medium and the surrounding structure or ground. Choosing the correct material impacts both the longevity of your raised bed frame and the health of your garden’s soil and plants. Understanding the specific function of a liner is the first step in selecting the safest and most effective material.
The Purpose of Lining Raised Beds
Lining your raised bed serves distinct purposes depending on whether the barrier is placed along the sides or across the bottom. A side liner is primarily a protective measure for the bed material, particularly wood. It prevents constant contact between wet soil and the frame, which significantly slows down rot and decay, extending the life of the wooden structure.
The side liner also helps contain the growing medium, preventing fine soil particles from washing out through gaps or seams during heavy watering or rain. Proper side lining ensures the soil level remains consistent and minimizes the loss of expensive planting mix. Without this barrier, moisture trapped directly against the wood can speed up decay if drainage is not properly maintained.
The function of a bottom liner focuses on managing the interaction between the raised bed and the native ground beneath it. A common use is to suppress pre-existing turf or weeds, preventing them from growing up into the cultivated soil. Bottom liners are also effective for pest control, acting as a physical barrier to burrowing animals like gophers, moles, and voles.
While a bottom liner can separate the imported soil from potentially contaminated native ground, it must permit proper drainage. A successful bottom lining strategy allows excess water to pass through freely, preventing the soil from becoming waterlogged and causing root rot. The choice of bottom material depends on whether the primary concern is weed suppression, pest exclusion, or soil contamination isolation.
Recommended Materials for Side and Bottom Lining
The most appropriate lining material depends on the location in the bed and the specific barrier function needed. Landscape fabric, also known as geotextile, is a versatile option favored by many gardeners. This permeable material works well for both side and bottom lining because it allows water and air to pass through while acting as an effective barrier against weeds and soil erosion.
For lining the sides of a wooden bed to prevent rot, a thick polyethylene plastic liner, such as a pond liner, offers the maximum moisture barrier. When selecting plastic, choose food-grade materials, identified by recycling codes \#2 (HDPE), \#4 (LDPE), or \#5 (PP). This material must only be used on the sides and should be stapled loosely to prevent moisture from being trapped between the plastic and the wood, which would accelerate decay.
Hardware cloth, a sturdy metal mesh, is the preferred material for bottom lining when burrowing pests are an issue. To be an effective barrier against small rodents, the mesh size should be no larger than \(1/4\) or \(1/2\) inch. Use galvanized steel or stainless steel hardware cloth to ensure longevity and minimize the risk of rust, which can introduce undesirable elements into the soil.
For an initial, temporary weed barrier at the bottom of the bed, layers of plain corrugated cardboard or newspaper are effective and biodegradable options. These materials suppress weeds long enough for the new soil to settle, eventually breaking down and adding organic matter to the soil. When using cardboard, remove all tape and glossy inks to avoid introducing synthetic or chemical residues.
Materials That Should Never Be Used
Certain materials commonly found around the home or garden center can pose serious risks to human health and plant vitality when used as a raised bed liner. Older treated lumber, particularly wood treated before 2004, often utilized Chromated Copper Arsenate (CCA), which contains arsenic. Using scraps of this old lumber or materials like railroad ties treated with creosote can cause toxic chemicals to leach into the soil, contaminating edible crops.
Many common household plastics and construction materials are not safe for food production environments. Non-food-grade plastics, especially those with recycling codes \#3 (PVC) or \#7 (Other), can leach endocrine-disrupting chemicals like phthalates and Bisphenol A (BPA) when exposed to heat and moisture. These chemicals can migrate into the soil and be taken up by plants, making them unsuitable for growing vegetables and herbs.
Synthetic fabrics like old carpet or certain tarps should also be avoided because they are treated with chemicals, dyes, or flame retardants. As these materials break down from sun exposure and moisture, they shed microplastics and potentially hazardous chemical residues directly into the garden soil. Always choose materials specifically rated for gardening or food contact to ensure the safety and purity of your homegrown produce.