A raised garden bed liner is a material placed between the soil and the interior walls of the bed frame, often extending to the bottom. This addition is a common consideration for gardeners concerned about the longevity of their structure and plant health. Deciding what material to use, or whether to use one at all, can confuse those building their first raised beds. The choice of liner depends entirely on the specific gardening challenges you are trying to address.
The Purpose of Lining a Raised Bed
A primary motivation for installing a liner is to preserve the structural integrity of the raised bed. When wood, the most common building material, is constantly in contact with moist soil, decomposition is accelerated. A barrier between the damp soil and the wooden frame slows down rotting, significantly extending the structure’s lifespan.
Liners also serve in pest and weed mitigation, creating a physical boundary between the garden soil and the native ground. A liner placed on the bottom prevents perennial weeds and grasses from growing upward into the cultivated soil. For areas with burrowing pests like gophers or voles, a liner provides a deterrent, blocking access to plant roots from below.
Recommended Lining Materials
For managing subterranean pests, quarter-inch galvanized hardware cloth is the preferred material for lining the bottom of a raised bed. The small, rigid 1/4 inch mesh stops rodents like gophers and voles from digging upward into the growing medium. This durable metal mesh offers physical protection while allowing plant roots to grow downward into the native soil for moisture and nutrients.
To prevent soil-to-wood contact on the sides, heavy-duty, woven landscape fabric is an excellent choice. This permeable material allows water to drain freely through the soil and out the bottom of the bed, preventing waterlogging. The fabric also keeps fine particles of potting mix and soil from washing out through gaps or cracks in the wooden walls.
Thick, food-grade polyethylene can be used to line the sides of a bed constructed from materials that may leach chemicals. This plastic acts as a non-toxic barrier, sealing off the soil from the bed walls, particularly when the frame is made from treated wood. If using polyethylene, only line the sides and not the bottom to ensure proper drainage.
Materials to Strictly Avoid
Avoid materials that can compromise both plant health and human safety. Thin, non-food-grade plastics, such as standard plastic sheeting or tarps, should not be used as they tend to degrade quickly when exposed to soil, moisture, and sunlight. These materials can break down into small fragments, contaminating the garden soil and potentially leaching undesirable chemicals like phthalates or BPA into the growing environment.
Materials like old roofing shingles or tar paper, which contain petroleum-based compounds, must be avoided entirely. These products are not intended for use in contact with edible plants and can release toxic substances into the soil over time. Similarly, scraps of older pressure-treated lumber should never be used as a liner, as they may contain highly toxic chemical preservatives like chromated copper arsenate, or CCA, which are clearly harmful.
Non-porous materials that completely block water flow, such as solid plastic sheeting used on the bottom without drainage holes, pose a significant risk. Trapping water creates an anaerobic environment that leads to waterlogged soil and root rot, detrimental to plant survival. The liner should never interfere with the soil’s ability to drain excess moisture.
Installation and Drainage Considerations
Proper installation focuses on securing the material while maintaining the necessary pathways for water movement. When lining the sides of a wooden bed, the material should be secured with a staple gun, positioning the staples just below the top edge of the frame. The material should extend to the bottom of the bed but should not cover the base, allowing the soil to connect with the native ground beneath.
If the raised bed is placed directly on native soil, the base should remain completely open or covered only with hardware cloth for pest control. Leaving the bottom open facilitates better drainage and allows plant roots to grow deeper, accessing a larger reservoir of water and nutrients. The hardware cloth, if used, is typically secured to the inside bottom edges of the frame to prevent it from shifting.
When a bed is built on a hard surface, such as concrete or a patio, where drainage is blocked, the lining material must be handled differently. If a liner is used across the entire bottom, it is necessary to puncture numerous drainage holes into the base. Without these perforations, rainwater and irrigation will be trapped inside the bed, creating a stagnant pool that kills plant roots.