A planter box is a controlled container environment for growing plants, typically constructed from wood or metal. Lining the box serves multiple protective functions, primarily shielding the structure from constant soil moisture. This barrier maximizes the planter’s longevity by preventing premature rot in wood or corrosion in metal. A proper liner also creates a stable, isolated environment for root systems, managing moisture levels and preventing potential contamination from the planter material.
Essential Liner Materials and Their Purpose
The choice of liner material depends on the specific function needed within the planter box. Materials fall into two categories: non-permeable, waterproof barriers and permeable layers for filtration. Both types are often used together to achieve the best results for plant health and structural preservation.
Non-Permeable Barriers
Non-permeable liners, such as heavy-duty polyethylene sheeting or pond liners, maximize the lifespan of wooden planter boxes. They prevent direct contact between the damp soil and the interior wood surfaces. For vegetable gardens, select plastics rated as food-safe, such as high-density polyethylene (HDPE) or low-density polyethylene (LDPE). These thick plastics are durable and do not break down or leach chemicals into the growing medium. Using a non-permeable liner is particularly important if the planter is constructed from older, chemically treated lumber, ensuring harmful compounds are isolated from edible crops.
Permeable Barriers
Permeable liners, such as landscape fabric or geotextile material, allow water to pass through freely. The primary role of this material is soil retention and separation. When placed over drainage holes, the fabric prevents fine soil particles from washing out or clogging the openings. This maintains consistent drainage over time, which is necessary for healthy root growth. Landscape fabric is also used to separate the main soil column from a drainage layer or to line the bottom of a box set directly on the ground.
Ensuring Proper Drainage and Soil Integrity
Managing water flow and preventing pest intrusion are necessary steps for a successful planter box, regardless of the liner material used. Drainage holes are mandatory, especially when using non-permeable plastic liners. These openings must be strategically placed in the bottom of the box to allow gravity to pull excess water out, preventing the anaerobic conditions that cause root rot. A sufficient number of holes, typically spaced every few inches, ensures that water does not pool at the base of the container.
Pest Barrier
A physical barrier against burrowing animals is an effective measure for maintaining soil integrity. Hardware cloth, a galvanized steel mesh with small openings (typically 1/4-inch or 1/2-inch), should be secured to the bottom of the planter box. This durable mesh prevents rodents like gophers and voles from digging up into the soil from below. The rigid structure provides a permanent, secure base that water passes through easily.
Avoiding Gravel Layers
Modern gardening science advises against adding a layer of gravel or stones to the bottom of a planter for drainage. This coarse material can create a “perched water table” just above the layer where the fine soil meets the gravel. This occurs because water does not easily move from finer potting soil to coarser gravel until the soil is fully saturated. Instead of improving drainage, this saturated zone increases the risk of waterlogging and root death. The best approach is to use a high-quality, well-draining potting mix throughout the entire container, relying on the drainage holes alone to manage excess moisture.
Safety and Materials to Avoid
Certain materials should never be used as a planter box liner due to potential safety hazards or lack of effectiveness. Lumber treated with older chemicals, such as chromated copper arsenate (CCA) or creosote, poses a risk of chemical leaching into the soil. If you have an older planter, a complete non-permeable plastic liner is the only way to fully isolate the soil from the treated wood. When selecting plastics, avoid any material where the chemical composition is unknown, particularly when growing edible plants.
Thin plastics, such as standard garbage bags or thin plastic sheeting, should be avoided. These materials degrade rapidly when exposed to constant moisture and soil microorganisms. As they break down, they can tear easily, compromising the water barrier and potentially releasing microplastic fragments into the soil. Choose thick, commercial-grade plastics labeled as food-safe for the best long-term results.
Highly biodegradable materials, such as thin burlap, cardboard, or newspaper, are ineffective as long-term liners. While they can temporarily suppress weeds or hold soil, they decompose quickly, often within a single growing season. This rapid breakdown negates the liner’s purpose of protecting the planter structure and maintaining soil integrity over multiple years. A durable, non-biodegradable material is preferable for permanent lining applications.