A planter box is an above-ground container designed to hold soil and plants, separating the cultivated area from the native ground. Whether the box needs a solid base depends on its placement and the gardener’s goals. The decision to include a bottom dictates how water will drain, the potential for root growth, and the necessary construction materials. Understanding these factors helps create the best environment for plants to thrive.
When a Solid Base is Essential
A solid base is required for any planter box that must be moved frequently, as the bottom contains the soil during relocation. This construction is also mandatory when placing a box on an impervious surface, such as concrete patios, decks, or balconies. A base prevents soil and water runoff from staining the material below and directs water to controlled drainage points.
The base requires multiple drainage holes to allow excess water to escape efficiently and prevent waterlogging. Proper drainage is important because standing water suffocates plant roots, leading to root rot. Elevating the box slightly using feet or risers ensures that the drainage holes do not become blocked, allowing for unimpeded water flow and air circulation.
An enclosed base also prevents plant roots from penetrating and potentially damaging sensitive underlying materials, such as wooden deck boards or waterproof membranes.
The Advantages of an Open Bottom Design
An open-bottom design transforms a planter box into a contained raised bed, providing direct contact with the native soil beneath. For boxes placed directly on the ground, this design offers superior drainage, as excess water wicks naturally into the earth below. The absence of a solid floor eliminates the risk of a perched water table, which occurs when water accumulates above the restrictive layer of a container bottom.
This open connection allows deep-rooted vegetables, such as carrots, potatoes, and tomatoes, to extend their root systems into the native ground. Access to the underlying soil provides additional moisture and nutrients, increasing the plant’s stability and resilience during dry periods.
From a construction standpoint, forgoing a bottom saves on material costs and labor, especially for large installations. Placing the wooden sides directly on a prepared ground surface can also minimize wood rot by allowing air to circulate freely around the base.
Managing Soil, Drainage, and Protection
Managing the interface between the soil and the bottom is important for long-term gardening success, regardless of the base type. For open-bottom boxes, a barrier of half-inch hardware cloth should be secured to the frame before adding soil if ground-dwelling pests like gophers or voles are a concern. This wire mesh creates a physical barrier against burrowing animals while permitting water and beneficial earthworms to pass freely.
A layer of permeable landscape fabric can be placed over the hardware cloth or the solid base. This prevents fine soil particles from washing out through drainage holes or into the ground. The fabric maintains the integrity of the soil structure and prevents clogging without impeding drainage.
It is important to avoid adding coarse material, such as gravel or broken pottery, to the bottom of the box. This common mistake does not improve drainage; instead, it increases the height of the saturated zone within the soil column, which can lead to poor root health.
To protect wooden planter box walls from premature deterioration, line the interior sides with heavy-duty plastic or a moisture barrier. This liner should not cover the bottom, as that would defeat the purpose of the drainage system. It acts as a buffer, minimizing direct contact between the damp soil and the wood, thereby extending the lifespan of the structure.