Large planting containers offer flexibility in design but present two major practical difficulties. Filling these deep vessels entirely with potting mix is often prohibitively expensive due to the sheer volume required. Furthermore, a large planter filled completely with saturated soil becomes extremely heavy, making it difficult or impossible to move and potentially compromising the structural safety of decks or balconies. Using an inert filler material at the base addresses these challenges by taking up space without adding significant cost or weight, maintaining portability and cost efficiency.
The Practical Rationale for Filler
The decision to use filler in large planters is primarily driven by economic and logistical considerations. Potting mix is significantly more expensive by volume than garden soil or compost, and a planter measuring three feet deep requires hundreds of pounds of this medium. By substituting the lower portion of the container with a lightweight, inexpensive filler, gardeners can dramatically reduce purchasing costs associated with large-scale container planting.
Weight management is the second significant motivation for this technique, directly impacting the safety and utility of the container. Saturated potting mix can weigh between 75 and 100 pounds per cubic foot, meaning a large container can easily exceed the load limits of a balcony or deck. Introducing materials that are largely air, such as crushed plastic, can reduce the overall weight by half or more. This weight reduction allows the planter to be safely placed in elevated locations and makes seasonal repositioning for sun exposure or winter storage a much simpler task.
Recommended Lightweight Filler Materials
The best materials for displacing volume in a large planter are those that are lightweight, inert, and non-degradable. Empty plastic beverage bottles and milk jugs are excellent choices because they are readily available and contain large pockets of air. These containers should have their caps screwed on to maximize air volume and resistance to crushing from the weight of the soil above. Clean, crushed aluminum cans or rigid plastic packaging inserts from consumer goods can also be utilized effectively to fill the lower cavity.
Gardening-specific items also provide an optimal solution. Upside-down plastic nursery pots, particularly those that are cracked or no longer useful for plants, fit well into the base of a larger container. Specialized lightweight foam blocks, sometimes sold as planter filler, are engineered specifically for this purpose and offer superior drainage capabilities due to their structure.
Using these materials ensures that the base of the container remains stable without introducing substances that will decompose or leach chemicals into the growing medium. The filler reduces the required soil volume and prevents the planter from becoming overly heavy. These materials do not require any specialized preparation beyond ensuring they are clean and free of residual food or chemical contaminants before installation. The air pockets they create must be separated from the potting mix to maintain soil integrity.
Materials That Should Be Avoided
Several common suggestions for planter filler should be avoided, particularly heavy materials like gravel or broken pottery shards. Contrary to the belief that they improve drainage, placing a layer of rocks at the bottom of a container worsens the situation through a phenomenon known as the “perched water table” effect. Water tends to move slowly from a fine-textured medium, like potting mix, into a coarse-textured medium, such as gravel.
Instead of draining freely, the water accumulates just above the transition layer, creating a saturated zone where plant roots can sit in standing water. This raises the level of saturation higher into the root zone than if the container were filled entirely with potting mix, leading to root rot and anaerobic conditions.
Using organic materials, such as wood chips, grass clippings, or old newspaper, is also not recommended for long-term plantings. As these organic fillers decompose, the process consumes nitrogen from the overlying potting mix, effectively stealing this necessary nutrient from the plant roots. Furthermore, the material breaks down over time, causing the level of the potting mix to sink significantly within a year or two. This necessitates constant topping off and disrupts the established root systems of the plants.
The Proper Execution of Layering
Once the appropriate lightweight filler material is selected, the transition between the filler and the potting mix must be managed correctly to ensure the long-term success of the container. The filler material should be placed into the bottom of the container, leaving sufficient depth for the root ball of the plant and future growth. Most ornamental and vegetable plants require a minimum of 12 to 18 inches of actual potting mix to thrive.
A barrier layer must be installed directly over the filler materials to prevent the fine particles of the potting mix from sifting down into the voids created by the filler. This barrier is typically made from a piece of landscape fabric, a section of window screen mesh, or several layers of newspaper. The purpose of this layer is not to impede water flow but to physically separate the two different material types.
Preventing the potting mix from migrating downward ensures that the air pockets remain open, maintaining the lightweight quality and preventing the air voids from becoming clogged with soil. If the soil sifts down, it could eventually compact around the filler, restoring much of the weight the gardener sought to eliminate. After the barrier is placed, the container should be filled with fresh, sterile potting mix to the desired level, allowing for a space near the rim for watering.