A planter filler is an inert, non-degradable material placed at the bottom of a large container to occupy space before adding the growing medium. This technique addresses logistical challenges associated with using oversized planters, which are popular for urban and patio gardening. Utilizing fillers is a common, cost-effective strategy employed by gardeners to manage both the weight and material costs of large-scale container projects.
Why Planter Fillers Are Used
One motivation for using fillers is to reduce the overall weight of the container garden. Large pots filled entirely with moist potting soil are heavy, making them difficult to move and potentially unsuitable for placement on balconies or decks with weight restrictions. Substituting heavy soil with lightweight, inert materials maintains the container’s visual size without excessive mass.
The practice also offers financial savings by reducing the necessary volume of expensive commercial potting mix. Using inexpensive or recycled filler material significantly lowers the total investment required. Fillers also help manage the phenomenon known as the perched water table.
The perched water table is a layer of saturated soil that forms near the drainage holes. Placing an inert filler raises the functional soil level higher up in the container, elevating the plant’s root zone. This repositioning ensures roots receive better aeration and are less likely to suffer from waterlogging near the base of the pot.
Recommended Non-Organic Filler Materials
The safest materials for use as planter fillers are inert, non-degradable, and chemically stable.
Empty plastic nursery pots, often accumulated from previous purchases, are excellent fillers when placed upside down. They create significant void space, remain stable, and allow water to flow freely while promoting air circulation beneath the soil.
Common household plastics like empty milk jugs and water bottles also function well as volume reducers. Secure the caps tightly on any plastic bottles to prevent them from collapsing under the weight of the soil. These items are lightweight, readily available, and will not decompose, ensuring the long-term structural integrity of the growing medium.
Specialized structural foam, such as custom drainage cells or lightweight foam blocks, offers a professional solution. These engineered materials are durable, resist compression, and provide maximum void space with minimal weight. Using such products helps ensure long-term stability and optimal drainage.
Polystyrene packing peanuts can also be utilized, but only the traditional, non-biodegradable, petroleum-based type is appropriate. Peanuts made from cornstarch or other organic materials will dissolve and decompose when exposed to moisture, causing the soil to settle dramatically and creating drainage issues.
Materials to Avoid Using as Fillers
A common mistake is using materials like gravel, rocks, pebbles, or broken pottery at the bottom of the planter to improve drainage. Contrary to popular belief, placing a layer of coarse material beneath the soil actually worsens drainage and increases the height of the perched water table.
This occurs because water will not move from the fine-textured potting mix into the coarse rock layer until the soil above is completely saturated. This resistance, known as the capillary break, leads to waterlogged conditions in the root zone just above the rock layer.
Organic materials should also be strictly avoided as fillers because they introduce instability. Items such as wood chips, pine cones, leaves, or cardboard will decompose over a growing season. As these materials break down, they create air pockets and cause the potting mix to settle unevenly, potentially damaging roots.
Decomposing organic matter also consumes nitrogen as part of the breakdown process, stealing this nutrient from the plant roots. This nutrient depletion can result in nitrogen deficiencies, leading to stunted growth and yellowing leaves.
Gardeners should also be cautious about using any plastic item that is not certified as food-grade or specifically designed for outdoor use. Unknown plastics may leach harmful chemicals, such as phthalates or BPA, into the soil, which can then be absorbed by the plants. Only use materials known to be inert and chemically stable.
Implementing the Filler Layer Correctly
Proper implementation requires a clear physical separation between the filler material and the potting mix. After placing the inert filler into the container, a barrier layer must be installed directly on top of it. Suitable barrier materials include landscape fabric, old window screening, or a section of burlap.
This separation layer prevents the finer particles of the potting soil from sifting down into the voids created by the filler material. If soil sifts through, drainage efficiency is compromised, and the weight reduction benefit is lessened due to compaction. The barrier ensures the lightweight structure remains intact while allowing water to pass freely to the drainage holes.
The depth of the potting mix above the barrier is important for plant health. Most plants require a minimum of 8 to 12 inches of actual soil depth for healthy root growth and proper moisture retention. Vegetables and deep-rooted plants require sufficient soil volume above the filler to thrive.