A large planter is typically defined as a container that is 15 or more gallons in volume, often exceeding 12 inches in depth or diameter. Filling a container of this size entirely with commercial potting mix is both costly and creates a substantial weight problem, especially when placed on decks, balconies, or other structures. The primary solution is the strategic use of filler materials. This practice reduces the volume of expensive growing medium needed, manages the overall weight, and ensures the long-term health of the plants within.
Why Fillers Are Essential for Large Planters
Filling a deep container solely with potting mix is detrimental to both your budget and the plants’ root health. High-quality potting mix is expensive, and large planters can require multiple bags to fill the entire volume, making the initial planting cost prohibitive. The resulting container also becomes excessively heavy once watered, making it difficult to move and potentially unsafe for elevated surfaces.
A column of deep, saturated soil compacts under its own weight and retains too much water at the bottom, a phenomenon known as the perched water table. This constant saturation limits oxygen availability, leading to anaerobic conditions where roots cannot respire effectively, resulting in root rot. Introducing a filler material creates a false bottom, effectively reducing the soil column’s depth. This allows water to drain more efficiently and provides the optimal growing environment for the root system.
Safe Materials for Base Layer Filling
When selecting a base layer, choose materials that will not decompose quickly or leach harmful chemicals into the soil. Non-biodegradable options are preferred for permanent plantings and include:
- Empty, capped plastic water bottles, milk jugs, or crushed aluminum cans.
- Non-dissolving foam packing peanuts or lightweight blocks of styrofoam.
These options significantly reduce weight, provided they are certified non-toxic and not the starch-based kind that dissolves in water.
For a more natural filler, you can use slow-biodegradable items such as chipped wood mulch, pinecones, or large pieces of bark. These organic materials will eventually break down, slowly contributing to the soil volume over several seasons, meaning the container may need to be refreshed periodically. Always avoid using materials that held chemicals, such as old bleach bottles, or wood scraps that have been pressure-treated with preservatives, as these can be toxic to plants.
Assembling the Layers
The process of assembling the layers must be done carefully to ensure the separation of the filler from the growing medium. First, cover the drainage holes with a physical barrier, such as landscape fabric or a fine mesh screen. This prevents the soil from washing out and clogging the drainage system. Next, add your chosen filler material to the desired height, leaving enough space for the required depth of potting mix.
The next step involves creating a separation layer directly on top of the filler material. A sheet of landscape fabric, mesh, or several layers of newspaper prevents the potting mix from migrating down into the filler layer. This separation prevents the soil from settling and maintains the air pockets created by the filler, sustaining proper drainage and aeration. Finally, the top layer of potting mix should be added, typically falling between 12 to 18 inches for most vegetables, shrubs, and deep-rooted annuals.
Choosing the Optimal Potting Mix
The most important layer is the actual growing medium, which must be a high-quality potting mix specifically formulated for containers. Standard garden soil should never be used, as its fine particle size contains too much clay and will quickly compact in a confined space, leading to poor drainage and a lack of oxygen for the roots. A proper container mix is a soilless blend designed to balance aeration, water retention, and nutrient availability.
These specialized mixes typically contain a combination of ingredients to achieve this balance. Peat moss or coir is included to help retain moisture, while materials like perlite or vermiculite are added to create air pockets and enhance drainage. Compost is often incorporated to provide organic matter and a slow release of nutrients. Using the correct potting mix ensures the plant roots have the light, fluffy environment they need to thrive.