A large planter is generally defined as any container exceeding 20 gallons in volume or having a diameter greater than 24 inches. Completely filling these containers with commercial potting mix presents two significant challenges. The quantity of soil required leads to considerable expense, and a fully soil-filled container can weigh hundreds of pounds, making it nearly impossible to move once planted. Strategic filling methods are necessary to reduce both the financial burden and the final weight of these sizable garden features.
Preparing the Container Base
The first step involves ensuring proper drainage, which is fundamental to plant health. Begin by checking the bottom of the planter to confirm that the drainage holes are clear and unobstructed. These openings allow excess water to escape, preventing the anaerobic conditions that lead to root rot.
To prevent fine particles of potting mix from washing out and clogging the drainage holes, a permeable barrier must be placed directly over them. Materials like fiberglass window screening, a coffee filter, or a small square of landscape fabric work effectively. This layer ensures that water can pass freely while keeping the soil medium contained within the vessel.
Strategic Use of Lightweight Filler Materials
The primary objective of using filler materials is to displace volume in the lower half of the planter without adding substantial mass or cost. These materials should occupy the space that roots are unlikely to penetrate, often the bottom one-third to one-half of the container’s depth. Using appropriate fillers significantly reduces the amount of expensive potting mix required.
Excellent filler options are those that are non-degradable and stable, such as clean, inverted plastic nursery pots. Place these pots upside down to maximize air space. Another effective choice involves using empty plastic water or soda bottles with their caps securely fastened to maintain structural integrity.
For the lightest option, specialized inert products like non-biodegradable packing peanuts are ideal for occupying large volumes. If stability against wind is needed, lightweight aggregate like lava rock or perlite can be used sparingly. These materials are permanent solutions that will not break down or compact.
It is necessary to avoid using organic materials as fillers, such as wood chips or grass clippings. These items decompose over time, causing the soil level to sink dramatically and creating air pockets that disrupt root systems. Their decomposition also consumes nitrogen from the potting mix, stealing nutrients intended for the plant.
Adding the Root-Ready Potting Medium
Once the lightweight fillers are securely placed, the space above them must be filled with a high-quality growing medium where the plant roots will reside. A second layer of landscape fabric can be placed over the fillers to prevent the potting mix from sifting down, though this is optional if the filler material is large and stable. The depth of this final potting medium layer is determined by the plant’s needs, often ranging from 10 to 18 inches.
For shallow-rooted annuals or herbs, 8 to 10 inches of potting mix is typically sufficient. Deeper-rooted perennials, shrubs, or small trees require 12 to 18 inches of medium for proper anchoring and moisture retention. This top section must consist of a specialized potting mix, which is distinct from native garden soil.
Potting mixes are sterile formulations engineered for container gardening, offering superior aeration and drainage properties. Garden soil, by contrast, is too dense and becomes compacted in a container, restricting water flow and oxygen exchange to the roots. The porosity of commercial potting mix allows for the necessary balance of water retention and gas exchange, promoting robust root development. This layer is the sole source of water and nutrients for the plant.
Post-Filling Steps and Maintenance
Before beginning the filling process, position the empty planter in its final intended location. Once filled with filler material, potting mix, and water, even a strategically lightened container will still possess a significant, often immovable, final weight. Attempting to relocate the planter after filling can result in structural damage or personal injury.
After planting, the container requires a thorough initial watering, often referred to as soaking. This deep saturation helps the potting mix settle around the filler materials, eliminating large air pockets that could cause future sinking. Over the first several weeks, the potting mix will inevitably settle and compress. Gardeners should monitor the soil line during this period and top off the container with fresh potting mix as needed.