Container gardening with large vessels presents a unique set of challenges that differ significantly from planting directly in the ground. A planter is generally considered “big” when it exceeds 15 to 20 gallons in volume or measures over 16 inches in both diameter and depth. Filling a container of this size entirely with specialized growing medium becomes prohibitively expensive, and the resulting weight can make the vessel impossible to move once watered. The primary goal when setting up a large planter is to manage the total weight, reduce the volume of costly potting mix required, and ensure a healthy root environment with excellent long-term drainage.
Preparing the Container for Success
The physical container must be properly prepared before any filling material is introduced. Adequate drainage is paramount for container health, as standing water deprives roots of necessary oxygen. A large planter should have multiple drainage holes, with the total area of these holes being roughly equivalent to one to two inches of diameter for every square foot of the pot’s base.
The drainage holes must be covered to prevent the growing medium from washing out and clogging the exit points. Simply placing a small piece of mesh screen, a coffee filter, or a square of landscape fabric over each hole is an effective solution. This barrier allows water to pass freely while keeping the fine particles of the potting mix contained within the vessel.
Strategic Filling Techniques for Volume and Weight Reduction
Filling the entire volume of a very large planter with potting mix is unnecessary for the plant’s health and creates an excessive amount of weight. The initial strategy is to fill the bottom one-third to one-half of the container with lightweight, non-degrading materials. This technique significantly reduces the amount of specialized potting mix needed, which saves money, and makes the finished planter much easier to handle or reposition.
Suitable filler materials include crushed plastic bottles, non-biodegradable foam packing peanuts, or inverted, empty plastic nursery pots. These items are ideal because they take up space without adding substantial weight and, crucially, they will not break down over time. The filler layer must be covered with a sheet of landscape fabric or a layer of newspaper to create a clear separation barrier before adding the actual growing medium. This separation prevents the potting mix from sifting down into the air pockets created by the filler.
A common but counterproductive practice is to use materials like gravel, broken pottery shards, or heavy rocks at the bottom of the container. While this may seem logical for drainage, it actually promotes a phenomenon known as the “perched water table” effect. Water moves poorly from fine-textured material (potting mix) to coarse-textured material (gravel) due to capillary action. This causes water to hold in the soil layer just above the gravel, creating a saturated zone where roots are likely to rot.
Selecting the Optimal Growing Medium
The material that occupies the remaining space in the planter, where the plant’s roots will grow, must be a specialized container potting mix. This is distinct from garden soil, which is composed primarily of mineral particles that are too dense for container use. Garden soil compacts quickly in a pot, restricting air flow and preventing proper water drainage, leading to root suffocation.
A quality potting mix is a soilless medium, often containing ingredients like sphagnum peat moss, coco coir, composted bark, and inorganic materials such as perlite or vermiculite. Perlite, a puffed volcanic glass, and vermiculite, an expanded mineral, are included to maintain essential air space within the medium. This lightweight composition ensures that water drains efficiently while still retaining enough moisture and air for healthy root respiration.
It is always advisable to use a fresh, sterile potting mix when filling a large container, rather than reusing old or garden soil. For long-term plantings, incorporating amendments like a granular, slow-release fertilizer into the mix can provide a steady supply of nutrients over several months.
Planting and Initial Care Procedures
Once the filler material is covered and the upper portion of the container is filled with the growing medium, the plant can be positioned. Dig a hole in the potting mix that is just large enough to accommodate the root ball of the plant you are introducing. The top of the root ball should sit slightly below the rim of the planter, allowing a small basin for irrigation.
After placing the plant, gently backfill the surrounding space with the remaining potting mix, firming it lightly with your hands to stabilize the plant. It is important to avoid pressing the soil down too firmly, as this can compact the medium and negate the benefits of the airy composition. The most important step immediately following planting is a thorough, deep watering.
Water slowly and deeply until the water flows freely from the drainage holes at the bottom of the planter. This process settles the new medium around the roots and eliminates any lingering air pockets that could dry out the roots. Finally, situate the container in its intended location, ensuring it receives the appropriate amount of sunlight.