A “large planter” typically refers to any gardening container exceeding 20 gallons in volume or 18 inches in diameter. Filling these significant containers entirely with expensive potting mix is often impractical and detrimental to long-term plant health. Preparing a large planter requires specific planning to manage material costs, the container’s overall weight, and the balance of root zone aeration and moisture retention. This preparation is paramount for creating a sustainable growing environment.
Addressing the Challenges of Large Planters
Filling a large container solely with standard potting media presents several difficulties. The sheer quantity of commercial potting mix required makes the endeavor financially prohibitive due to the high cost. Completely filling a large planter with moist soil significantly increases the container’s total mass, making it nearly impossible to move once planted. This excessive weight can also create problems for balconies or decks not rated for heavy loads.
The volume of soil at the bottom of the container risks severe compaction over time, leading to poor oxygen exchange and waterlogging that suffocates deeper root structures. As the weight of the soil column presses down, the fine particles settle into a dense mass, restricting the movement of water and air. These issues necessitate using materials that displace volume without adding excessive density or cost.
Choosing Effective Bulk Filler Materials
The primary goal of bulk filler is to occupy space and reduce weight without interfering with the drainage hole. Non-degradable options provide the most stable long-term solution, as they maintain their structure and do not compress or decompose. Clean, empty plastic containers, such as upside-down nursery pots or crushed plastic bottles, are excellent choices for occupying the bottom third of the planter. These materials are lightweight, free, and will not break down, ensuring the soil level remains stable for years.
Another effective non-degradable material is foam packing peanuts, provided they are made of polystyrene and not the water-soluble, cornstarch-based variety. Polystyrene peanuts are inert and exceptionally light, offering substantial volume displacement while maintaining air pockets. Confirming the material’s composition is important, as biodegradable peanuts will quickly dissolve and cause the soil column to sink dramatically after the first few waterings.
Some semi-degradable materials, like clean, dry bark mulch or wood chips, can be used but require specific precautions. Their slow decomposition process requires nitrogen from the surrounding environment. To prevent these materials from robbing nitrogen from the plant’s active root zone, they must be completely separated from the potting mix above.
A layer of landscape fabric, window screening, or an old t-shirt must be placed directly between the wood filler and the growing media. This barrier prevents finer soil particles from washing down and clogging the air pockets created by the bulk materials. Avoid using materials that compost rapidly, such as kitchen scraps or yard waste, as their decomposition is too unstable for a contained environment.
Gardeners should avoid the common practice of adding rocks, gravel, or broken pottery to the bottom of the container. While this seems logical for drainage, these dense materials add significant, unnecessary weight and actually impede effective water movement.
Creating the Ideal Top Layer and Drainage System
Once the bulk material is in place and separated by the barrier layer, the remaining volume must be filled with a high-quality growing medium. This top layer is the environment where the plant’s roots will actively grow and absorb nutrients, so selecting the proper media is important. A sterile, commercial container mix is highly recommended, as it is specifically formulated to resist compaction and maintain air space.
The best potting mixes typically contain a blend of components like peat moss or coir for water retention, and perlite or vermiculite for aeration and lightweight structure. Garden soil, topsoil, or dense sand should never be used in large planters because these materials compact severely when repeatedly watered, rapidly eliminating the air pockets necessary for root respiration.
A common misconception involves adding a drainage layer, such as gravel or broken pottery, above the drainage hole. This practice, intended to improve drainage, actually creates a “perched water table.” Water will not move from the finer-textured potting mix into the coarser gravel layer until the entire soil column immediately above the gravel is completely saturated.
This saturation raises the water level higher into the root zone, significantly increasing the risk of root rot. The only requirement for effective drainage is a clear, unobstructed drainage hole at the base of the planter. After filling, ensure the soil surface is set approximately one to two inches below the rim. This space allows for sufficient room to water thoroughly without overflowing.