The practice of container gardening often leads to a dilemma when choosing large planters for the patio or yard. Fully filling a container that is 15 gallons or larger with commercial potting mix quickly becomes expensive, as these specialized mediums are sold by volume. The resulting weight of the filled container is also a significant issue, making it nearly impossible to move once saturated with water. Strategic filling methods address both the financial burden of buying excessive soil and the logistical problem of managing an extremely heavy planter.
Preparing the Base for Proper Drainage
The first layer of any container setup involves securing the drainage holes to ensure a functional system. Water must be able to escape freely from the bottom of the pot, but fine particles of potting mix should not wash out with it. A common misconception is that a large shard of pottery or a piece of gravel is sufficient to cover the hole, but this actually impedes water flow.
A more effective method is to cover the drainage opening with a piece of mesh, window screening, or landscape fabric. These materials act as a permeable barrier, preventing the growing medium from clogging the hole while still allowing water to pass through unobstructed. Using a coffee filter or a paper towel can also serve this purpose in the short term, though these will break down more quickly. This foundation establishes the proper exit point for excess moisture.
Effective Volume Reduction Methods
The most efficient way to reduce both the volume of potting mix needed and the overall planter weight is to displace space in the lower half of the container. The ideal filler materials are lightweight, non-biodegradable, and chemically inert, meaning they will not break down or leach toxins into the soil. These materials are placed where the plant’s roots are not intended to grow.
Empty plastic containers, such as water bottles or sealed milk jugs, are excellent options because they are extremely lightweight and take up a substantial amount of room. It is important to place the lids tightly on any plastic bottles or jugs before use to maximize their volume displacement and structural integrity. Non-biodegradable packing peanuts, made from polystyrene foam, also offer significant weight savings. Crushed aluminum cans can be used, though they are slightly heavier than foam or sealed plastic and should be rinsed clean before placement.
Common Filler Materials to Never Use
Intuition often suggests using rocks, gravel, or broken pottery in the bottom of a container to improve drainage, but this practice is counterproductive to plant health. Adding a layer of coarse material beneath the soil actually creates a “perched water table,” a phenomenon that hinders proper drainage. Water moving through the finer-textured potting mix stops when it hits the coarser layer because the capillary action holding the water in the soil is stronger than gravity at that interface.
This layer of saturated soil sits directly above the gravel, effectively reducing the amount of usable, aerated soil depth for the plant roots. The roots are then forced to grow in a constantly waterlogged environment, which can lead to root rot and oxygen deprivation. Similarly, adding large chunks of decaying organic matter, such as logs or thick sticks, is not advisable in a closed container system. Microorganisms require nitrogen to break down high-carbon materials like wood, and they will draw available nitrogen from the surrounding potting mix. This process, known as nitrogen immobilization, can create a temporary nutrient deficiency for the plant, leading to yellowing foliage and stunted growth.
Determining the Necessary Potting Mix Depth
The purpose of the volume-reducing filler is only to occupy space where the plant’s roots will not extend. Therefore, the most important consideration is ensuring a sufficient depth of quality potting mix above the filler material. The required depth depends on the type of plant being grown, as different root systems have different needs.
Shallow-rooted annuals and many herbs typically require a minimum depth of six to eight inches of growing medium to thrive. Plants that develop a deeper taproot, such as carrots or certain perennials, need a soil column closer to 10 to 12 inches deep. For larger, more permanent plantings like shrubs or small trees, a depth of at least 12 inches is necessary to support a healthy root mass.