Using non-soil materials at the base of large containers is a common gardening practice to reduce both expense and weight. Gardeners often seek a lightweight “filler” material to occupy the substantial volume in the lower portion of deep pots. This strategy prevents the need to purchase and lift excessive amounts of potting mix. The material most frequently considered for this purpose is Styrofoam, a brand name for Expanded Polystyrene (EPS) foam. The core question is whether this material, often available from packaging, is a safe and effective choice for a plant’s container environment.
The Direct Answer and Practical Advantages
Using large pieces of Styrofoam blocks or sheets at the base of a planter is generally considered acceptable, but it is not the ideal long-term solution. The primary advantage of using this material is its low density, which significantly reduces the overall weight of a large container. A container filled entirely with moist potting mix can weigh hundreds of pounds. By substituting the lower third or half of the container volume with Styrofoam, the planter becomes much more manageable for repositioning. This technique also provides a considerable cost saving by requiring less purchased potting mix. For temporary or seasonal plantings, these practical benefits often outweigh other concerns, especially since polystyrene is readily available as recycled packing material.
Understanding Drainage and Water Dynamics
The functional problem with using Styrofoam or other coarse material at the bottom of a pot relates to the physics of water in soil. Contrary to common intuition, adding a layer of coarse material, like Styrofoam, rocks, or broken pottery, does not improve drainage. Instead, it creates a boundary layer that interferes with the soil’s natural capillary action. Water moves downward through the fine pores of the potting soil until it reaches the transition point where the fine soil meets the coarse foam. This raises the level of saturated soil, a phenomenon known as the “perched water table,” where water collects directly above the Styrofoam layer. By introducing Styrofoam, the gardener effectively raises this saturated zone higher into the container, reducing the vertical distance of aerated soil available for healthy root growth. Roots that grow into this perpetually saturated layer are deprived of oxygen and become highly susceptible to root rot.
Chemical Stability and Environmental Concerns
Polystyrene, the polymer that makes up Styrofoam, is largely chemically inert and extremely stable, which means it resists decomposition within the planter environment. This stability prevents the material from rapidly breaking down and collapsing the soil column. However, the foam is not biodegradable, persisting in the environment for hundreds of years. The material contains residual components from its manufacturing process, such as styrene, ethylbenzene, and toluene. Studies have shown that these aromatic compounds can leach out of polystyrene, particularly when exposed to heat. While the cool, dark environment of a planter base minimizes this risk, the potential for long-term, low-level chemical release into the soil remains a consideration. Furthermore, Styrofoam is prone to physical breakdown into tiny beads and fragments, which are a source of microplastic pollution. When the planter is eventually emptied, these small plastic pieces become mixed into the soil, making them difficult to separate and recycle.
Recommended Alternatives for Planter Fillers
Gardeners looking to reduce a container’s weight and soil volume without the drawbacks of foam have several superior options.
Plastic Bottles and Jugs
One effective method is to use clean, empty plastic beverage bottles or milk jugs with their caps tightly secured. These containers displace volume without adding significant weight, and their solid surfaces do not interfere with water movement in the same way coarse particles do.
Inverted Nursery Pots
Another excellent alternative is to invert unused plastic nursery pots or smaller flower pots at the bottom of the large container. These create large, stable air pockets and are easily removable when the planter is disassembled.
Placing a layer of landscape fabric over any of these fillers before adding soil helps to prevent potting mix from sifting down. These alternatives offer the desired weight reduction while being easier to manage, safer for root systems, and more readily recyclable or reusable.