The practice of using household materials to fill the bottom of large plant containers has become a popular method to reduce the cost and weight of potting soil. Items like plastic bottles, gravel, or broken pottery are often repurposed for this task, but packing peanuts are a common candidate. Determining whether this common shipping material is a safe and effective choice requires understanding its composition and how it interacts with the soil environment for the long-term health and stability of your plants.
The Core Question: Identifying Packing Peanut Materials
The suitability of a packing peanut as a planter filler depends entirely on the material it is made from, as there are two distinctly different types in circulation. Traditional peanuts are made from expanded polystyrene, often referred to as Styrofoam, a petroleum-based plastic that is non-biodegradable. This type of peanut is chemically inert, meaning it will not break down or leach toxic substances into the soil over a typical growing season.
The second type is the modern, eco-friendly version, which is starch-based, usually derived from corn or wheat. These biodegradable peanuts are designed to dissolve rapidly when exposed to water, which makes them completely unsuitable for use in a planter. A single watering session will turn the starch peanuts into a sticky, gelatinous residue that compacts the soil and clogs drainage holes. If you are unsure which type you possess, a simple water test will provide the answer, as the starch-based peanuts will begin to dissolve within minutes.
Understanding the Role of Planter Fillers
The main practical reasons for adding any filler to a large planter are to reduce the volume of expensive potting mix needed and to decrease the overall weight of the container, making it easier to move. However, a widespread misconception suggests that adding a layer of material like peanuts or gravel improves drainage, which is scientifically inaccurate. Water movement in a container is governed by soil physics, specifically the concept of the perched water table.
The perched water table is a saturated layer of soil that forms at the bottom of a container due to capillary action resisting the pull of gravity. When a layer of filler is placed at the bottom, it essentially moves the entire soil column higher up in the pot. This action raises the saturated water layer closer to the plant’s roots, reducing the amount of aerated soil available for healthy root growth.
Specific Drawbacks of Using Packing Peanuts
Even if you use the inert polystyrene peanuts, they present several long-term performance and stability issues within the planter. Polystyrene peanuts are extremely light, and over time they tend to shift and compress under the weight of wet soil, which causes the potting mix to settle unevenly. This settling can lead to instability for larger plants and create air pockets, making repotting a messy and difficult process.
Polystyrene is also hydrophobic, meaning it actively repels water. When used as a filler, the peanuts can create dry zones within the lowest part of the container’s profile, making it difficult for water to permeate evenly through the soil. The combination of shifting, compression, and water repellency undermines the goal of a stable, consistent growing environment. Using expanded plastic in a garden setting is environmentally questionable given its non-biodegradable nature.
Recommended Alternatives for Lightweight Planters
Instead of dealing with the issues caused by packing peanuts, there are more stable and effective alternatives to reduce soil volume and container weight. One highly effective method is using clean, sealed plastic bottles or jugs placed strategically at the base of the container. These items are larger, do not compress, and remain stable under the weight of the soil.
For a more robust filler, materials can be used that provide excellent void space without shifting or dissolving. These include:
- Crushed aluminum cans
- Inverted, empty nursery pots
- Lava rock
- Specialized foam inserts
These options achieve the primary goals of volume reduction and weight management without the associated risks of instability or poor water distribution.