Why Is Styrofoam Added to Potting Soil?

Styrofoam is a versatile material used in numerous ways. From manufacturing and construction to packaging and insulation, this unique material has even found a place in gardening and horticulture. Specifically, that place is in potting soil. 

Styrofoam is added to potting soil to improve drainage and aeration, yet studies have proven that it does more harm than good. It is practical, however, for filling space in large pots. Note that styrofoam is not to be confused for perlite—the little white “foam” balls in commercial potting soil.  

Despite its usefulness, styrofoam has significant drawbacks, especially when you use it in gardening. In this post, I’ll explain why people sometimes add styrofoam to potting soil and why you should avoid this practice.I’ll also explain the foam balls in your potting soil and their purpose.  

Adding Styrofoam to Potting Soil: Helpful or Harmful?

Adding styrofoam to potting soil has been a common suggestion in many green thumb communities for decades as a means for: 

  • Aerating the soil
  • Improving water retention
  • Encouraging proper drainage.

Unfortunately, what started as a gardening hack with the good intentions of repurposing a non-biodegradable material has backfired.

Not only is styrofoam ineffective for these functions, it often makes issues worse and leads to additional complications. 

Adding styrofoam to potting soil is harmful, causing soil compression and leaching toxic chemicals into the soil. Styrofoam is non-biodegradable and harms the environment.

Styrofoam in Potting Soil Leads to Soil Compression and Waterlogging

Many gardeners have added styrofoam packing peanuts to the bottom of containers or mixed them into potting soil to improve drainage, help retain moisture, and aerate the soil.

In theory, it seems plausible that the airy foam material would allow air and water to pass through. Yet, it does the opposite. Styrofoam is more likely to retain moisture and become waterlogged. And instead of aerating the soil, this leads to compression of the soil. 

In addition to this, the plant roots can grow into styrofoam, where they are devoid of nutrients and at risk of waterlogging and rotting or death.  

Styrofoam Leaches Harmful Chemicals Into Potting Soil 

It’s bad enough that styrofoam lacks any nutritional value for your plants and thus, creates pockets of “nothing” that the roots could otherwise use for nutrients. But this synthetic material is also made from toxic chemicals. 

Manufacturers use a chemical called styrene to create polystyrene, and this chemical is the standard material used to make many styrofoam products, including packing peanuts. 

The CDC has noted that exposure to styrene comes with numerous side effects, including:

  • Issues with balance and concentration
  • Impaired memory
  • Feeling drunk
  • Tiredness and slowed reaction times
  • Changes in color vision
  • Vision loss.

Prolonged styrene exposure also links to potential health issues such as cancer.

On top of this, studies have shown that styrene leaches from styrofoam and other items made with polystyrene via water.  

When it comes to your potting soil, this means that every time it rains, or you water your plants, trace amounts of harmful chemicals seep into the soil, slowly contaminating it. So, not only does styrofoam not feed your plants, it poisons them. 

Styrofoam Pollutes the Environment

Styrofoam is essentially non-biodegradable, considering the millions of years it is said to take to decompose. Thus, it poses a continual threat to the environment. This threat becomes a gradual and persistent attack when styrofoam finds its way into topsoils and oceans worldwide. 

It would be best if you also kept drainage in mind. When heavy rain floods your garden bed and washes the styrofoam away, it may pollute the land and waters around it.

Unfortunately, this happens more often than not, as the styrofoam is very buoyant and less dense than the soil. Hence, it easily “floats” to the surface of flooded soils. 

Another use for styrofoam in the garden is to insulate delicate plants, such as roses, from the harsh winter conditions. Styrofoam cones or boxes are placed around the plants to protect them from wind and cold temperatures. And while there is some merit to this method—even potential success—it becomes a source of pollution more often than it serves its purpose. 

Besides the fact that styrofoam loses insulation efficiency in freezing temperatures, the flaw in this idea is that heavy snow may easily damage these cones and boxes. So, not only are your plants left unprotected, styrofoam pieces are carried away by the wind and scattered about your yard, leaching chemicals into the environment. 

Notably, styrofoam makes up a substantial portion of ocean debris and pollution. Eliminating the use of it in your potting soil is an easy way to help ease this pervasive issue.   

Use Styrofoam To Fill Up Space in Large Potting Containers

Perhaps the one non-harmful way to use styrofoam alongside potting soil is as a space filler in large potting containers. 

In particular, many use styrofoam containers for planting and then place that inside a larger, more aesthetically pleasing potting vessel. Yet, you could also place big blocks of styrofoam at the bottom of the pot to prop up a smaller container. Markedly, this is a great way to reduce the amount of soil you use (and thus, the amount of money you have to spend on it). 

However, planting in styrofoam containers or placing blocks under the soil does not eliminate the potential for contamination. Therefore, it requires some additional effort to keep styrofoam from causing damage. 

For one, you must ensure that the styrofoam has no direct contact with the potting soil. You can cover the styrofoam with a plastic tarp or duct tape—as long as you create a waterproof barrier between the potting soil and the styrofoam. 

It is also best to keep drainage in mind. This vital requirement may require you to cut some holes out of your styrofoam container (and cover them with tape) before planting in it. Also, be sure not to block any drainage holes in the larger container or ensure an unobstructed path for water to escape. 

The Small White Balls in Potting Soil Are Not Styrofoam

There is a possibility that you purchased some potting soil, opened the bag, and after a quick inspection, came to the internet to inquire why styrofoam is in the mix. 

If so, know that you are not alone. After seeing the small, white, foam-looking balls found in many potting soils, it is a common misconception for beginning gardeners to think they are styrofoam. Rather, this is perlite

What Is Perlite?

Quite the opposite of styrofoam, perlite is a natural, amorphous glass formed from a volcanic glass called obsidian. Also referred to as “volcanic popcorn,” perlite puffs up when heated, similar to how popcorn does, to take on its styrofoam-like appearance. 

You add perlite to potting soil for all the reasons people suggest styrofoam. The differences are that perlite comes with the bonuses of being an organic material and is successful in its tasks. Plus, it provides nutrition, as it is made mainly of silica and other elements that plants need, including sodium, potassium, and aluminum. 

It also efficiently keeps water readily available to surrounding roots. Many mistakenly believe this is because it absorbs moisture—a common false appropriation from comparing it to foam. Instead, perlite has a rugged surface with little crevices that collect and hold water. 

Another reason people opt for perlite in potting soil is to improve aeration, as it is incredibly light and helps keep the soil loose and prevent compacting. It also improves drainage and creates a better environment for plant roots. 

Not to mention, perlite naturally decomposes over time. So, unlike styrofoam, it will not cause harm to the environment.

Final Thoughts

Styrofoam has been added to potting soil for decades to work like a substitute for perlite, yet its failure to serve this purpose is plainly evident. Not only is styrofoam ineffective at aerating soils or otherwise improving soil quality, but it also causes harm to organic materials in the soil, including plants. So, the next time someone suggests you add styrofoam to your potting soil, perhaps you should share this post with them instead. 

Continue reading here to learn more about potting soil, and its ingredients: Is Potting Soil the Same as Fertilizer?

Alexander Picot

Alexander Picot is the principal creator of TheGrowingLeaf.com, a website dedicated to gardening tips. Inspired by his mother’s love of gardening, Alex has a passion for taking care of plants and turning backyards into feel-good places and loves to share his experience with the rest of the world.

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