Is Compostable Plastic Really Compostable?

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Robert Pavlis

Everyone wants to be green and now you see a lot of compostable plastic on the market. Is compostable plastic really compostable? Can I add it to my compost bin? How long does it take to decompose? These are all good questions that gardeners should be able to answer.

My Master Gardener group had a year end party and one member brought some compostable plastic drinking glasses. As I was collecting them at the end of the evening I started to wonder how well they decomposed. I had previously looked into compostable tea bags and was surprised by what I learned. Are these drinking glasses just some more greenwashing?

In this post I’ll discuss compostable plastic, compare that to biodegradable plastic, and explain the role of all of these in the garden.

Is Compostable Plastic Really Compostable?
Is Compostable Plastic Really Compostable?, image by aaron.bihari

What is Compostable Plastic?

I thought this was a simple question, but its not. Different organizations use different definitions and the general public has their own idea. As a gardener, I assumed I could drop it into a hot composting bin and it would decompose, but that has nothing to do with the definition.

Microbe Science for Gardeners Book, by Robert Pavlis

Compostable plastic is defined by the standards association ASTM International (ASTM) as “a plastic that undergoes degradation by biological processes during composting to yield carbon dioxide (CO2), water, inorganic compounds, and biomass at a rate consistent with other known compostable materials and that leaves no visible, distinguishable, or toxic residue.”

Note that the time frame, or the condition of composting are not part of the definition.

Many assume that compostable plastic is a new type of plant-based plastic. Some compostable plastic is made from plant material such as corn, potato, tapioca, soy protein, and lactic acid but others are made from petroleum, including BASF’s product Ecoflex.

Compostable plastic that has been in a compost pile for 18-24 months
Compostable plastic that has been in a compost pile for 18-24 months

Biodegradable vs Compostable

Biodegradable is defined as ” a product that completely breaks down, given the right conditions and presence of bacteria or other living organisms, to its basic components.

The key words here are “the right conditions”, which can be defined for each biodegradable product.

When the right conditions are a composting environment, the the terms compostable and biodegradable mean the same thing. When the degradation needs an environment other than composting the two terms are not synonymous.

The term bioplastics is also confusing, misused and misunderstood. Things are further complicated by the fact that there are no labeling standards, at least in the US, for terms such as biodegradable, degradable, compostable, starch-based or plant-based.

Will Compostable Plastic Break Down in a Composting Bin?

PLA compostable plastic
PLA compostable plastic clamshell container

The short answer is probably not. If it is labeled as “home compostable” it might, but most compostable plastic does not carry this label.

Building Natural Ponds book, by Robert Pavlis

For example, Polyactic acid (PLA) is a popular compostable plastic used to make drinking cups, clamshell containers and plastic cutlery. It requires 140 F (60 C) to decompose, which is much hotter than a backyard compost bin can maintain long enough and throughout the complete pile.

Commercial composting facilities carry out composting in closed chambers and they routinely reach the higher temperature needed, however, many of these facilities run on a 30 day cycle, and PLA takes 60-90 days to decompose. At the end of 30 days, the plastic is still intact and gets filtered out and sent to landfill.

Many consider PLA to be biodegradable, but its not in landfill. PLA is also very stable in soil and water.

The CBC reports that “Ecoflex, PLA, and two other kinds of biodegradable plastics all survived a year in either seawater or freshwater without breaking down, a 2017 University of Bayreuth study showed. A 2019 University of Plymouth study found that “compostable” bags buried in soil were still there after 27 months, and “biodegradable” bags could still hold groceries after three months in the ocean.”

Being “plant-based” does not mean the plastic will decompose like yard waste.

Are Tea Bags Compostable?

I contacted Unilever about Red Rose, a popular tea brand in Canada, to find out what material is used to make their tea bags. They confirmed it’s PLA and went on to say, “it’s un-bleached 100% plant-based material (corn) that is 100% compostable”. In the same email they also said “home composters may not have high enough temperatures or appropriate moisture content to decompose the tea bags” – at least they were honest about that part.

Many tea bags these days are not home compostable, and neither are compostable K-cups.

Is Compostable Plastics Good for the Environment?

Consumers buy compostable plastic because they believe it is a better option for the environment, but is it really?

Most municipalities in N. America do not have composting facilities. Of the 150 in the US that do composting, most do not meet the requirements for composting PLA. What does this mean? Your PLA probably ends up in landfill and if that happens it is no better for the environment than other types of plastic.

Recycling regular plastic is a better option for the environment, provided it is actually collected and recycled.

The public is being greenwashed about compostable plastics.

Which is Better – Regular Plastic or Compostable Plastic

Here is a good example of the value of science.

Plastic made from plants sounds so organic, and when they are labeled with “compostable” they become irresistible to the public who imagine plastic drinking cups melting away in the compost bin. Many manufacturers are quick to enforce that vision. The media also jump on the bandwagon with headlines like “Calgary Co-op to eliminate plastic bags from liquor stores as compostable bag program takes off”.

Now science steps in and asks, which option is better for the environment? Testing is still going on, but it appears that traditional plastic might be better, given our current composting capabilities. Much of this however depends on how we handle the waste, not the actual product. If every municipality used 90 day commercial composting, and consumers actually put the material in the right bins, compostable plastic would be better for the environment.

It seems to me that our focus is on appearing to do the right things, and not on actually doing what is right for the environment.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

15 thoughts on “Is Compostable Plastic Really Compostable?”

  1. Thanks for writing this. Seedlings purchased last year had plastic compostable wrappers around the peat pots they were planted in. I put a few of these in my worm ‘composter’ and others directly in the garden soil to see what would happen.

    What happened? Nothing. The plastic was still in the composter, and I found it in the garden as I turned the soil as well. No discernible change to the wrappers. Still as firm and crispy as the day I purchased the plants.

    Now, I’ve done the same thing with tea bags and they broke down as hoped, which probably means they weren’t made from this new compostable plastic material.

  2. I wonder why the bottom of the so-called biodegradable bags always tore if I had something wet in them for longer than a few hours. That’s why I stopped using them.

  3. Thank you for an expose on compostable plastics that can be understood by everyone.
    I still remember the first major consumer use of PLA packaging by Frito Lay. They quickly changed their Sun Chip bags, not because they were misleadingly labeled compostable (and I can attest, they weren’t), but consumers thought they were too noisy when you opened them!

  4. We have some gardener experiments going on here in the UK on this issue. I find tea bags never break down so now I let then dry when they can be torn and only compost the tea.
    Sometimes I have put caddy liners on the compost heap but they Are so slow to completely break down I find I’m pulling out the ‘rags’ of them months later.
    You are spot on saying it’s not so much the use of plastic that’s the problem as how we dispose of it.


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