Why Your 'Compostable' Plastic Isn't Composting

You did everything right. You bought the compostable bags, the ones with the little leaf logo and the promise of breaking down naturally. You tossed them in your compost bin with your vegetable peelings and garden waste. Six months later, you dig through your compost, and there they are, looking the same. What went wrong?

Nothing went wrong. The system is working exactly as designed. The problem is that "compostable" doesn't mean what most people think it means.

The Promise vs The Reality

When you see "compostable" on packaging, your brain translates that to "this will break down in my compost bin." It's a reasonable assumption. After all, that's what compost is for, isn't it?

But here's what "compostable" actually means in most cases: this product will break down in an industrial composting facility operating at sustained temperatures of 55-60°C, with specific bacteria present, under controlled conditions, over a period of weeks to months.

Your garden compost bin? That's a completely different environment. And those products aren't designed for it.

The Different Types of "Compostable" Plastic

PLA (Polylactic Acid): The Most Common Imposter

PLA is made from plant materials like corn starch or sugar cane, which sounds promising. It's what most compostable cutlery, cups, and bags are made from. The plant-based origin leads people to assume it will behave like plant material in their compost.

It won't.

PLA requires industrial composting conditions to break down. In your home compost bin, where temperatures fluctuate and conditions vary, PLA will sit unchanged for years. I mean that literally. Years. Some studies show PLA products remaining intact after three years in a home compost.

Certifications to look for: ASTM D6400 (US), EN 13432 (Europe), BPI Certified, or the Seedling logo. These all mean industrial composting only.

PHA (Polyhydroxyalkanoates): The New Hope

PHA is a newer material made from a canola oil base. Unlike PLA, it genuinely can break down in home composting conditions. The catch? It's expensive, relatively rare, and still in limited production. You're unlikely to encounter it unless you're specifically seeking it out.

Certifications to look for: TÜV Austria "OK Compost HOME" or Australian Standard AS 5810. These are the gold standards for actual home compostability.

Starch-Based Blends: The Disappointing Middle Ground

These blend plant starch with other materials to create a supposedly compostable product. The starch portion will break down, but what's left often doesn't. Performance is inconsistent, and you can end up with a partial breakdown at best.

Oxo-Degradable: The Outright Scam

This deserves its own section because it's genuinely harmful greenwashing that's still legal in some places.

Oxo-degradable plastics are conventional petroleum plastics with metal salt additives. When exposed to oxygen and sunlight, they fragment into smaller and smaller pieces. Companies market this as "biodegradable" or "compostable."

It isn't. What you get is microplastics. Thousands of tiny plastic particles that persist in the environment indefinitely, contaminate soil and water, and enter food chains. The EU banned Oxo-degradable plastics in 2021. Several UK regions and other countries have followed suit. But it's still sold in many places, often with misleading green marketing.

How to spot it: Look for terms like "degradable" without "compostable," "oxo-biodegradable," "photo-degradable," or claims about breaking down with no certification backing it up. Avoid these entirely.

What About Garden Waste Collections?

Surely your council's garden waste collection accepts compostable plastics? They go to industrial facilities, after all.

In theory, yes. In practice, most don't accept them. Why? Contamination concerns. Council workers can't easily distinguish between genuinely compostable plastic bags and regular plastic bags. Rather than risk contaminating entire batches of compost with plastic, most facilities refuse all plastic-looking materials regardless of what they're made from.

Check your local authority's guidance, but don't be surprised if compostable bags are explicitly banned from garden waste bins.

The Industrial Composting Problem

Even if you want to do the right thing and send your PLA products to industrial composting, there's a problem: you probably can't.

Industrial composting facilities that accept PLA products exist, but they're not widely accessible to individual consumers. They primarily serve commercial clients. Unless you live in an area with specific compostable plastic collection services (rare in the UK), your industrially compostable products have nowhere to go except general waste.

Which means they end up in a landfill, where they won't compost either because landfills lack the oxygen needed for decomposition.

Compostable Bags: The Specific Disappointment

Compostable bin liners and food waste bags deserve special mention because they're widely sold with the implicit promise that they'll work in your home system.

Most won't. They're typically made from PLA or starch blends and require industrial composting. Your local food waste collection might accept them, but many don't for the contamination reasons mentioned above.

The cruellest irony? The biodegradable bags that would actually break down in your garden compost (paper-based options) often aren't strong enough to handle food waste without leaking or tearing.

How to Spot Genuinely Home-Compostable Products

If you want products that will actually break down in your garden compost bin, look for these specific certifications:

• TÜV Austria "OK Compost HOME" (the most rigorous and reliable)
• Australian Standard AS 5810
• Belgian "Vinçotte OK Compost Home"

Importantly, these are different from the industrial composting certifications. If it just says "compostable" or has a Seedling logo without specifying home composting, assume it needs industrial facilities.

The problem? Products with genuine home composting certification are rare and expensive. That's the honest truth.

So, What Should You Actually Do?

For Your Home Compost

Don't add any plastic-like materials to your compost bin, even if they claim to be compostable. Stick to actual plant material, paper, cardboard, and traditional organic waste. Your compost will be better for it.

For Food Waste Collections

Check your local authority's specific guidance. Some accept compostable bags, but most don't. If they don't, either empty food waste loose into the caddy and rinse it regularly, or use newspaper to line it instead.

For Single-Use Items

Here's the uncomfortable truth: if a single-use item is made from PLA or similar materials, it's probably going to end up in general waste regardless of what the packaging claims. At least PLA is made from renewable resources rather than better petroleum, but it's not the environmental solution it's marketed as.

The better option is always to avoid single-use items entirely when possible. Reusable containers, cloth bags, proper cutlery you wash and reuse – these are genuinely sustainable choices.

Why This Isn't Common Knowledge

Because there's an entire industry with a vested interest in you not knowing. Compostable packaging allows companies to charge premium prices and appeal to environmentally conscious consumers without actually solving the waste problem.

The regulations around what can be labelled "compostable" are surprisingly lax in many jurisdictions. A product can be certified industrially compostable, marketed to home consumers with green imagery, and technically be telling the truth while completely misleading the buyer about what will actually happen to it.

It's greenwashing, but legal greenwashing.

Our Honest Position

We stock some products that use plant-based plastics like PLA. We're transparent about what they are and what they aren't. Where we sell these items, we make it clear: they won't compost in your garden bin, most councils won't accept them in garden waste, and they'll likely end up in general waste despite being made from renewable materials.

Are they better than petroleum plastic? Marginally, because they use renewable feedstock. Are they the environmental solution they're marketed as? Absolutely not.

Reusable is always better. Always.

When single-use is unavoidable, make informed choices. Understand that "compostable" usually means industrial facilities you can't access. Look for "OK Compost HOME" certification if you actually want to compost at home, but be prepared for limited options and higher prices.

And if anyone tells you their product is "biodegradable" without backing it up with specific, third-party certified claims about where and how fast it breaks down? They're either ignorant or lying. Either way, give it a miss.

The Bottom Line

That bag still sitting in your compost bin isn't there because you did something wrong. It's there because the compostable plastics industry has spent millions convincing people their products will behave like food waste when they won't.

The sustainable choice isn't finding the perfect compostable alternative to plastic. It's using less plastic altogether, reusing what you can, and being deeply sceptical of any packaging that promises to solve the problem with clever materials science.

Sometimes the honest answer isn't the one people want to hear. But it's the one that stops you wasting money on products that won't do what you expect, and prevents you from inadvertently contaminating good compost with plastic fragments that'll persist for years.

Because that's what transparency looks like in a market drowning in greenwash.