Not all bioplastics are biodegradable. Roughly half the bioplastic on the market today is chemically identical to conventional plastic and will persist in the environment just as long. The word “bioplastic” only means the material was derived from biological sources like corn, sugarcane, or other plant matter. It says nothing about whether the product will break down after you throw it away.
Why “Bio-Based” Doesn’t Mean “Biodegradable”
The confusion starts with the prefix “bio.” A bio-based plastic is made from plant-derived carbon instead of petroleum. A biodegradable plastic breaks down through the action of naturally occurring microorganisms. These are two completely separate properties, and a bioplastic can have one without the other.
Plant-derived versions of common plastics, including bio-polyethylene, bio-polypropylene, and bio-PET (the material in most plastic bottles), are chemically identical to their petroleum-based counterparts. They’re made by converting plant sugars into ethanol, then polymerizing that ethanol into the same molecular chains found in fossil-fuel plastic. The European Bioplastics Association has stated plainly that these materials are not biodegradable and pose the same environmental threat as traditional plastics. Meanwhile, some petroleum-based polymers actually are biodegradable. The source of the raw material and the ability to decompose are independent traits.
Bioplastics That Do Biodegrade
The bioplastics most commonly marketed as biodegradable fall into two main categories: polylactic acid (PLA) and polyhydroxyalkanoates (PHA). They behave very differently from each other once discarded.
PLA is the most widely used biodegradable bioplastic, found in compostable cups, food containers, and packaging. It’s typically made from corn starch. In an industrial composting facility, PLA breaks down in roughly 6 to 12 weeks. In soil, full degradation takes about a year. But PLA needs high temperatures and active microbial communities to decompose. In cooler or less biologically active settings, it stalls. In seawater, one study observed no weight loss at all after 45 days. Ocean temperatures rarely reach the threshold PLA needs for hydrolysis, so the primary breakdown mechanism in marine environments is slow photo-oxidation from sunlight rather than microbial digestion.
PHA is a newer class of bioplastic produced by bacteria that store carbon as polymer granules. It degrades in a much wider range of environments. Bacteria capable of breaking down PHA have been found in soil, freshwater, oceans, and even extreme environments like geothermal springs. In one study, roughly 58% of a PHA film degraded after 160 days in seawater. Certain PHA copolymers reached nearly 99% degradation in soil and over 81% in lake water after just five weeks. Degradation rates increase in warmer months: seasonal studies in the Sea of Japan and the Baltic Sea confirmed that PHA breaks down faster in summer due to higher microbial activity. Even at cold temperatures (5°C), degradation can continue if certain nutrients are present in the water.
The Industrial Composting Problem
Most biodegradable bioplastics are certified for industrial composting, not your backyard compost bin. Industrial facilities maintain temperatures above 58°C (136°F), controlled moisture levels, and dense microbial populations. These conditions don’t exist in a landfill, a roadside ditch, or the ocean.
When biodegradable plastics end up in natural environments that can’t support complete decomposition, they don’t simply vanish more slowly. They fragment. Factors like UV light, temperature swings, and inconsistent moisture cause the material to crack into smaller and smaller pieces, producing biodegradable microplastics. These microplastics carry many of the same ecological risks as conventional microplastics. Some researchers have noted that because biodegradable plastics fragment more easily than conventional ones, they may actually generate microplastic pollution faster in environments that lack the conditions for full breakdown.
What Happens in Recycling Streams
Biodegradable bioplastics create a second, less obvious problem: they contaminate conventional plastic recycling. PLA looks nearly identical to PET, the plastic used in water bottles and food containers. When PLA ends up mixed into a batch of recycled HDPE (the plastic in milk jugs and detergent bottles), even 1% contamination significantly reduces the quality of the recycled product. At 10% PLA contamination, the tensile strength of the recycled HDPE drops by half, making it essentially unusable. After UV exposure, which simulates time spent outdoors before collection, just 2.5% PLA contamination cuts tensile strength by 51%. This means a small number of mislabeled or improperly sorted bioplastic items can ruin an entire recycling batch.
How to Tell What You’re Actually Buying
If you’re trying to make a better choice at the store, the label “bioplastic” alone tells you almost nothing useful. Look for specific certification marks. In the U.S., the BPI (Biodegradable Products Institute) certification confirms a product meets the ASTM D6400 standard for industrial composting. In Europe, the equivalent is the EN 13432 standard, often shown as a seedling logo. These certifications mean the product will break down under industrial composting conditions, not that it will degrade if littered or landfilled.
Products labeled “bio-based” without a compostability certification are likely made from plant-derived versions of conventional plastic. They can be recycled through the same streams as their petroleum-based equivalents, but they will not biodegrade. If your area lacks an industrial composting facility that accepts bioplastics, a certified compostable item will likely end up in a landfill, where low oxygen and cool temperatures prevent meaningful decomposition. In that scenario, it behaves much like any other piece of plastic.
The Bottom Line on Biodegradability
About half of all bioplastics on the market are not biodegradable at all. The other half are biodegradable only under specific conditions that vary widely by material. PLA needs industrial composting heat to break down in weeks; left in soil, it takes a year, and in ocean water, it may not degrade meaningfully. PHA is the most promising option for real-world biodegradation across soil, freshwater, and marine environments, but it remains a small fraction of the market. No bioplastic currently available disappears quickly in every environment. Where and how you dispose of it matters as much as what it’s made of.