When you toss a supposedly compostable cup, fork, or plate into a compost bin, can you count on it turning into compost?

Not necessarily.

Many items that claim to biodegrade don’t break down in the amount of time allowed by the commercial composting facilities taking that waste.

Other items may be labeled “compostable,” “biodegradable,” or “plant-based” as a greenwashing tactic. Either way, the stuff that don’t completely degrade often winds up in a landfill – despite the good intentions of consumers.

With cities such as Portland, Seattle and San Francisco launching programs to separate food waste and compostables from garbage and recycling, more and more consumers are being asked to determine whether they should put items labeled “compostable” into their curbside compost bins.

It’s a surprisingly tricky question, according to Michele Riggs, who tests products for compostability at Cedar Grove Composting in Everett, Wash.

“There are a lot of  products that say they’re compostable,” she said. “There are a lot that say they’re biodegradable and bio-based and plant-based. All of these terms are different, and a lot of people superimpose them. It’s really hard to determine what’s compostable and what’s not. Knowing your labels for real compostable products is a must.”

A product may very well be partly made from plants, Riggs said, “but that has nothing to do with its compostability.”

And biodegradable? Riggs said that could be anything. Only specific types of biodegradable products will break down in a composting process.

Anne LeCocq, an environmental manager for Recology, which handles Portland’s food waste, said roughly 1 percent of the compost received at the Nature’s Needs composting facility near Portland is “contamination” that doesn’t turn into compost. A portion of that contamination is plastic that’s labeled “compostable” or “biodegradable” but doesn’t break down fast enough.

Nature’s Needs sends compostable waste through a 60-day composting process. It sorts out the items that didn’t degrade and adds them to a second batch of compost.

“Some of that material will break down if you send it through a second time,” said LeCocq. “But some items will take longer than 120 days. They are compostable in theory, but not in our stream.”

So, how do you know if the products labeled “compostable” will actually compost?

The best way to make sure a product will actually break down in a commercial composting process is to look for the Biodegradable Products Institute, or BPI, label, said Riggs. Or you can check for a label indicating the product has met the compostability standards ASTM D-6400 or STMD 6468.

“Those are specifications that point to test methods showing these products actually do biodegrade – that microbes are eating them up,” she said.

At Cedar Grove, Riggs will label potentially compostable products with a piece of electrical tape and put them in a mesh bag with a portion of food waste. The bag goes into a pile of yard waste for two months.

When she checks back to see what’s left of the items, she’s hoping to see lone pieces of electrical tape. But among the thousands of products her company has tested over the years, she said, “more fail than actually pass in the end.”

The Cedar Grove list of acceptable items is a handy guide for consumers trying to determine which products are likely to degrade in the composting facility near them. But Riggs notes composting conditions vary across the country.

Rachel Oster, director of external affairs for Recology, said product labels can be confusing for composting facilities, too.

“At our facilities, when plastics go across the line and we do our best to remove them before they’re sold as compost, it’s hard to tell the difference between compostable plastic and hard plastic,” she said.

Long term, it will be important for truly compostable items to be clearly marked for both consumers and compost facility operators, she said.

“If the products are not just marketing to the consumer but really trying to close the loop, then we are going to need to know which products those are so we can leave them in the loop,” said Oster. “I don’t think we can really get to zero waste without looking at the containers that stuff comes in and how to turn those back into the earth the same way we do with compost.”