It sounds about as likely as a “healthy cigarette.”  I’m talking about “compostable plastic.”  I mean, the one thing we’ve all known for sure about plastics is, they’re bad for the environment, because they don’t bio-degrade.  Right?

Now comes this new generation of plastics that does biodegrade, and some of them even can go into a compost pile.

I’ve seen the product labels for a couple years, but I didn’t really take it seriously.  Someone told me, they’re made out of corn.  Yeah, corn and what else?

Compostable cups, fork, and other containers with the Cedar Grove seal.

It turns out, those corn-based plastics do in fact degrade into compost, according to the local experts at Cedar Grove Composting.  That’s the company that takes the yard waste from a million or so customers in King and Snohomish counties.  In the Seattle area, Cedar Grove has defined the category of composting, taking our leaves, branches, and clippings and selling it all back to us as bags of compost.  They also take all of our table scraps, from residences and many businesses.

Cedar Grove runs its own tests — by simply putting a so-called degradable plastic item into the compost pile, and checking to see if it turns into dirt.  “If it degrades and disappears then we approve it,” says vice president Jerry Bartlett.  Companies pay hundreds of dollars to get their products tested.

More than 400 plastic cups, forks, plates, and other items passed the test and do in fact turn to dirt.  Bartlett tells me they leave no visible residue, although no chemical testing is done at the end of the cycle.  On the other hand, 75% of the so-called “biodegradable” plastics submitted for testing do NOT turn into dirt. These are banned from the compost pile.

How does it work?  Through heat, mostly.  These industrial-scale compost piles get up to 170F degrees.  (If you put one of those compostable spoons in a bowl of hot chowder by the way, don’t leave it sitting in too long — the heat will start melting it.)

Apparently, many other new plastics are still made of traditional petro-chemical plastic, with additives that speed up the decomposition.  What might normally take 1,000 years, now takes 20 years.  But, Treehugger reports there are concerns that these actually may do more harm than good, by leaving tiny residual particles of plastic that then can enter the ecosystem.

So, if it says “compostable,” it’s probably made from corn or another plant-starch.

My story on KPLU has a few more details and links.