So there you are, tossing your corn-based plastic cup into your corn-based plastic garbage bag and then tossing your corn-based plastic garbage bag into your regular old garbage bin. OK, so it's going to the landfill, but still, it's biodegradable and that makes you all green and virtuous and stuff, right?
WHY IT'S BAD:
Turns out that compostable plastics are only green if you actually compost them. If you toss them in the landfill, they're not going to decompose any more than a conventional plastic bag will. Or at least you hope they won't. In the oxygen-deprived atmosphere of the landfill, decomposing organic materials produce methane, a greenhouse gas 72 times more potent than carbon dioxide over a 20 year time frame. Landfills are the largest human-made source of methane emissions in the United States, with a greenhouse-gas impact equal to one-fifth of that produced by the nation's coal-fired power plants.
WHAT'S THE ALTERNATIVE?
If it's not going to be composted in an industrial-grade composting facility, don't use compostable plastic. Throwing compostable plastic in with recyclable plastic just contaminates the recycling load, and trying to compost corn plastics in your home compost pile doesn't work (home compost piles don't get hot enough). If the garbage is headed to the dump anyway, you might as well use conventional plastic garbage bags. The real low-impact solution: work to eliminate your landfill-bound garbage entirely by composting your food waste, recycling your recyclables, and avoiding products with plastic packaging.