A couple stores I frequent now use, or are starting to use, plastic bags and cups that say they're compostable. Are these safe to add to my compost pile or are they just more environmentally friendly for landfills?
I suspect the answer is still no, they're not suitable for your compost heap. There is currently a programme in action at Imperial College in London which is making good progress on producing plastic bags and the like made from degradable polymer, which should be compostable.
However, most 'biodegradable' plastic bags currently are polylactide based, and these need high temperatures of the sort achieved in industrial facilities to ensure breakdown rather than what you might achieve in your compost heap. The current crop of these plastics otherwise requires sunlight and air to break down - placing in landfill or in the middle of a compost heap means the breakdown process will be slow or non existent. This polylactide product has metals added which help it to biodegrade, but according to a Swedish study recently, the process is also halted by cool or cold, damp conditions. Since that description applies to most of the Northern hemisphere during the winter, 'biodegradable' in this case appears to be a very loose term indeed.
Given optimum conditions, polylactide based plastics should, in theory, biodegrade within 3 years - in reality, that is not what's happening because they're not in optimum conditions. There is also a big question mark over the metals they contain and what happens to those within the environment, never mind your compost heap. So on balance, I'd not recommend putting them in your compost heap unless you are sure they are polymer based and, so far as I'm aware and have been able to ascertain, these won't be available for another 5 years or so, unless someone out here has already perfected the process and has marketed them.
UPDATE 14th September: sorry only just seen your comment that the cup you have is made from PLA - PLA is polylactide, not polymer (vegetable origin) so I wouldn't be putting it on my compost heap. Although you could try one, I'd love to know if/how long it takes to degrade, and whether you have fragments of plastic in its place rather than a whole cup. Which is what I'm betting will happen, no idea how long that'll take, I suspect 3-5 years, but it'd be interesting to see.
Unless it is made from something organic like corn or hemp, I would say no. Plastic (other than from corn, hemp, etc) is made from petroleum, and those plastics advertised as biodegradable or compostable are simply petroleum processed in such a way that it breaks down faster than regular plastic. When petroleum-based plastic breaks down, it does not turn into something organic, it's just smaller bits of petroleum. All petroleum-based plastics will break down in this manner eventually, and they will build up in the environment - a good example of this is the 'Great Pacific Garbage Patch' - creating a toxic environment for local flora and fauna.
In short, unless it's made completely from materials you would put in your compost in their raw form, I wouldn't add it in it's processed form. I wouldn't add crude oil to my compost, so I wouldn't dream of adding plastic made from crude oil to my compost either, especially with it's links to cancer and other nasties.
Unfortunately industry has done a poor job of educating the public about compostable 'plastics'. It all started with Sun Chips who have since eliminated their 'PLA'/Compostable chip bag campaign. PLA/Ingeo/Earthfirst or other PLA 'compostable' plastics do not compost in a residential compost pile and must be commercially composted. While Europe has done a great job of establishing hundreds of commercial composting facilities to deal with the deluge of compostable 'plastics' the US has fallen short in this area and, to date there are few commercial composting facilities available to the public.
Your home compost will likely never get hot enough to compost the plastic. These are intended for industrial operations.
Just a few corrections to the above: PLA is a polymer bioplastic which is made from starches (mostly corn in the US), so in theory, assuming it doesn't contain any harmful additives, it should be "compostable" However, it's not likely to get hot enough to compost at any reasonable speed (if ever) in your home compost heap.
As far as I know, here in North America there aren't really any facilities accepting bioplastics for composting or recycling (though I could be wrong), so the reality of it is that it's just eco-chic, and probably just making a different kind of plastic garbage.
A recent (2015) study on additive based biodegradable plastics shows they don't biodegrade very well: Additives to make plastic biodegradable don’t cut it. Hardcore version: Evaluation of Biodegradation-Promoting Additives for Plastics (Abstract only, but with nice graph). They tested two plastic types with five different kinds of biodegradation enhancers, sadly not named in the abstract, and found:
The results of the experiments can be summed up very simply: we found no evidence that the additives we tested resulted in any significant biodegradation of either the polyethylene film or the PET sheet in the environments we used. There were no significant differences between the samples with and without the additives, and no evidence of substantial biodegradation of any of the samples.
Maybe you'll get lucky, and the bags your grocery store gives out will turn into prime compost within months, but as of 2015, the odds seem against that.