This didn't happen over night, pay attention to what is happening, do not continue to place grass clipping in a plastic container, not sure what you think will happen by placing the mix on a plastic tarp. Emulate nature.

I collect all my kitchen scraps in plastic garbage cans, and yes, it becomes quite putrid. solutions I use:

1) apply materials to garden when it is cold.

2) dig a trench, empty the contents into trench, cover.

so long as the lid is on and tight, there is little smell.

Transfer the putrid stuff to your compost bin and as you do so add anything dry and 'brown' that you have: cardboard, paper, dried leaves, straw, woodshavings, whatever. You don't have to dry the sloppy stuff first as long as you add the browns. Then stop putting lawn mowings in a plastic bin. Either leave them on the lawn or add to the compost bin as you go. Meanwhile build a second bin ready for when the first is full and needs to be left alone.

My first recommendation is to not collect grass clippings. They are best left on the lawn. If you want to compost what you have now it would be a good idea to mix it well with some good carbon materials.

Yep, all good recommendations with options in how to proceed.

It's been raining here off and on for about 2 weeks. The neighbor cut his grass yesterday. I wish he would raise his deck, but like many in this area, anything over 2" - at any time - is considered messy (oh, they hate me lol.) Because he cuts it so short, the invasive grasses that are creeping into my garden proliferate - but that's a different story. The rain made his grass grow, and instead of raising the deck, he brought it back down to 2 inches. But, since he doesn't spray (thank goodness,) what's bad for his lawn is good for my compost pile!

Took over 2 hours to rake and haul... yep huge lawn... but I now have a giant pile of clippings sitting open in the driveway. As we learn from the OP, putting them in a container is a no-go, and I don't like to leave grass clippings piled up on their own. I could spread them out thinly over top of the garden, but my mulch is perfect just as it is for now. I am out of last year's leaves, so what to do??? Thank goodness for the wood chips I had delivered over the last few weeks. Even though the chips had a LOT of nice, green, shredded leaves already mixed in, the chips are like a super-brown... off to the side will go a new pile of wood chips and grass clippings. The cool thing is that it's such a good sized pile that with the chips mixed in, it will be almost 2 cu. yds., plenty large enough to heat up from day one!

I guess the point of my story is that from experience I've learned that grass clippings can be fussy. Best to deal with them immediately. Either mulch them back into the yard, use as a thin layer over garden areas, or be ready with some browns to make a pile. Once they sit for a length of time, I find that they become difficult to work with. They aren't 'bad', just very active and in want to decompose quickly.

Spreading them out for a day or two won't hurt and will let some air get at them - it's the lack of air that makes it putrid. But spread right in ground instead of on tarp. Floral_uk's suggestion is the best though - toss it in your bin with plenty of browns in layers. Plain cardboard or egg cartons work quite well. You won't think the cardboard will break down but it will. Given the stat the stuff is in the more dry brown stuff the better. In future leave as much of the yard waste on the ground as you can - if it's been mowed it will disappear quicker than you think and your lawn will thank you.

Grass clippings are notorious for packing down and getting slimy and smelly, even in an open pile. Their shape is such that they stack together with few air spaces; they have a high moisture content and a high nitrogen content (all that chlorophyll); taken together these things mean there is a ton of bacterial food available in the mass of clippings but not enough oxygen in the material for anaerobic bacteria to live, so the nitrogen gets gobbled up by a bloom of nasty, stinky anaerobic bacteria. If you don't leave the clippings to decompose on the lawn, be aware of these factors and mix them with browns that have a coarser texture to allow air into the pile. You don't have to hot compost them but you will need air in there to combat the stank.

And for Naomi (the OP): I think everyone who composts has had this happen. It's not a big deal, it's fixable, and you'll learn to avoid it in time.

As you can see, in general, you can't store up greens like you can browns. They start composting without you, and if you haven't put them in a proper pile with some browns, this is what happens! Fortunately it's fixable.

Thanks you for your thoughts! To clarify, we hadn't intended to keep the clippings in the can...had just put them there as temporary storage until bin was set up. Bin set up got delayed. Lesson learned! Per your suggestions, we've incorporated some of it into the bin and the rest is spread out in the ground (didn't have enough browns at the ready to put it all in).

if you put a layer of dirt on top of the compost it will act as a lid and cut the smell down some. I remember as a kid (40 years ago) in the spring when it warmed up Mom was glad to open the windows for some fresh air...until the wind shifted and started blowing the compost pile stink into the house. I would get sent out to "cap" the compost heap.

The smartest, most efficient, and most natural solution to this issue is to stop bagging the clippings entirely and allow them to remain where they can provide the most benefit! The soil will be better off for it, and your lawn regains some of the nitrogen you applied as fertilizer.

Properly cut grass will decompose quite rapidly once it comes into contact with the lawn. I honestly do not understand why (some) people insist on collecting the clippings....there is no justifiable reason to do so.

Now, if you are mowing too infrequently, and the cut grass looks like hay windrows, then you should change your mowing practices. ;-)

If you mow to evenly distribute the clippings, you hardly notice them. I mow by spiraling inward, throwing the clippings out onto the area already mowed, so the coverage is very even. Unless the lawn is crazy long, after a day or two there is no sign of them. Of course a mulching mower will do an even better job.