Posted: 24/05/2015 at 19:55
Have you grown mixed salad leaves or anything like that? It could have come in a mixture like that. Or possibly even in some birdseed.
It looks as if there's a brassica of some sort flowering in the bottom left hand corner (with a yellow flower) - if you've not grown any veg that could have come in bird seed too.
Once the chard has flowered and it's seeds are ripe the plant will die. Unless you want it to seed around the place I'd pull it up and compost/bin it.