OK to let sit, but not too long with the lid on, there are stories over on the main Roses Forum that are pretty scary! :-))

If you "dump" it, then do it over the garden!

It won't hurt hurt anything to let it sit. Except it might start to ferment. Either way, it smells like dog vomit, so it doesn't much matter. I like to bottle mine up in used milk gallon jugs and save it for when I need it. Couple years ago I filled the freezer with it. DH really liked that a lot. haha But I used it during the winter to water my house plants and boy were they happy that year! Cheryl