In Persian, we say "When the reed blooms" when we want to express that an event:
- Never happens. (This is only the opinion of the speaker so it's not a fact)
- It's very unlikely to happen.
It's going to take so long to actually happen. (It's so long that you'd be better off giving up expecting it)
3.1. We also use it when someone is always postponing something they have to do so you never actually see it happen.
Kyle: When are you going to pay my money back?
Cartman: When the reed blooms.
Now I don't know if I got the plant's name right because there are some other versions like:
In any event, they have mentioned this plant in the idiom because apparently some versions of it actually never bloom. Others believe it's because farmers harvest them before they get the chance to bloom (e.g to make music instruments), so it never happens. I'd love to know the equivalent idioms or expressions for this.
PS: I only know "When pigs fly" but I'm not a fan of it because the word pig sounds a bit rude to me and partly because it implies a strong impossibility which doesn't work for the #3 use.