It's shoo fly plant (Nicandra physalodes).

Thank you for asking this question , F/G .
We have them growing very happily around the garden , and one is blooming spectacularly in the greenhouse . (I never planted it there .)
We were given a few about 20yrs ago , and the lady was called Pat Powell . For lack of another name , that is what we have called it . She said that we would never be rid of it ; how true , but it is very pretty .
We have passed the mantle on to our son , too , when we saw him the other day , he said that he had this persistent plant around his garden , yes , a Pat Powell !
Thanks Myron for the name , I was going to ask myself !

Wow. Thanks Myron. Now I know the name, I remember buying it, but it was years ago. The seed must have been buried in the soil which was recently dug over to erect a new fence.

Yeah it's Nicandra someone told me if you grow them in the greenhouse they deter white/blackfly , I've done this and never had either whether its a coincidence , and they also smell like cats wee. I don't actually plant them they appear in the cracks every year.

I've heard that some bird seed mixes contain shoo fly seeds, so that could sometime be a reason for the plants popping up, especially in the vicinity of bird tables.

That would explain it. It's growing just under a bird feeder. I wondered if that could be the case when I wrote my message but dismissed it as it seemed impossible. Why would they use those seeds in bird mixes? I've been feeding birds for years and this is the first time.

The fruit of the shoo fly plant contain a lot of seeds, so I think that one of the reasons they're added to bird feed would be to bulk the feed up. Also,most birds go mad for the fruit when it's ripe so this would make sense.

I just hope the seeds are good for them!