any idea what this flower/herb/weed is??
 

This mysteriously showed up in one of my planters alongside nasturtiums and cucumber. I have been gardening for years but have no idea what this is. It had light purple flowers that were pretty but I didn't get a photo of them. this is the seed pod and stem.

Did it get any blooms?

yes the flowers were light purple/lavender and shaped somewhat like a morning glory or single flower hollyhock flower as I recall. It took a while before I realized the plant had flowers because the flowers did not last long, they quickly turned into these balloonish seed heads.
The plant was thin and spindly with many long thin stems.

Yes, this is called "flower of an hour" "Venice Mallow"or Hibiscus trionum. weed weed weed, but lovely smelling none the less.

YES! That's it! Thank you so much for replying. I have been searching for this for months. I wonder how it got in my planter...must have been from one of the composts I bought. Interesting...