I just looked for this, very pretty. Similar to Beefsteak plant or Perilla frutescans according to Illinois widflowers but yours looks shinier so I'll keep looking. (maybe another Perilla)

Thank you. This pic is just after a rain shower. That might explain why it's shiny. I see that Perilla is in the mint family. I now think I know were it came from (My former neighbor used to grow various kinds of mint. She passed away and this is right on the border between our houses.) Maybe I can use this in lemonade.

That's the closest I can find Bathgate although another Perilla, Purple shiso looks very similar. Edible too it says although I wouldn't try it unless I 100% knew what I was eating!!

Does a crushed leaf smell minty?

It did say mint family Pennyfarthing if it's Perilla.

I took a couple leaves intending to use them for lemonade, but they weren't minty at all. The smell is rather pungent, a little off-putting.

Could it be Atriplex which is I believe classed as a weed? Pretty though and not long lasting.

No...Atriplex has alternate leaves, while this one is opposite. I would have said shiso, myself, but it apparently doesn't smell right--though shiso smells more like Thai basil than mint, to me.

As mentioned, I don't want to risk eating it unless I know for sure. You might be right Tug. It could be shiso. Here's what I found about shiso...

'Shiso has a unique flavor: pungent and grassy, it contains strong flavors of spearmint, basil, anise and cinnamon.'

thanks everyone for your input