Frosty disasters!

I had confidently put my busy lizzies in the greenhouse, expecting them to survive temps. down to freezing... sadly they didn't. I only managed to rescue one, and although they are easily replaceable, I can't find seed for the variegated leaf double flowered one. Help, please? The doubles don't seem to set seed for me, is this because they're hybrids?
Contrary to my expectations the Bacopa is doing ok in the greenhouse, though none have survived outside. [I have one under a plastic dome and fleece whose roots may survive]. :(

 

Hi Liz

If its peppermint
https://www.fotolibra.com/gallery/image/?image_id=34254

I have only ever had it as a plant and took cuttings from it, never seen seed available

 

Same here liz put lizzies in green house went away came home no lizzies. I think the cold snap was the worse for a good many years.