“When is the latest I can sow Hardy annuals in Autumn?”…if I had a penny for every time I have been asked this…I would have about 17p, The answer has a good deal of variables…mainly being how far north or south your flower patch is. I met up with some flower farmers down here in Cornwall who sow hardy annuals right up to Halloween (in a poly tunnel)…I myself am happy to sow outdoors up to the September equinox (usually about the 21st) in my Cornish patch. However if I still lived in Edinburgh I would have my seeds in the ground by now. I would also consider fleecing the beds in mid Winter.
The weather also plays a part of course…the milder the Autumn…the later you can sow.
Here is a swimmingly good list of other related Autumn sowing posts:
Uncle Archie’s Seven Point Plan to sowing autumn annuals.
Have fun one and all.