Posted: 19/09/2017 at 10:42
Thanks to everyone for their advice, really helpful for me as well. My Verbena Bonariensis self seeds and regrows every year and I just leave it to its own devices, although I ususally cut it back to the ground where it is a nuisance, such as along the paths.
I also have heavy clay soil and after trying leaving Dahlias in, lifting them, storing them dry, storing them in dry compost and all the other options I have given up and treat them as annuals, buying them as small plug plants in spring. For me, and it is probably my lack of skill and knowledge, it is the easiest option.
I also have Asters, which I cut back to the ground once the frosts brown them off.