Posted: 21/09/2012 at 15:00
Nothing to lose- but not sure if it'll work. Never tried with Helianthus.
Trim the broken stem to just beneath a leaf node, remove most of the foliage above this cut- leave some- & also any flower buds that were forming. Should then have a piece say 6ins long. I'd dip the bottom, cut end, into some hormone rooting powder, if you have any, but no panic if none & then pop the cutting into some moistened, gritty compost near the edge of a pot. I'd probably fix a plastic bag over the pot too to save evaporation & then put the whole into a coldframe, or sheltered corner by a wall & wait & see. You can tell if it's worked by seeing some new growth through the bag, but watch for the whole rotting off.
Just checking in book & the normal way for propagating this plant is by division in Autumn/Spring, but I suppose you can treat it like a penstemon or Fuschia both of which I treat as I've described. J.