Do you perhaps mean Jerusalem artichokes, the ones whose roots you can eat? If so, you can divide and replant them now I would say.

Makes a great vegetable, but be aware of the (some say amusing) side effects. High in Inulin, a starchy compound which cause most people to produce excess wind. We called them 'fartichokes' when I was a kid. Phil J

Dreeny, I answered your second question on this subject - I think we need to establish exactly what plant you're referring to - can you post a photograph please? Only sunflowers are annuals, not perennials.

"Fartichokes" is excellent, sheer class Phil. I told the kids who of course had a good chortle at that.
Apparently my Dad & Sis can testify that they had dramatic effects on my poor Mum!

Bamboo - I'm still in the dark re the original question, but I too have some unidentified sunflower impostors (kind of, but much smaller) so I'll try and post a photo.
Dreeny - have you done a google image search and found your flower's name yet?

The number of plants which produce yellow daisy like flowers of varying sizes is very large, so Dreeny's could be any one of a number of plants, all of which vary in height, shape and size of flower, etc.

I suppose Heliniums< I have two, one a shorter darker headed plant in front of
the very tall specimens, which my Dad certainly grew in the 1940,s and possibly before that. I chopped most of the heads yesterday because they were leaning on the ground, but I will look tomorrow and see if there is one still lurking and
get a pix.
Dreeny

I agree with that Bamboo by now, but at least the UK 1940s-likely list was a bit shorter! I think people had more fundamental things on their minds than browsing at the garden centre. Saying that, there weren't any yet...