The narrow leaves look like weedkiller damage. I would prune that entire stem out back to where you get normal leaves. Have you put fresh horse manure round the plant?
No - I never use weedkiller, and though I gave it a manure mulch it was very well rotted. But I've only had the garden a year so can't speak for previous owners. Thanks for the advice, I'll try a hard prune.
Thats weird. Can you save seeds off it to see if it comes true and not a one off? It looks like the mutation called hen and chickens. There is a poppy that does this and it comes true from seed.
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No - I never use weedkiller, and though I gave it a manure mulch it was very well rotted. But I've only had the garden a year so can't speak for previous owners. Thanks for the advice, I'll try a hard prune.
Cosmos, all the flowers on this particular plant seem to be doing this