Had exactly same problem as Hotwax this year. Seed sown March, then planted out May with a trowel of compost for each plant. Purity and another variety. All the Purity plants lush and tall and very late to flower, the other earlier to flower in some cases but not in others, very frustrating! I had pinched some out, but it didn't seem to make a difference. Some still full of large buds and flowers so I'm enjoying while they last.
My cosmos purity flowering late this year too. But I'm still happy with them. They have been flowering since mid September and putting on a lovely display. I'm only deadheading every week. I think the cooler weather is allowing the flowers to last longer on each plant. I'm dreading a hard frost where they will all be wiped out.
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Finally first flowers - 8th November!