Yes, my sweet williams are just coming into flower (in their second year). I'll see how long they last. They're in a wee bed I just dug in spring last year and are in with antirrhinums which are also coming into their second year's bloom. I'm in the happy position just now of replacing a different bunch of antirrhinums after their 6th year (which admittedly was poor, so I chucked them). I'd thought the sweet williams would be slightly shorter than the antirrhinums, so planted them in front, but they're exactly the same height! Not complaining though, they're all lovely. Must try wallflowers, I love the scent.
Do you think that planting into regular, unconditioned clay would be too rich for cosmos? Perhaps with a load of grit dug in?
With my limited knowledge of cosmos, I'd say you'd deffo have to improve the drainage on clay. Grit, yes, but if you have any, dig in some compost too. If it's only to improve soil structure rather than fertility you could use the contents of last year's containers. Maybe even some grass cuttings??
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Must try wallflowers, I love the scent.