For me zinnias always grow taller than advertised, and I've only grown cosmos once so I really cant speak to that. My saved seed zinnias regardless of named variety always go 4' or so. The alleged 3' plants grow to 4' too. Smaller blooms, less foliage, but height is always there. I've got some candy cane mix listed at 30'' pushing 4' sown in late June. Proper cutting, in my experience, does increase bloom. I'll take a picture this week off a really skinny planting that's had a few cuttings. Nothing great but it's quite a showy bed. I've cut it's best and it's bulking up, it's a happy bunch. It could be better, veggies out back are on center stage.
