Cosmos are very easy to sprout from seed, but it’s best to sow them directly in the garden. I usually buck that advice and give most of my cultivars a head start by sowing them indoors about 4 weeks before our last frost date. Sometimes it works and sometimes it doesn’t.
Cosmos grow very quickly once they sprout, and you can’t pinch them back because they tend to grow about 3 or 4 inches before the first set of leaves pops out. Those are only the cotyledons and they feed the young plant. You can only pinch back a seedling above the second set of leaves that unfurl. Cosmos may grow 6-8 inches high before that happens. The plants often struggle to stand up straight, and floppy plants don’t last long when I’m around. I usually behead them accidentally while sliding the plants trays in and out.
I have had very good luck with the Sonata, Versailles and Cosmic series of cosmos. Last spring I was impressed with ‘Pink Pop Socks’. I tried ‘Rubenza’ two years ago, and the poor plants never made it out of the basement.
I’m going to stick with the tried-and-true again this year, but because ‘Pink Pop Socks’ (pictured above) performed so admirably, I’m going give its sister ‘White Pop Socks’ a shot.