Pinching them back will not make them much shorter if it makes them shorter at all. All pinching back does is make a plant bushier because it will send out new growth. The plant will still try for the height that is in it's genetic code,

Yes it will delay blooming because the plant would have to replace the growth that was taken away.

I think you'll have to take them as is and plant a dwarf variety next year

when you have multiple plants ... you experiment with multiple uses ...

take one area of your plethora.. and pinch them back.. and SEE WHAT HAPPENS ... if they are all in one glob.. do half a glob each way ...

have no fear.. what the worse that can happen???? [well according to the link. you can have more plants regardless..]

just make sure you can ID dormant buds.. and pinch just above them.. they are usually in the leaf nodes ... though it has been a long time since i have seen a cosmo in person ...

ken

Here is a link that might be useful: i googled cosmo leaf node and came up with this link ...

Last year I discovered that larkspur sends up short, blooming side branches when laid down on the ground well before bloom time. I wrote about it here and someone mentioned that cosmos have the same tendency. You could try weighting or tying down your cosmos and see if it will make a long, low line of blooms for you. My larkspur was gorgeous, winding in and out of a clump of day lilies. This year, I have purposely flopped some larkspur down in several places. Let us know if it works! We'll all be trying it.

Hepatica

Hepatica, love this notion. I've been looking for cosmos seedlings to come back from last year. So far, none, but maybe they emerge late. Could experiment using the small plants. Adore the flowers.

Last summer, I pinched back the sparkler series, the plants got thicker and it put out seeds (like crazy).

I got many cosmos roots by accident last year. I put some cuttings with flowers in a vase. Almost immediately they started producing roots.

I am going to try rooting cuttings early this year.