This would be the telling feature then. Something I didn't remember.
<<Unopened flower buds of coreopsis are boxy looking, very distinctive.>>
I grew cosmos long before I grew the coreopsis and wondered about it being cosmos;however, the coreopsis is a perennial while the cosmos is an annual that reseeds itself. I thought it more likely that the coreopsis (tick weed) would be added to a wildflower mix. A guess really. In my memory the double coreopsis has a more dense foliage, but I also tried a "paint" variety which is single and it had more sparse foliage.
I think they're both pretty.
KY Zone 6
From: Linda Mann <firstname.lastname@example.org>
To: iris-photos <email@example.com>
Sent: Mon, Nov 1, 2010 3:47 am
Subject: [iris-photos] Re: Wildflower ID/companion plant - not!
Unopened flower buds of coreopsis are boxy looking, very distinctive.
Flowers of yellow cosmos and coreopsis are hard for me to tell apart
from a photo.
I thought cosmos were going to be a great iris companion plant, drought
tolerant, airy foliage, self sowing. Several generations later, self
sowing in the iris patch, they grow up to 6 ft tall with dense foliage.
Beautiful in bloom, but too enthusiastic! smothering irises. First
generation was <3 ft.
Linda Mann east TN USA zone 7