Cosmos. (Cosmos sulphureus 'Bright Lights')
Chrysanthemums. (Chrysanthemum rubellum 'Sheffield Pink')
When my friend, Christine, asked me today just what was the difference between a cosmos and a chrysanthemum, I wasn't really sure how to explain it, so I thought I'd spend a little time researching and sharing what I found out. Okay, so maybe these aren't the best examples of how these two different varieties of flowers could be thought to be similar, but these are the varieties I have in my gardens, so we'll go with them. (Of course, I couldn't find my picture of cosmos, so the one above is from the seed company catalog.)
According to the scientific classification (you remember that, right? Linneaus? The guy who would have gone nutso with a p-touch?), these two beauties are the same until you get down to the tribe classification, that's when the split happens. Not very helpful - right?
Okay, so then I looked at where the plants came from. Cosmos come from Mexico and South America while chrysanthemums came from China. That seems pretty clear, but since my flowers don't have labels growing out of them (sorry Linnaeus!) this probably isn't helpful either.
Cosmos and chrysanthemum leaves sort of look the same, lobed like an oak leaf. Cosmos in my zone are usually reseeding annuals. Chrysanthemums are perennial bushes. Cosmos start blooming in summer. Chrysanthemums start blooming in fall. Cosmos generally have a single row of about 8-10 flower petals that create a single ring around the center of the flower. Chrysanthemum generally have anywhere from 2 rows of petals (like a daisy) to a gazillion petals (like a pompom) that create multiple rings around the center of the flower.
And then I noticed this: cosmos flower petals are sort of squared off at the ends, and chrysanthemum petals are pointed.
Cosmos petal - squared off
Chrysanthemum petal - pointed
So when you're walking through the garden and you see one of these beauties, take note of the petals. If they are squared off, it's probably a cosmos. If it's pointed, it's probably a chrysanthemum.
And that's the best non-scientific, non-botanist way I can come up with to explain in 15 seconds or less the difference between a cosmos and a chrysanthemum.