What's wrong with my Cosmos

[PickledMuffin]

it's a pretty new plant, bought it circa 3 weeks ago and planted in the post with compost. but last two days the leaves are turning yellow. in partial sun. Any ideas?

bump

It looks great just probably a bit pot bound. Have a look underneath and see if the roots are poking through. I think it needs a bigger pot, more water and food. I suspect it's not getting enough nutrients in that little pot.

Looks fine to me, often the lower leaves are lost as it grows up. Might need a bigger pot though? Depending on variety, they can get pretty enormous!

I’d guess overwatering/drainage issue. On the assumption that ceramic outer pot doesn’t have any holes in the bottom, the inside pot won’t be draining properly. Take the plastic pot out, look underneath to see if the soil you can see through the holes looks soggy. If so, leave it somewhere dry and shady until it has a chance to dry out. Don’t feed it!

Would also suggest that pot is too small and it's probably hungry to get its roots out, any chance it could go in the ground? They really do get huge if allowed to and the floral reward is so worth it.