2020-05-13: Added comparison of seedpods (easiest way to distinguish the two) and sap (not reliable).
I’ve been seeing a lot of misidentifications – or perhaps wishful ones – of the invasive Chelidonium majus, greater celandine as the Eastern U.S. native Stylophorum diphyllum, celandine poppy. Here is a visual guide for distinguishing them.
Habit
Both grow to similar height and width, holding their flowers just above the foliage when blooming.
Sap
All parts of both species exude a brightly colored sap when broken or crushed. However, I find the color is variable, ranging yellow to orange, and not distinct enough to be diagnostic.
Foliage
Both have deeply pinnate leaves with lobed leaflets. The lobes on the native celandine poppy are more open, almost oak-like, than on the invasive greater celandine.
Flowers
Fruit (Seedpods)
I find this the easiest way to distinguish the two species.
Very helpful post. I have a LOT of the Celandine Poppy. Actually, anyone who starts out with a few will likely end up with a lot, because of how it spreads by seed. Which is fine, though it does tend to smother other spring-blooming flowers, especially shorter ephemerals. Still, the flowers are wonderful.