That's what I think it is too. I have many growing at the top of our ravine. They seem to spread very readily.

It certainly does look like Celandine poppy. But, Stylophorum diphyllum is definitely not 30 inches tall, more like a foot...foot and a 1/2. The sap of diphyllum is yellow. However, the larger Stylophorum lasiocarpum has red sap. Maybe that's what you have? Any rate, that's my guess, if it's 2 1/2 feet tall. Chinese wood poppy. Stylophorum lasiocarpum. Please let us know when you find out.

OK, my guesstimate at measurements is not very good usually, so I might be off on the plant height. I found several sources and it does look like Celandine poppy. Thanks for pointing me in the right direction! It's beautiful--I would love to have room for a bunch of them.

I have my celandines planted with species brunnera, because of the overlapping bloom period. The light, airy blue flowers of the brunnera combined with the larger glowing yellow of the celandines makes a fabulous mix, (if I say so myself). : )