Hi Ann

I suspect what you have is not marsh marigold, but Ranunculus ficaria (aka LESSER CELANDINE, CELANDINE BUTTERCUP) which is considered an invasive wetland plant. I too have had my yard and garden beds invaded by this innocent looking little monster. It is incredibly hard to get rid of... but one thing I have discovered - you need to dig it up and remove the little tuber or potato like thing that it grows from. If you do not, that will divide and you will have tons of them the next spring. I resorted to using Round-Up and a paint brush to paint the leaves of them where they have gotten wedged inbetween the eyes of some hostas and irises. In the lawn I am just spraying them and will reseed when I know they are dead.

You also need to be cautious about disposal of them. Do not put them into a compost bin and do not toss them in a trash dumpster or can. They can survivg a long long time out of the ground living off the little tuber thing and it doesn't help to contribute to the spread of them. If I am digging them, I put them in a plastic bag and give them a squirt of herbicide just to be sure they are dead and then they go in the trash.

Here is a photo of it:
http://www.ibiblio.org/herbmed/pictures/p11/pages/ranunculus-ficaria.htm

LESSER CELANDINE, CELANDINE BUTTERCUP (Ranunculus ficaria) has spread from gardens to carpet our floodplains with small yellow flowers in spring. It comes up in winter, giving it a head start over most native spring wildflowers.
Control: It is not yet known whether digging is effective - the small reproductive corms break off very easily. Try digging (1) before the plants flower. Otherwise, use Rodeo (10 or 11), preferably in February to protect native plants, frogs, and salamanders which become active in March.

Good Luck and if anyone else has suggestions for control or removal of this beastie I would love to hear about it.

Alexa

Here is a link that might be useful: Ranunculus ficaria

I forgot to mention.. it is considered a "moderate threat" by the PA Dept of Conservation and Natural resources.

Alexa

Here is a link that might be useful: PA DCNR

Thanks, Alexa... I think. That's not great news. And you're right. Judging from the picture you offered, I have the insidious buttercups and not the marigolds. Sounds like I've got my work cut out for me! Oy!
Ann

Yeah - the first time that I had ever seen them was last spring, the spring after my sister had moved into her new house. She has a creek at the back of the property and we were wondering what the heck they were (and soon discovered the truth). Seems they are out and about all over the place now. I never remember seeing such as much as now - even as a kid playing by the creeks in the area. Amazing how plants like this can spread.