This plant in this photo apprears to be Lesser celandine an invasive plant though it may be confused with the native yellow marsh marigold also in the same family. The yellow marsh marigold does not have an tuberous root like the lesser celandine.
Please read the Washington State Noxious Weed Control Board description for Lesser Celandine Ficaria Verna. You can read about control methods such as mechanical, cultural, biological and herbicide which varies with the amount of infestation and comfort level working with increasing amount of control method interventions. Information is also available in the Pacific Northwest Management handbook and the Oregon Department of Agruculture lesser celandine information. Best of luck with this effort.