You might be tempted to conclude that it is the native Stylophorum diphyllum (wood poppy), which has similar leaves and flower. You would unfortunately be incorrect. This is actually the introduced species Chelidonium majus (greater celandine), which resembles Stylophorum diphyllum primarily because they are both members of the family Papaveraceae (poppy family). The native variety, however, has much larger flowers, its leaves are more deeply incurved, and its seed pods are round and hairy rather than the long and slender ones we see in this species.
|Chelidonium majus population in bloom|
|Chelidonium majus seed pods|
So why was it introduced here? This plant was originally brought over for medicinal purposes, used for a whole range of illnesses and problems including stomach ailments, warts, and menstrual problems . Note, however, that there is not much evidence for its use in these capacities: it is possibly effective (some limited evidence) for stomach upset, and there is insufficient evidence for the remaining claims .