Creeping Woodsorrel (Oxalis corniculata, 酢浆草).
These buttery yellow ground flowers begin to creep out of the green crowded earth every Fall and more appear whenever it rains until at least late Spring. According to Joseph Needham's Science and Civilisation in China (vol. 3 p. 678), medieval Chinese scientists discerned how to locate copper deposits in the earth from the trace accumulations that build up in their lemon-flavoured trifoliate leafs.