Alternate leaves, purple flower inflorescence is capitulum. Bracts are present.
Oppositely arranged, whorled, thin, simple/linear leaves.
This one was difficult. I believe it is some type of water pennywort. Thr leaves are more deeply lobed than some of the other water pennyworts. It is seeming to thrive in the more moist areas of the yard and grows quickly. Some of the stems seem to have a pinkish-tan coloring.