buttonweed (Diodia virginiana)

Diodia virginiana is a plant species in the Rubiaceae, common name Virginia buttonweed. It is a spreading, matted perennial with opposite leaves, often mottled because of a virus that attacks the foliage. Flowers are white, cross-shaped with 4 petals. Fruits are green, often floating on water. The species can become a nuisance weed, hard to eradicate because of underground parts that remain behind when you try to pull up the plant.