Is this a weed?
This looks like chickweed Stellaria media to me. http://agpest.co.nz/?pesttypes=chickweed
This weed propagates both by seed and the creeping stem, which forms a mat on the ground and roots in the soil it's on. Yank it out before it flowers. Mulching will help to keep it at bay. For more information and options, visit Hortsense: http://hortsense.cahnrs.wsu.edu/Search/MainMenuWithFactSheet.aspx?CategoryId=6&ProblemId=2047