Plant or weed

Hi living on the coast of Mexico i have a box out-side on my patio last month we added some more soil hoping to grow some herbs but nothing lives very long seeds grow a couple of inches then die , But some how in the new soil a green shut came up and grew , and its still growing , Now i kind of recognise it ,as this weed ? grew every were in my Garden in BULGARIA , any way that's another story i have put a picture of the plant so you can all see and help me out weed plant herb ,? does have small yellow flowers , love to know more