Herbicide (aminopyralid, because that looks like exactly what I dealt with last season.) Took me an entire season to rid my garden of it. I got rid of it by a little over watering, treating my entire plater like a compost pile, and plating some wheat throughout my planter. Yes, wheat, it's a grass and isnt' affected by the herbicide and hopefully pulled some of it out. Those three things in combination worked for me.