Mu'adh had written to the Prophet to ask him about greens - which is "herbs" - so he (pbuh) said, "There is nothing due on them."  

أَنَّهُ كَتَبَ إِلَى النَّبِيِّ ﷺ يَسْأَلُهُ عَنِ الْخُضْرَوَاتِ وَهِيَ الْبُقُولُ فَقَالَ لَيْسَ فِيهَا شَيْءٌ