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."
Jami` at-Tirmidhi Book 7, Hadith 22
← Previous Hadith that two women came to the Messenger of Allah, and they each had a bracelet of gold on their forearms. So he said to them: "Have you paid their Zakat?" They said, "No." The Messenger of Allah said to them: "Would you like for Allah to fashion then into two bracelets of Fire?" They said, "No." He said: "Then pay its Zakat."

Jami` at-Tirmidhi Book 7, Hadith 21 Next Hadith → the Messenger of Allah said: "For what is watered by the heavens and steams, the Ushr is due, and for what is watered by irrigation, half of the Ushr."

Jami` at-Tirmidhi Book 7, Hadith 23