Asim bin Damrah relates from Sayyiduna Ali (may Allah be pleased with him) that there is no zakah on green produce and vegetables.
It is reported from Sayyidah Aishah (may Allah be pleased with her) that the Sunnah has been established on this: there is no zakah on the vegetables and the like that the earth grows.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7484