A "planted question" is a rehearsed question. The word "planted" was cleverly used here because the joke is referring to an agricultural presentation. "Agricultural" is a word related to agriculture, which is the practice of growing plants and livestock. A plant is a living organism that grows in the earth and has a stem, leaves, and roots. "Plant" is in the word "planted". This joke plays on the word "planted".
Why did the agricultural presentation go so smoothly ? They planted questions .