Why do Zyra plants interrupt Nunu W?

Finished a game not too long ago vs. Zyra, was probably one of my more frustrating Nunu matches. Zyra could pretty much just plant down a W and pop Q when I'm relatively close or pretty much on top of me, aka before I have enough time to react and turn or leaving me with 0 reaction time, and her plants completely block Nunu's W. While we're on the topic, is it intentional for W to be blocked by minion waves? It felt like a weird interaction when otherwise it rolls over them, and also only collides with jungle creep and monsters.