I don't imagine plants would be very conspicuous in the Yeerk Pool...Maybe if the Animorphs had themselves form a bouquet and get themselves sent to Visser Three for Valentines day?
No. Even better! The Animorphs convince Visser Three to convert to Christianity (or to just celebrate Christmas), by having him go through a "Christmas Carol" scenario, where they all pretend to be the ghosts of Christmas past, present, and future. After he sees the error of his ways, Visser Three gets into the spirit of Christmas. The Animorphs morph his Christmas tree, and assassinate him over the milk and cookies and presents!
I am convinced. Animorphs must definitely morph plants!
Also, in regards to murdering innocent real life Taxxons: for shame, Saffa. For shame!