Enchanted items can do anything evocation or thaumaturgy can do, so you don't need thaumaturgy at evocation's speed and methods to work thaumaturgy instantaneously with enchanted items.
Secondly manipulating plants is a lot like manipulating lightning (see the box "Mommy, where does lightning come from?" on YS254). It could potentially fall under several different elements. Water for manipulating the liquid inside them, earth for it's thematic proximity to plants, air again for thematic proximity, and possibly even spirit for manipulating them by force if you want to stretch it. But just because it's not an element in and of itself doesn't mean you can't do it with evocation.