Um. Ok. I think you're operating under the assumption that Dresden magic works like Eragon magic, where energy for spells must come from something's life energy. That's not the case. In fact, Thaumaturgy, like Wards, don't have to cost any stress to the faster at all.
Now, you could use the plants to enhance the Ward's strength by two by saying,"I have a bunch of magically prepared plants that can help with these Wards.