To be clear, we're talking about any organism found on earth that's classified under the kingdom Plantae. Assume these plants won't be genetically modified at all when they're used in the military and that the army's composition is 85% human and 15% plant.
I'm not asking for specific plants to use (although that would be helpful to include in your answer), but rather, how living plants could be used in warfare in the first place. The usage of the plants can also be indirectly impactful; for example, planting tall trees and annoying vines in a buffer zone between two military camps can act as an obstruction for an opposing army trying to invade. So far, however, that's the only idea I've thought of. How could plants be used in an army?