The helicopters detect heat from the overturned soil at the plant's base due to the lack of vegetation.
Pretty close, its the dark soil that is freshly turned/watered absorbing more of the sun's energy, making it appear to be a white ring on FLIR.
I've heard a CAMP pilot on another forum tell us the only plants he can't see from the air are ones under camouflage netting. He says NOTHING else works, and camo netting is amazing.