You don't need to use your Cenarian Plant Salve when getting NightDragon or Whipper Root. Basically if the timer on the plant is up you cleanse it and it will bring you to a second screen to confirm. The plant, however, will be cleansed. You leave the second window open, pick the plant then cancel. Voila, you keep all your Salve and you get the plant and you get to keep the plant "corrupted" so next guy can cleanse it for you but cant pick till timer is up.