What is The Twig Key?

The twig key is symbolic of temptation. When Kip defies the Night Gardener, the Night Gardener fashions a special key for Kip to use on the green door. Despite Kip using the key to the door, bringing himself face to face with greed and the balm for his leg, Kip refuses to use the balm because he knows it will take away his soul and leave him powerless against the tree. Thus, Kip manages to defy temptation and remain morally just.