The amount of water needed for a Christmas tree depends on the size of the tree and how long it has been cut. A good rule of thumb is to give the tree at least one quart of water per foot of height. If the tree is dry, you may need to give it more. You should check the water level daily, especially if the tree is in a warm or dry environment. You can also use a tree stand with a water reservoir to help keep the tree hydrated.