Christmas trees need to be watered regularly to prevent them from drying out and becoming brittle. The amount of water they need will depend on a number of factors, including the size and type of tree, the temperature and humidity in the room, and how much sunlight the tree is receiving. As a general rule, it's a good idea to check the water level in the tree stand every day and add more if needed. If the water level is low, add enough water to fill the stand up to the recommended level.