Like all living organisms, trees need water. All of the water they need, they take up from the soil through their root systems.
Trees use water is a number of different ways.
Water coming into the root system brings along many of the essential elements (nutrients) the tree needs to live and grow.
Water evaporating from leaf surfaces draws more water up through the vascular tissue (the xylem), at the same time cooling the leaf so that photosynthesis can continue; when leaves get too hot the chemical processes stop working right and no new sugars are made.
Water is fundamental to processes going on inside each and every cell, both as the internal fluid of the cell, and as an “ingredient” in biochemical reactions occurring within and around the cells.
Trees need a lot of water, but how much? Unfortunately we don’t really know, in terms of gallons per week – good research on this question hasn’t been done, and the answer depends on many variables. Often we suggest trees be watered every other week, but with a lot of water spread over a lot of soil and allowed to soak in.
What is really important to the tree is the combination of a large volume of water into a large volume of soil. While trees can be grown using drip irrigation systems, to adequately water a tree of large size, one would need dozens of emitters spread throughout the root zone. Drip systems are usually set to run too often, but not long enough, to adequately water a tree. Finally, the bigger the tree, the more water it needs. As the years go by and the tree grows, it will need more and more water, so any automated irrigation systems need to be tweaked every couple of years. Add emitters out to the edge of the branch canopy (the drip line) and beyond; old emitters at the base of the trunk can be capped over time.