Water is a critical requirement for all plants, and they need a lot of it. Pulling water in from the soil through the roots is not only how plants get water, but also how they obtain the nutrients they need to grow, which is dissolved and suspended in soil water. Each day they take in a great deal of water through their roots, sift out the nutrients, and then release it as vapor through their leaves in a process called transpiration. Forests, being made up of a lot of really big plants standing shoulder to shoulder, accounts for some of the high humidity we experience each summer through this process. A mature maple tree can take up enough water to fill three bathtubs per day.

Water is especially important for newly planted trees, as they often have minimal roots, so dry weather is especially stressful. For well-established trees, getting enough water keeps them healthy and better able to fend off insect and disease problems. It will also keep their growth rate high, and that means value. Big trees in a landscape add worth to a home, so keep them cranking with water when needed.

It's best to water slowly so the feeder roots can take up the moisture as it's provided. That way the tree benefits from all the water and none is wasted from runoff. For big trees the simplest way is to drag the hose out to the tree, turn it on to a slow drip, and leave it for a few hours or overnight. A soaker hose on the end that oozes water out slowly is ideal, that way you can coil it around under the tree and get more coverage. Move the hose to a different tree every night and keep at it until you water all of the trees, or until the rain comes. For small trees a 5-gallon bucket with a nail hole in the bottom should suffice for drip irrigation.

The way a tree takes in water is an interesting process that was only figured out a few decades ago. The pipeline used to move water is xyloem cells (pronounced zi-lom). They run from the roots, up the trunk, through the branches and twigs, up the leaf stem, and finally to the leaf itself. Water is not pumped up a tree but rather pulled up because of two attributes: water clings to itself (cohesion), and it will always move from a high-pressure area to a low one. When heated by sunlight, water in tree leaves evaporates and is released into the atmosphere through openings called stomates. The escaping water creates a low-pressure area in the leaf and so water moves in from the leaf stem to replace it. There is now a pressure drop in the stem, so water moves in from the twig, and so it goes until finally a pressure drop in the roots causes water to move in from the soil. Once water enters the tree it is called sap.