Today marks the hottest day of 2021 so far, with temperatures expected to be close to 100° by the afternoon. Seems to me to be a great time to go over how water moves from the ground all the way up into the canopy of a tree.
Water is taken up from the ground into a tree’s root system and it’s the smaller, more fibrous roots that absorb the majority of the groundwater. The larger roots take up very little water but instead provide the main support for the tree.
From there, the water then travels up through the “vascular” system of the tree, which lies just below the bark. This network is made up of specialized cells called xylem and phloem. Phloem is responsible for transporting nutrients throughout the tree that aid in processes like growth, reproduction and photosynthesis. Xylem is actually made of dead cells that act as plumbing for the tree. Water is pumped up through the hollow xylem, very similar to the pipework in your home.
So how then does water defy gravity to move upwards to the very top of the canopy? The answer lies in the leaves. There are tiny pores (mostly on the undersides of leaves) called stomates that are crucial in gas exchange as well as a process called transpiration. Heat from the sun causes the water in the leaves to evaporate from the stomata. Negative pressure (suction) is created, similar to how liquids can be drawn up through a straw.
Water is made up of two hydrogen atoms and one oxygen atom, giving it the chemical name H2O. This particular molecule is also polar, meaning one side of the molecule is positive and the opposite side is negative. Positive and negative charges attract so water molecules have the ability to link together like a chain to be drawn up through the xylem all the way from the soil to the canopy.
On very hot days like today, a tree will do it’s best to conserve water, sometimes by closing its stomata or even dropping some of its leaves altogether. The best way that we can help is to provide the tree with as much water as possible during hot, dry times. A slow deep watering directly to the root zone is much more beneficial to a tree than relying solely on irrigation, which is usually evaporated or taken up by the surrounding landscape plants before the tree’s roots get a chance.