Any tree planted within the last two years must be watered now. My suggestion is to water a small tree thoroughly every two weeks. Trees that have been planted more than two years ago may need attention now also. They can be watered every three weeks. Very large trees can generally get away without water for four weeks but be careful. One formula for application volume is 10 gallons per inch diameter of tree trunk. The temperature is so high now (100+ degrees) that all plants need more frequent watering than they normally require. To water trees, it is advantageous to understand where the roots are located, the physical nature of the roots and water delivery process to the leaves.
How do leaves get water?
A tree's primary source of water is an area from the surface to about a foot and a half deep, located around the tree. To understand what the root system looks like, simply look at the tree and picture the tree limbs as roots which are usually a mirror image of the tree limbs. The length of the roots will be longer than it corresponding limb and will extend beyond the drip line of the leaves. Now think of the leaves as fine roots needing to find water. The tap root will penetrate much further, but the shallower roots are its source of life and are the key to the tree's health. 90%+ of the roots of a tree are within the first foot of soil! In broadleafed trees, water is delivered to the leaves of the tree from the roots by a physical process called transpiration which leverages a basic physics process called capillary pressure. A pressure that counteracts gravity pull is produced by the capillary walls consisting of cells especially made to move water and nutrients from the soil to the leaves. In the leaves, during the day, the stomata under the leaves constrict to preserve water. But as inevitable evaporation occurs in the heat, water must be replaced by the capillary pressure pushing the water into the tree leaves, keeping a pressure in the capillaries. Without water in the root system, the capillary pressure can fail, causing the tree to die. The first sign of lack of pressure in the capillary system is the wilting of the leaves, next a browning, and then a total failure of the capillary system. A pine tree is different. It moves water from live cell to live cell, using a pass-the-baton type process. It is much slower than the capillary process, so pine trees die easier when the water dries up. By the time the leaves turn color on a pine tree, it is too late to recover. Pine and Cypress trees react to a shortage of water in the heat by shedding many of its leaves, thereby reducing its consumption and thirst.
How to water your trees
So as the soil dries, the process of moving water up the trunk of the tree to the limbs is diminished, stressing a tree in its ability to cope with drought. The tree needs your help. A small tree can get by with watering every 2-4 weeks. I use a two gallon bucket. One bucketful for a seedling, two bucketfuls for a 3 foot tree and three bucketfuls for a 10 foot tree. One recommendation is to provide a tree with 10 gallons per caliper (diameter) inch. To do this effectively, you must have the base of the tree mulched as an shallow upside down bowl structure so that the water is held within the irrigation area intended. Get the water to the area on the ground that mirrors the ends of the branches. This is where the ends of the roots occur below the ground. The base of the tree needs to be mulched to conserve evaporation from the ground. The larger the tree and the older age of a tree makes the process more difficult. A large tree needs considerably more water and it must be delivered slowly. A drip hose wrapped in concentric circles around the tree will soak the ground. That is the best way to make sure water reaches one full foot below the surface. It may be more advantageous to use a needle approach however for the large trees, where the water is delivered directly to the roots a foot underground. A water measurement device to one foot deep is handy for you to know if you have reached the depth intended. One can also purchase watering devices for trees at any tree nursery, but they are not required.
Conserving water and being a good neighbor
When the time comes to water trees, we almost always are under a water alert from our water utility for watering lawns. We are either asked to voluntarily conserve water or required to water the lawn only on prescribed days in order to maintain water pressure to homes and have emergency water in case of fire. Drip watering devices do not utilize a lot of water and therefore are not a threat to the water pressure if used with low pressure. However, it is wise to monitor the consumption of your watering process by taking a water meter reading before and after you have watered a tree, so that you understand how much water you are using and what the delivery rate is. If the usage is more than about 10 gallons per hour, you should not be drip watering on days that your home is not allowed to water the lawn.
Watering your lawn is not the way to deliver water to your trees, unless they are seedlings and even that is insufficient for seedlings. If you water your lawn and do not water your trees, the root system will die at normal feeder root depths and only live on the very surface. High wind can then blow your tree down much easier! One inch of water on a lawn is a significant watering but will not reach very deep for a tree's use. Remember Hurricane Ike!
Take care of your trees. They take decades to grow back. The loss of deep roots has proven to be a significant risk to homes and fences in high winds. I recommend reading the references below for additional information.
Additional Resources
1 Architecture of a tree
2 Caring for Trees During Drought
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