How trees should be watered is vastly misunderstood by the public at large. I speak with people almost every day who do not know how to water trees. In fact, people who think they know how to water trees are too often wrong. They water in ways that either do nothing for the trees or actually do damage to the trees.
So, first let me dispel some of the more common misunderstanding by listing the things that should not be done when watering trees.
1. Expect your lawn irrigation system to water your trees.
2. Water your tree near the trunk.
3. Believe you have to water the entire root zone.
4. Think you have to water your trees every day.
5. Think you do not have to water your trees.
6.Think that the brief showers and thunderstorms we have in Austin are sufficient water for your trees.
All this warrants more explanation, so I will do that. First, I want to explain how to water with a list like the one above that will be a list of "Do's."
1. Water your trees when there has been no rain for about a month.
2. Make sure your tree gets a good drink.
3.Water with you garden hose.
4. Water in the evening and into the night.
Now explanation is even more warranted! I can do that. Please read on.
How To Water A Tree
Austin, Texas has soil and weather conditions that pose a bit of a challenge to those who wish to water trees. There is little soil in this area with a few exceptions. (Nothing about trees or soil can be written in stone. So, I will just do what I can to get information out there that will help.) What soil there is has so little water retention capacity that it dries out very quickly. The rains come in short down bursts that drench the soil beyond its capacity to hold water and after the soil is saturated the rest of the rain is lost as runoff or penetrates the soil where it runs off along the rock below the surface. The trees get some water, but not enough. It is like giving a person a sip of water when they are dying of thirst. The trees get a good drink only when it rains for a day or more.
Trees drink water from the macro pores (small spaces between soil particles) of the soil. The very small roots called "hair roots" do the drinking and they are found out toward the ends of the roots. The roots near the trunk are very large, like the limbs are where they branch out from the trunk, and get smaller and smaller as they get further from the trunk. That is why it is said to water the trees out near the drip line, which is the outer reaches of the limbs where the water drips from the leafs at the extremity of the canopy. There are hair roots a little closer in and a little further out, so we can give the trees water and nutrition in the area just inside the drip line and just outside it. Trees are all different and species of trees vary greatly in root structure. But you can be fairly certain that you are water you trees when you water under the drip line. Water applied near the trunk is mostly a waste though there are likely to be some hair roots from adjacent trees in that area that will benefit from the water.
The next thing to know is that roots provide water to all parts of the tree. It is not true that a certain root only supplies a certain part of the tree even though there is some factual understanding in the notion that certain parts of a tree are first to receive the production of certain roots. The affects of this are uncertain but appear to have some influence in the survival of a certain part of a tree when lack of water stresses a tree to death or near death. We see this when we see a tree with one major artery alive while the rest of the tree is dead. I think that when there is only a "sip" of water available to a tree, the parts that get the water first are the last to die or the only ones to survive. So, when you want to water a tree you can do so by watering a section of the root zone provided you supply ample moisture over a sufficient amount of time.
Water must be available to the roots long enough to sufficiently hydrate the tree and all its systems. A tree can hang on to life surprisingly well with just a small amount of water. I have seen trees in our area go over six months without a drop of water without dying. It is a strength the trees have to more or lesser degrees depending on the tree's condition. Different species of trees have various degrees of ability to withstand the lack of moisture, "drought tolerance" as they say. Here again, nothing is written in stone. Drought tolerant trees can die quickly if they are suffering other types of stress at the same time as the water shortage. A poorly developed root system will cause a member of a drought tolerant species to die before an example of a less tolerant species with fewer stress factors. There are many variables that come into play, as in all aspects of life. That being said, one must consider the fact that the roots pick up the water and the leafs draw the water up to them where they transpire it into the atmosphere along with other things, like oxygen.
The transpiration process replenishes the tree's systems. It removes waste and toxins. It re-supplies the cells and helps the tree to regulate itself and to defend itself. This and much more must go on sufficiently for the tree to be considered to have gotten a "good drink." In our area that means making water available to the root system for a sufficient period of time and in sufficient quantity.
Grass is better at getting water from a brief shower than trees. A good way to help your trees get plenty of moisture in our hot dry climate is to remove the grass that is growing out under the canopy and replacing it with a shallow layer of good mulch. It appears that few are willing to make this change to the landscape, so more water is required. Competition for water is ubiquitous. There is no getting away from it, so we have to make sure there is enough for all takers, and done right this can be a water frugal process.
Water demand by trees varies by season and by the conditions of the seasons. Usually, there is no need to water a mature tree. Only when it hasn't rained a full day in a month is it necessary to water a tree. Stressed trees require more frequent watering according to their condition, including the stress of being planted. Extremely hot dry weather will warrant a bit more frequent watering. The length of time to make water available varies as well, but as a general rule it is safe to recommend watering a mature tree for at least eight hours, bigger trees longer, smaller trees a little less. Giving a giant live oak a watering over its entire root system that last for twenty-four hours is a good drink. This is rather impractical for all but Mother Nature. We can make sure the trees survive by getting water to them sufficient for survival.
There is a lot to the biology and even the mechanics of watering a tree. Considering all the details I have brought forward here and many more, it can be seen that watering must be done in a way that is deliberate and calculated. I can't prescribe a technique that fits every situation, but I can get close. If you still have questions after reading this, please call me for help. I love to help trees. Now I will attempt to describe what must be done. I am tailoring the technique to the Austin area, but with a little thought you can translate the information for use in your area.
When To Water A Tree
When it is time to water, go out with your garden hose and find an area out near the drip line and beyond. If there are many trees in the area they will have roots mingled in some areas. Assume that the tree has a normal root structure to find the area where the roots might be mingled in the greatest quantity, if you want to water them all. Begin by running the water into the area at full volume until you have watered the soil to a depth of eighteen inches or to the rock below the soil, which ever comes first. In our area there is usually only a few inches of soil on top of the rock layers. Saturate a pie shaped section of the circle described by the tree's canopy.
It is hard to find a tree in an urban area that has a root circle described by the canopy that is not interrupted by structures or property lines but for this explanation we will assume a perfectly placed root zone making a nice circle.You will probably have to imagine the circle. Now, the distance from the trunk to the ends of the branches is the radius of the canopy. Divide that distance by three and increase the radius of you circle by the amount you get. That is one third added to the radius of the canopy. That will define the size of the circle you are to work with. It varies widely by circumstances and species. A tree that is growing next to a house will have its roots mostly on the side away from the house. Other man made and natural obstacles impede the development of a natural full root system. Consider these factors when selecting a place to water. Once you have defined your circle, slice it like a pie into three to six pieces depending on the circumstances and water one of those slices of soil. If you keep the moisture available long enough, the entire tree and maybe its neighbors will get a good drink. It is, of course, better to water the entire roots zone, but that is quite a bit more difficult. Nature does it best with good, slow, long rain.
When the soil is porous and shallow like it is here, you will have to saturate the area to be watered and then stop the flow of water into the soil. If you have saturated the soil, any more water added is lost to the tree you are trying to water. If you have deep soil, it will take longer to saturate the soil, but you will not have to add water as frequently as you will shallow porous soil. Water runs through most of the Austin soils as quickly as it would a screen door. So, you will soon saturate the soil. Wait a while then come back and add water to the watering zone. A good sprinkler can do a good job once the soil is saturated. Soaker hoses usually put too little water into the zone, especially during our hot weather. If you put the coils of hose an inch or two apart it might work provided the output volume is sufficient. How will you know if you are watering enough? There is really only one way.
There are tools that can be bought that make the job easier, but they are all version of the same thing, unless you get into electronic measuring devices, which actually do the same thing you can do with your eyes. Apply water at a certain volume that you can reproduce easily for a certain amount of time that you measure over a given area then dig a hole in the area and see how far the water went. If the water is not to the desired depth, keep watering until it is. Once you do this and keep a written record, unless your memory is astonishing, you will know how much and how long to water. After that just keep adding water frequently enough to maintain the proper moisture level, which means wet, and keep it wet for a day more or less according to the conditions of the weather, the soil, and the tree. Keep the soil very wet without allowing it to run off or run through the soil as best you can.
If your watering area is inclined and the soil is very hard, it will run away down the slope instead of into the ground. You can use this sometimes. Let it run down the slope if it is running in the right direction, over roots. If the water is not penetrating the soil, break the surface with a hand tool so it can soak into the soil. You can dig little ditches in the first inch or two of soil to channel the water where you want it. Don't cut any roots. You may damage the hair roots some, but they grow back. In fact, they change out every year like leafs do. If you do damage a root, don't worry, it will repair itself.
It is very hard to over water a tree in our area. In places where the soil has a high clay content or where the soil is deep and rich you could conceivably over water a tree. All you have to do to make sure you are not over watering is to allow the soil to dry out for at least four or five days between waterings. Soil expands when it gets wet and contracts when it gets dry. This helps aerate the soil and insures that the tree does not drown. Bear in mind that I am not saying water then wait four or five days. I am saying wait four or five days after the soil has become completely dry. Planted trees, again, are the exception. It is most common to water only once a month if the tree is well established and in good health
How To Water A Tree During A Drought
During a drought it is essential to supply trees with enough water to survive. I have seen pecan trees damaged and/or killed by a drought that lasted seventeen months. I have seen hackberry and chinaberry trees be the first to die during a prolonged drought. This year (2011) the drought is killing the cedar elm trees first. Even with all my years of experience and study I cannot explain why one species is the first to die one year and another the first to die another year. There are too many variables to make it the answer easy to find. Timing is certainly of vital consequence and many other factors are involved. This makes it necessary to observe the condition of the trees. With a little effort it becomes easy to detect drought strees in trees. So, get the vital drink to those that need it most and then water the other trees. Remember, during a severe drought you goal will be to help the trees survive. Like people crossing the desert, even a sip regularly will keep one alive.