Irrigating Desert Trees

I?ve written quite a few articles of late, and it seems no matter what the actual topic is, the sub-theme of appropriate irrigation seems to pop up quite often. Appropriate irrigation systems and conscientious water management practices are so very necessary for maintaining long term plant health and landscape viability. When dealing with ?desert? trees, there are some special considerations that come into play. A few common, and often entirely preventable, irrigation mistakes can make the difference between a healthy and vibrant desert landscape, and one filled with sick, malformed or dead plants.
 
One of the most important points I want to drive home here is that when you try to get a handle on what your plant water issues are, think of your irrigation practices in terms of two equally critical and codependent parts: how much to water, and how often to water. The single most common shortfall I see in the maintenance of desert landscapes is that they are watered way too often. The next most common mistake is that when they are watered, the run times are too short, and wetting patterns are less than ideal.
 
As I?ve said many times in the past, the plants we maintain are often our best teachers, if we just pay heed to the lessons they give. A mesquite I?ve come to know well just recently taught me a bit about the true water needs of desert trees. This particular mesquite was well irrigated as it grew and began to mature, with root access to many dozens of drip emitters for quite a few years. The landscape of which the tree was a part then underwent a complete renovation, in which all of the existing irrigation and all the other plants were removed. It was a long-term project, and the water was shut off in March and wasn?t turned back on until September. Other than a few paltry showers, the tree received no irrigation for 6 months, during the hottest parts of the year. And it never skipped a beat. After 6 months of drought, the tree looked every bit as healthy and vibrant as it did at the beginning.
 
Are you watering your desert trees twice a week, or worse yet, every day? If so, and the trees aren?t newly planted or transplanted, you?re watering way too often. Please notice here that I didn?t say you?re over watering, just that you?re watering too often. That?s because of the other part in the appropriate watering equation, the quantity of water you deliver when you do irrigate.
 
I?m a firm believer in the principle of water deep, water wide, and water infrequently. To put it delicately, water the hell out of your trees, just don?t water very often, especially with desert trees. Those periods of time between irrigation cycles are critical to allow oxygen to re-enter the soil and reach the roots. And never forget that air in the soil is just as critical as water, for all but true bog loving plants?definitely not desert trees.
 
So what are some of the steps we can take as landscape managers, designers and builders, to help insure healthy trees and vibrant landscapes?
 
To start with, it helps so much if the plant material around our desert trees is appropriate and compatible with the tree?s water needs. Select plants that thrive in dryer climates, and avoid the urge to include moderate and, especially, high water use plants, as they will force you into over watering your tree.
 
Some people claim, (and I used to include myself in this camp), that the use of plants with different water needs can be sensibly accomplished if one just zones the plants with different water needs on valves dedicated to that plant type. This works best if the different water-use types are well segregated from one another. If, however, the desert tree is on one valve which is not run very often, and it is closely under-planted with material that is watered much more often, the tree roots do not distinguish between the valves, and it will pick up and use the water intended for the under-plantings. Appropriate zoning of valves in relation to water needs is a very good thing, but only works well if there is adequate segregation between plant types.
 
If few or no plants are installed under the canopy of the desert tree in question, then it becomes critical that enough emitters are installed (assuming drip irrigation) to support and encourage a wide and divergent root system. In order to accomplish this, space emitters closely enough together so that the wetting pattern from one touches its neighbors. Remember that water from a drip emitter spreads more widely beneath the soil surface than what is visible from above. Lay the emitters out in a cross or star pattern of at least four rays, so that when the valve is run, a wetting pattern of four or more lines is continuous from the base of the tree, and extends to, and even beyond, the edge of the canopy. This is important, not only for the delivery of needed water and nutrients to the tree, but also because roots provide structural stability, and a root system well spread in all directions is much more capable of holding a tree upright in high winds. Because tree canopies grow in diameter, and it seems all-to human nature to not come back to finish a job, I believe in installing all the emitters a tree will need right away, even if some of the emitters are initially unused by the tree. If however, you?re certain you or somebody else will come back later and add more emitters as needed, you can install just enough emitters to get the tree to the point where irrigation expansion is desirable. Hmmmm? you?re certain somebody will come back later to add more emitters. Yeah, right.
 
If the tree does have under-plantings, plan the layout of the emitters to take those emitters into account, and use them as part of the trees drip system.
 
A quick word or two on the practical irrigation differences between soil types is appropriate here, I think. Because water will travel laterally further through a clay soil than it will through a sandy one, space your emitters accordingly; approximately 2 to 3 feet on sandy soils, and 4 to 5 feet on clay. Also, know that clay soils hold water well, and can hold lots of it, whereas sandy soils allow quicker oxygen penetration, but can hold less water when irrigated. Thus you should water a tree with clayey soils less often, but for longer periods of time than you would with a sandy soil. Remember too, that the infiltration rate of water into clay can be very slow, so that even with drip irrigation, run-off can occur. If this is the case, a cycle and soak approach to setting the irrigation timer works well, wherein the valve is run for, say, half an hour, shuts off, and then runs again a couple of hours later, off again, etc., until the whole run-time is fulfilled.
 
Another important point to consider when irrigating desert trees, is what are the consequences of over-watering them? Unfortunately, there are several side effects at least, and none of them are good.
 
Because nature is very sparing in delivering rainfall to desert trees in the wild, these species have evolved very water-adventitious natures. In other words, these types of trees have the tendency to suck up every drop of water available to them, as quickly as possible, because as an evolved species, it has ?learned? that to do so is very helpful to life in a desert climate.
 
As a result of this, over-irrigated desert trees develop massive canopies in a very short time, and the canopies often extend beyond the ability of the root system to anchor them in times of high winds. Situations like this are further exacerbated by the lack of development of the root system itself. Because the roots are getting plenty of water to support new canopy growth and replace water loss through evapo-transpiration, there is little incentive for the tree to invest energy into further root development. Big canopies, under-developed roots, and high winds result in trees lying on their sides, pivoted on inadequate root balls.
 
Another consequence of over-irrigating desert trees comes in the form of poor branch attachment and more brittle wood. The above-mentioned rapid growth of these trees canopies results in branches that are poorly attached, with narrow v-shaped crotches being the norm. Lush canopies, weak crotches, and high winds, and you have chunks, sometimes major portions of the canopy, lying on the ground beneath the tree. And when branches are ripped off due to wind, and not careful pruning, the ensuing wound can be devastating and ugly indeed.
 
Yet another point to consider in all this is the cold sensitivity of a lot of species of desert trees. It has been my observation, and one that?s been very much confirmed by many of my peers in the industry, that cold sensitive desert trees are much more likely to sustain damage, and to sustain greater quantities of damage, if they?re ?pumped? full of water going into a severe cold spell. This point was beautifully illustrated to me just last winter by a couple of Acacia smallii?s, close to one another in age, about three or four years old from 24 inch boxes, living in the same neighborhood, and in similar microclimates. One was heavily irrigated last fall, and the use of the irrigation system continued on into the winter. The other had its irrigation system shut off in late September, and received no water other than rainfall. Even though last winter was not a particularly cold one, The Acacia that was thoroughly irrigated suffered dieback of almost all above ground tissue, including the death of its three or four inch trucks, and had to be cut back to the ground this past spring. It quickly started to develop new shoots, however, the caretaker (me) now has to choose and develop new trunks, and I?m constantly on my hands and knees at its base, removing the hoards of suckers that continue to develop. The other Acacia never even skipped a beat, suffered no cold damage at all, and is now a much larger, more valuable, and easier to maintain tree.
 
There?s one final point I want to make, and it?s one that I?ve mentioned before in another article. Consider the use of indicator plants in determining the appropriate frequency of irrigation. Indicator plants are those in the landscape that are the first to show signs of drought stress, and the whole system is programmed to meet their needs. If you can make the determination as to which species in a particular zone is the most water thirsty, and it?s one that clearly shows signs of drought stress, if you remain aware and observant, you can use that plant to tell you how often you need to water. If your indicator plant shows it?s first signs of stress after 10 days without water, you may want to water about once a week. Remember though, that if you increase the run time of the valve, the indicator plant may develop a more extensive root system and, as a result, become more drought tolerant, so that in the following year, it may only show signs of stress after two dry weeks, and you can then water only once every week and a half or so. You may even consider replacing the most water thirsty plants in your landscape with ones less so, and thus increase the drought tolerance of the landscape as a whole.
 
Desert trees perform and look better if they?re appropriately irrigated. Those same good irrigation practices result in money savings to the ratepayer, and better environmental stewardship. Sounds like a win-win situation to me.