This is an article I wrote to outline some of the best ways to keep trees healthy, happy and growing.
The first thing we need to understand is how much water a tree needs. To make things simple I like to classify trees into one of two categories: desert trees and everything else. In our low desert environment a desert tree, such as Mesquite or Palo Verde, will probably need no additional water other than the natural rainfall. When initially planting desert trees it is usually helpful to supplement natural rain fall with irrigation just for the first Summer, as this will help them recover from the shock of being planted outside of the container. Beware of planting desert trees in a lush, maintained lawn, as extremely rapid, chaotic growth can be the result. I once had a wonderful client who had to have her mesquite trees trimmed three times per year since they were planted in her lawn area, which gets frequent shallow watering. I think Mesquites planted in a lawn area are some of the fastest growing trees on the planet, but unfortunately this rapid growth is weak and brushy.
For everything that is NOT a desert tree there is a simple formula for watering established trees: water a tree two times per month in the Summer and once per month in the Winter. If you keep an eye on the tree and notice wilted leaves then it is way past time to water. Once or twice per month may not seem often enough. It is, as long as each the trees get a lot of water each time. A good, easy way to water a tree is turn the hose on its low setting on the uphill side and walk away for 4-6 hours. It should be enough water to soak the ground to a depth of three feet or more. This is important, as it will help help flush salts, and other chemicals that can make for poor soil in excess, below the level where roots can absorb them. Deep watering will also encourage the roots to grow deep. Roots are not going to grow where there is little water, there's just nothing in it for them. The worst thing for a tree is shallow, daily watering like that which occurs in many lawns. The trees that grow in a lawn with constant easy access to shallow water are most likely to blow over in a storm because their roots are not deep enough withstand heavy winds. It may help to picture a tree's roots as underground roads that anchor the heavy trunk and the wind-exposed canopy.
The next thing to consider is soil quality. There is a myth that soil quality has something to do with the structural makeup of the soil. In some case this may be true, as is the case with sandy soils that will not hold water. For the most part, all soil problems can be alleviated through the addition of a magic ingredient. This ingredient is organic matter. When a person takes a handful of rich, black soil and say something farmery like “this is the soil of the gods” or something dumb like that, what they really should say is “this dirt has lots of terminated organic matter in which plants will grow very well. ” Good soil does not usually just happen on accident; it is created through the constant addition of water, which causes microbial growth, and the addition of organic material that is a natural by product of decaying plant matter. So if you have “bad” soil you can almost always fix it simply by adding mulch and water. An alternative (or supplement) to adding mulch is to plant nitrogen fixing plants such as legumes, buckwheat or clover. These are basically magnetic bean types of plants that are able to absorb nitrogen from the air. When they die they release that nitrogen into the soil. One cheap source of organic material is the wood chips from tree services like ours. Be prepared though, if a tree service brings you a load of wood chips it may be a lot more than you anticipated or can easily handle. You may need to spend a Saturday hauling them into your backyard. It is not necessary or even advisable to work the wood chips into the soil. An above ground application 4-6 inches deep is a good way to do it. A surface layer of wood chips will also have the added benefit of retaining water. I estimate that mulched trees use less than half the water of trees whose soil is exposed directly to the air. It always amazes me when I dig down under the mulch and find water and worms and decaying matter, even if the trees have not been watered in a month, in such contrast with the Arizona heat blazing away above. You will be surprised at how quickly your mulch disappears. I have an unproven theory that worm castings covering the chips are what causes the wood chips to rapidly disappear; to be integrated into the soil below. Whatever the reason, you will probably need to reapply the wood chips annually to get the best results.
If leaving mulch on the ground is not feasible, then the next best thing is some kind of fertilizer. There is lots of hype about how great “deep root fertilization” is but it does not need to mean that “shallow root fertilization” is not just as effective. If the fertilizer is water soluble, which they all are, then the fertilizer will be transported to the roots via water. It just makes sense that the fertilizer will dissolve in the water and make its way to the roots. Deep root feeders are really just a way to make tree fertilization more cost effective as more trees can be fed this way versus the spreading of a granular fertilizer above ground.
The debate about organic versus inorganic is a tired one and I have to admit I do not really feel too strict about it either way, especially for shade trees that are not producing edible fruit. On an instinctive level I would say that something organic is better and less harsh than something that came out of a deep pit mine. The organic fertilizer will have a wider variety of micro-nutrients and will tend to have a sustained slow release as compared to so-called synthetic fertilizers which tend to be rapidly available and quickly washed out of the soil. Where I work we use organic fertilizers because that is what the majority of our customers want. I've never had anyone specifically ask for “inorganic” fertilizer, whereas many people want organic so that is what they get.
The last thing I can say is to keep an eye on your trees. I have often been asked to consult on trees that are seriously declining but the change happened so incrementally that the home owner did not even notice. A casual glance up every month or so can really help solve some problems before they get serious. If you notice a lot of dead branches or the leaves turning yellow that might be a sign that it is time to fertilize or change the watering schedule.
Good luck nursing your trees along!