I recently had the honor and privilege — probably the right term is challenge — to expound on pecan water requirements in a court of law. Those of you who practice that profession and/or those of you who have done that sort of thing in the past know exactly what I am talking about.
The real question is where you start to explain how and why water is important to pecan trees. I personally think you begin with the leaves, because it is the number of leaves on the tree that dictates how much water a tree uses. The more leaves, the more water the trees use; so if you reduce leaf number you will reduce the amount of water that the tree needs.
Jody Worthington figured out that you could set a tub of water in the orchard and the amount of water lost from the tub was proportional to the amount of water the tree used. In early season when there are small leaves and it is not as hot, the trees don’t use as much water as evaporates from the tub; but in the heat of summer when the trees have a full crop of leaves and pecans, and it is windy and hot, the trees will use the same amount of water as evaporates from the tub. So, if you really wanted to figure out exactly how much water trees were using and apply that amount, that would be great. But this is complicated and so what we have done in Extension is to simplify the process and if you work out the math, it comes out about right.
We recommend one inch of water per week in the beginning of the season and continuing until the nuts are full sized. Typically this is March until late June. Then in July, August, September and October, the trees need two inches per week as the trees are now putting the kernel inside of the nut. As the nuts mature, water is needed to cause the nuts to open on the trees. Stress or lack of water in late season can cause many, many problems. Since there are more than 4 weeks in some months, we have put the water amounts in tabular form (Table 1).
Realize also that the trees are living organisms and go dormant for the winter, but in reality the roots are still functioning and should not be overly stressed during this time. Hence, we recommend two inches every 6 weeks during the dormant season (Table 2).
Lastly irrigation systems are not 100 percent efficient; unfortunately not all the water applied gets to the trees for a number of reasons. Usually evaporation is the worst culprit for water lost. System efficiency can range from 50 to 95 percent; in our example we are using 80 percent (Table 3). Also this assumes that the water does not have a high salt content. The more salt in the water, the less water that is available to the trees; in addition if the salt content is too high, it will be toxic to the trees.
Furthermore, I was able to help dispel the myth that average rainfall data can be used to reduce the projected irrigation requirements of the trees. In other words, subtract the average rainfall from the numbers above and use that as your potential irrigation requirement. Wow! Averages are the median of extremes and what good is the rain if it all comes at once. As all pecan growers know, every year is different and offers specific challenges. In 2015, we had a very wet spring, but the summer was brutal and then it started to rain a bit again in the fall. So, if you did not have irrigation water available in the heat of the summer, you would have been in a world of hurt.
Average rainfall amounts in 2015 were at least 7 to 15 inches above normal in many areas. Don’t get me wrong, rainfall is a wonderful thing, but you can never predict or count on an amount. The great thing about rain is that it can affect ALL the roots of the trees and typically it contains a bit of nitrogen as well. However, realize that not all rainfall is effective rainfall; depending on the soil type it has to rain at least 0.25 in order to be effective and if it rains over an inch an hour, most of the rainfall will runoff and not into the soil. So even though it rained 3 inches in less than an hour, only a very limited amount actually went into the soil; which brings us to the last point and that is why native pecans typically do very well.
Many native pecans sit on a minimum of 20+ feet of prime sandy loam, well-drained soil along rivers and creeks. Usually we think of native pecans as being in the “bottom” and such areas are called pecan river bottoms for a reason. This is because they are in the lower area near the river or at the bottom. Typically when it rains over 1.5 to 2.0 inches, in many soils the water will run off at that point meaning that a 1.5 inch rain becomes at least a 3-inch or more rain in the bottom. So these areas usually accumulate a lot more water than it actually rains, if it rains more than the soil above or around the bottom can absorb.
It should come as no surprise then as to why pecan trees are native to these soils. Simply put, these soils have the ability to hold and store a lot of water for later use by the pecan trees, which is critical to their ability to survive and produce quality nuts.
As pecan growers we need to be good stewards of our resources and I would encourage you not to waste water on under-performing trees and/or soils. There are many, many native trees that could be removed leaving the remaining water for the better trees.