The pecan scientific community has collectively and rightfully stressed the importance of late-season irrigation. Larry Stein, Jody Worthington, George Ray McEachern in Texas and scientists in other states contributed to our modern understanding of the necessity of late-season irrigation to improve pecan kernel quality and reduce stress disorders like shuck decline and stick-tights.
Darrell Spark’s title to a Pecan South article, “Water, Water, Water in September” has become an often-quoted phrase by Extension workers and pecan growers in the Southeast. Most pecan growers in the industry today embrace this cultural approach to pecan growing. It makes sense that September is both the critical time when layers of kernel dough are being made in nuts and a time of sparse rainfall in many growing areas.
The pecan industry has better irrigation technology than ever. Inadequate-capacity irrigation systems of the past have been upgraded by many growers who realize the need to irrigate with higher rates to meet the demands for producing high-quality pecans. While irrigation systems are the most important tool for producing pecans, they can also do equal harm if mismanaged. Thus I want to offer another quote to consider for the late-season period of growing pecans: avoid, avoid, avoid flooding in September.
Pecan trees growing on river banks and in bottomland areas lead many people to believe that pecan is a wetland species. True, pecans grow naturally in areas that are occasionally flooded. I have canoed under native pecan trees on my parent’s place in Young County, Texas, when the Brazos River got out of bounds. Pecan growers in the Western irrigated region flood irrigate by design, covering the orchard floor with a few inches of water that infiltrates over a few days. Brief, comprehensive flooding is not only survivable by pecan trees but effective at providing the water needed for them to produce good quality pecans.
Some of the most productive pecan orchards in the world are flood-irrigated, but flood irrigation, like the occasional flooding of creeks and rivers only briefly deprives tree roots of oxygen. Place pecan trees into boggy, swampy, perpetually wet soil with low oxygen and a series of negative effects will be seen — reduced vigor, dieback of the treetops, and outright death. Water hickory (Carya aquatica) is better adapted to continually wet or marshy soils than pecan, but it too grows better in well-drained soils.
Understanding pecan root health and function is key to more effective and efficient irrigation system management. Most of the water uptake that goes into meeting transpirational demand (cooling the tree) and nut development is by tiny feeder roots. Feeder roots are soft and delicate when they are also at their best at absorbing water. Those that live and thrive grow thicker and tougher, ultimately taking up less water and forming new feeder roots to take over the water absorption task. Lack of moisture, excessive heat (over 90 degrees F), and lack of oxygen will cause feeder roots to stop working or die. Oxygen levels are greatest in the upper portions of the soil profile from diffusion from the air or infusion from swift-moving, aerated water. Because of the oxygen-rich nature of the 12 to 24 inches of soil, feeder roots thrive in this area and are thus susceptible to death from asphyxiation if oxygen is depleted long enough.
The role of oxygen to roots cannot be underestimated. It is as essential as water. Malcolm Drew, Texas A&M Professor Emeritus and expert in root physiology, describes 4 functions of oxygen for tree roots: 1) the process of respiration which breaks down sugar and produces needed root energy, 2) absorption of minerals and water, 3) manufacture of plant hormones by root tips that are used throughout the tree, and 4) exclusion of sodium and other potentially toxic ions (Drew and McEachern, 2012, Texas Pecan Handbook). Drew further states that small pecan roots “die rapidly” without oxygen. Any significant loss of feeder roots during late season, whether from too little moisture OR too little oxygen then decreases water and nutrient uptake, which limits kernel development. Hormone-driven processes like shuck opening may also suffer.
How then should pecan growers manage irrigation systems to prevent depletion of soil oxygen? The key is to deliver the recommended amount of water the tree needs for developing nuts in one or more applications, which prevents oxygen levels from being limiting. Soil oxygen levels of 8-10 percent are generally considered a minimum threshold for supporting root health. Soils that have 25 percent water, 25 percent air and 50 percent soil particles are considered ideal (Drew and McEachern, 2012). In a New Mexico State study, clay soil oxygen levels fell below 2 percent to a depth of 20 inches after 5 days of maintaining water of at least 0.4 inches over the soil surface (Kallestad, et al., 2007). In this same study, lower tree photosynthesis rates were observed within 5 days after flooding and were half of maximal at 34 days of continuous flooding. Using this example as guidance, managers should avoid continuous application of irrigation that saturates the soil pore space continually for 5 days or more.
While there are essentially no flood irrigation systems in the Southeast, pecan growers in the region can simulate flooding with drip and sprinkler systems with a “turn it on and forget it” approach to irrigation. If water application rates exceed infiltration rates, there is the potential to depress soil oxygen. Cycling irrigation systems on and off, so that oxygen rates can replenish is a wise approach, and with irrigation controller technology available today, there is really no good reason to run a system continually using manual valves. Battery and solar-powered controllers are available for use where electricity cannot be provided. Good, alluvial pecan soils — sandy loams — that drain well pose little risk for heavy/lengthy irrigation cycling, but soils with increasing amounts of clay must manage irrigation systems more closely to prevent soil waterlogging.
Belding Farms in Fort Stockton, Texas, is considered one of the best pecan orchards in the state and perhaps the industry from their consistent good yields of high-quality pecans. Belding Farms is flood irrigated, but orchard manager Glenn Honaker revised the flood irrigation approach in recent years to water one half of each tree row per flood cycle in order to prevent oxygen depletion to an entire root zone of any one tree and reduction of photosynthesis (Fig. 1). This approach and concept can be easily adapted to pressurized systems where any number of on/off cycling is possible.
Oxygen is a valuable and essential component to the production of high-quality pecans. It is also measurable in air, water, and soil. A number of soil oxygen sensors are available that can be placed in the root zone much like soil moisture sensors to allow constant onsite monitoring of changes in the soil “atmosphere”. With many pecan orchards in Texas and the Southeast producing a big crop of pecans this year, there is no question that the trees need to be “watered, watered, watered” this September, with rates of at least 2 acre-inches on mature trees where the crop is heavy. In the process of applying these rates, it’s important to remember that pecan trees go best on the river bank and not in the river itself.