Watering Pecan Trees
Pecan trees extract the moisture they need from soil in the root zone daily during the growing season. The amount a tree withdraws may vary from a gallon or less for a young tree to as much as 150–250 gallons per day in a fully mature tree. This water demand is the tree’s peak (maximum) water use on the hottest day of the summer. Water consumption is decreased before and after this maximum use. “Consumptive use” is generally expressed in a withdrawal rate of a fractional amount of an inch of water per day for an area, enclosed by the drip line for a small tree or the total field for a mature orchard. Consumptive use varies with the tree’s growth and its stage of the nut-bearing cycle, but it generally falls in a range of 1/4" per day in the early season to a maximum of 1/2" per day during the month of August in Las Cruces area, which occurs at the nut’s watery stage. Consumptive use by pecans varies with different geographic areas and climate conditions, but growers should plan for the possibility of applying the maximum amount which might be needed at the most critical time. As soil moisture is depleted, it becomes more difficult for plant roots to extract needed moisture. When about 50% has been depleted, soil moisture must be replaced. In addition to reducing plant stress, this also gives leeway in watering operations with respect to the time required to cover the orchard.
The first step in understanding the water needs of a pecan orchard is knowing how much water can be stored in the root zone of the tree. If you are not sure of the water-holding capacity of your soil, contact your local Soil Conservation Service work unit for an evaluation. They can furnish you with a reliable estimate after a field visit, or by using existing soil survey maps. Generally, sandy soils can store only about 1 inch of water per foot of depth. Heavy soils, such as clays, can store more than twice that much, or 2.7 inches per foot of depth. Loams, which are mixtures of sands, clays, and silts in varying proportions, will fall in a range between 1 and 2.7 inches per foot of depth. Information on soil types and root zone depth of your trees will give a realistic amount of water that can be applied at any one irrigation without causing water to be lost by seeping to depths deeper than the roots can reach. Growers should check the soil moisture depth after each irrigation. Water should penetrate at least three feet.
Water, particularly where nut quality is concerned, has more of an effect on pecan production than any other factor. Drought stress affects nut size and filling, as well as leaf and shoot growth. Adequate soil moisture is important at bud break in order to stimulate strong, vigorous growth, from bloom through shell hardening for nut size, and during the nut filling stage for optimizing kernel percentage. If trees do not receive adequate soil moisture levels late in the season, shuck split and energy reserves are affected. Pecan trees extract most of their water from the upper 32 inches of the soil profile. Though they are deep-rooted, most of the deep water available to the tree is survival water and is not useful for fruit production. The deeper the available water, the more energy the tree must expend to obtain it. As energy is diverted from the leaves and nuts, the tree will shed leaves, drop nuts, or only moderately fill the pecans. The nut sizing period normally occurs from May 1 through August 15. Although, not a critical water use stage for the pecan, serious drought conditions during this period can affect yield. The most common visible effects of an extended drought during this period are excessive nut drop at the two and three leaf stage and "shell hardening" on small nuts. Additionally, lack of sufficient water during the nut sizing period causes small nuts and may lead to physiological split of the nuts resulting from a sudden influx of water during the nut filling stage in some varieties. The nut filling stage occurs from August 15 to the first week of October. The most critical period for water use is during the first two weeks of September. Lack of sufficient water during the nut filling stage will lead to poorly filled nuts, which will result in poor nut quality. Reports from other areas of the country indicate that as much as 350 gallons of water per day can be required by each tree during the nut filling stage. Based on this recommendation, if an orchard has a plant density of 12 trees per acre (60' x 60' spacing), then 4200 gallons per acre per day may be needed. For a density of 20 trees per acre (46.5' x 46.5') 8400 gallons per acre per day may be needed. Therefore, pecans can have high water requirements, using as much as 48 inches of total water (including rainfall) during the growing season. Drip irrigation system design capacity for a mature pecan orchard should be 3600-6000 gallons of water per acre per day. The difference between the amount of water needed by the tree and the amount supplied by rainfall is its supplemental water requirement. This requirement will vary depending upon the type of irrigation system used and the orchard’s canopy density. Because of evaporation losses, solid-set sprinkler irrigation can require as much as 3 times more supplemental water as drip or micro-irrigation. Solid set irrigation systems should have a design capacity of 1.5–2.0 inches per week.