A Detailed Guide to Irrigation Timing and Appropriate Water Amounts for Walnut Trees
Walnut trees are tall with broad leaves, have high transpiration rates, and require a lot of water. Insufficient water can seriously affect tree growth and development as well as flower bud differentiation and nut yield. Walnuts can tolerate relatively dry air, but they are very sensitive to soil moisture conditions. Soil that is too dry or too wet is not conducive to walnut growth and development. Soil drought hinders root absorption and the transpiration of branches and leaves, affecting physiological metabolic processes. Severe drought can cause fruit drop and even premature defoliation. Young trees experiencing early drought followed by a rainy late season are prone to excessive vegetative growth, leading to branch dieback after overwintering. Excessive soil moisture and poor aeration weaken root physiological functions and cause poor growth. The groundwater table in a walnut orchard should be below 2 m from the surface. When planting walnuts on slopes, terraces and other soil and water conservation measures must be constructed. Drainage problems must be solved in areas prone to waterlogging.
In regions of China with annual rainfall of 600-800 mm that is evenly distributed, the water requirements for walnut growth and development can basically be met. In most walnut-producing areas in southern China, annual precipitation exceeds 1,000 mm, and irrigation is generally unnecessary except in drought years. In northern regions, annual precipitation is mostly around 500 mm and is unevenly distributed, often manifesting as spring drought with little rain, so timely irrigation is necessary. Research shows that when the maximum field soil water-holding capacity falls below 60% (soil absolute water content below 8%), timely irrigation is needed.
Based on the critical water requirement periods of walnuts, there are mainly 3 irrigation timings.
The first irrigation is around bud break in spring. In northern regions, from late March to early April, walnuts must complete life processes including bud break, shoot growth, leaf expansion, and flowering, requiring an adequate water supply. This coincides with the spring drought season in the north. If soil moisture is poor, timely irrigation should be carried out. The purpose is to alleviate spring drought, allow autumn fertilizer to continue exerting its fertilizer effect, and promote tree growth and fruiting.
The second irrigation is after the beginning of summer and before flower bud differentiation. In northern regions, May to June is the dry season with water shortage before the rainy season arrives. This is the period of fruit enlargement and rapid tree growth, with growth increments reaching more than 80% of the annual total. Female flower buds have also begun to differentiate, and physiological metabolism within the tree is very active. If water is insufficient, it will not only cause substantial fruit drop but also affect flower bud differentiation. During this period of drought with little rain, timely irrigation should be carried out.
The third irrigation is after fruit harvest and before leaf fall. From late October to before leaf fall, irrigation can be combined with autumn basal fertilizer application. It should be thorough and penetrating, which helps the basal fertilizer decompose and rot, aids the recovery of injured roots, promotes new root growth, and supports tree nutrient storage, laying the nutritional foundation for bud break, flowering, and fruiting the following year.
In areas with abundant water sources, one more thorough irrigation can be applied before the soil freezes, commonly known as "freezing water," which can improve the tree's cold resistance and is very beneficial for walnut trees overwintering.
The most appropriate irrigation amount should, in a single irrigation, bring the soil moisture within the root distribution zone to a level most favorable for walnut growth and development. Generally, a thorough irrigation requires wetting a soil layer of more than 1 m. The irrigation amount can be calculated based on soil water-holding capacity, soil moisture before irrigation, soil bulk density, and the required soil wetting moisture, as follows:
Irrigation amount = irrigation area x required soil wetting degree x soil bulk density x (field water-holding capacity - soil moisture before irrigation)
It should be noted that water management in walnut orchards should promote early growth and control late growth, meaning more watering in spring and attention to drainage during the rainy season. The purpose is to control excessive late-season shoot growth, promote tree vigor, and improve flowering and fruit-setting rates.