Irrigating almonds through the summer

Almonds require a lot of water during the summer. This is due to the high temperatures experienced and the crop stage of the tree. It is not uncommon for an acre of almond trees to have evapotranspiration rates exceeding 0.30” (or 7.5 mm/ha) per day across the growing regions in California.

Almond kernel development through the season. The yellow arrow indicates kernel weight gain in June and July. Source: Almond Production Manual, Micke, 1996.

Regardless of what is believed, almonds are sensitive to water stress during the summer. Moderate to severe stress through June and July has been shown to reduce kernel weights. This has been demonstrated in multi-year and multi-location experiments across California, in which researchers have found that kernel weight begins to decrease once the average mid-day stem water potential (SWP) for June and July becomes more stressed than -15 bars. Kernel weights were reduced by as much as 20%. This reduction is due to the reduction of photosynthate that is being produced and converted to fats within the almond kernel.

Based on the risk of crop loss, it would be easy to think, “why should we stress almond trees in June and July?” This is due to the various farming demands of the system. Mild to moderate stress levels during the onset of hull-split have been shown to reduce hull-rot. Shaker damage can be reduced by the reduction of over-irrigation in wet-lying areas. Finally, drier fields make it easier to access with larger equipment, easing harvest processes.

There is a strong relationship between prolonged stress during June and July and kernel weights.

Applying tree stress isn’t as easy as reducing or cuttingthe irrigation duration or frequency. In many cases, excessive stored moisturemakes it difficult to readily apply stress. Trees will need to “suck out” thismoisture in order to make them more responsive to deficits at hull-split andharvest. Once trees are more responsive, mild to moderate stress levels areeasier to achieve by reducing the duration of the irrigation by a few hours. Atharvest, responsive trees make it easier to time shakes as water applicationsonly need to be suspended for a day or two prior to shaking.

Gradual reductions in water applications are needed startingmid-June to reduce stored moisture. Once trees begin to drop to -15 bars, fullirrigation should resume. At the onset of hull-split, the duration of theirrigation can be curtailed again to apply mild to moderate stress for hull-rotreduction (~-15 bars, SWP). After this period, full irrigation should resumeuntil harvest. At that time, if the trees don’t have access to stored soilmoisture, a day or two without irrigation should provide enough stress tominimize damage at shaking. After shaking, water should be applied as soon aspossible to reduce the impacts on next year’s crop.

Even though challenging, this isn’t impossible to perform.Several operations have begun to implement practices of approaching thehull-split and harvest period with reduced soil moisture levels. These farms canapply water to meet (and often exceed) the tree’s daily needs. This function ofthe system is critical if trying to manage tree stress. As an illustration, alarge farm operation has been able to maintain tree stress levels to -15 barsfor two weeks at the onset of hull-split. When it is time to shake, they do notirrigate for 1-2 days prior to shaking. After shaking, irrigation immediatelyresumes. They have a five-year moving average of 3000 lbs/acre and consistentkernel crack outs in the range of 29-33%.

Please note that this type of irrigation is not for allfarmers and all irrigation systems. It takes careful management to maintain anorchard without stored soil moisture. Prior to pursuing this type of strategy,determine the following:

• That the output of the irrigation system meetsthe maximum summer demand of the trees;
• If there is the ability to monitor plant stressand soil moisture levels;
• And if the operations have the ability toproperly time the water application when it is needed.

As irrigation systems improve in their ability to applywater at the needed amount, farming practices must also improve. By beingbetter water managers, we can reduce the impact of water stress through thesummer while reducing hull-rot and shaker damage, while increasing kernelweights. As this hull-split period approaches, ask the following question: “Howlong do I have to reduce my water applications in order to hit the targetedstress level?” If the answer is longer than a week, consider working to improvethe response time of the trees to water stress by reducing soil moisture levelsprior to the targeted deficit period.