Determining When To Add Water

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One way of recognizing when irrigation is necessary is to look at the terminal bud growth and determine when it has stopped.

Most people know that to optimize plant growth and productivity, avoiding plant water stress is important. This is true whether you are talking about the Zinnia or tomato plants in your garden or the fruit and nut trees in your orchard. However, are there circumstances where intentional imposition of stress by withholding water may be beneficial? That depends on what the definition of beneficial is.

With almonds, the benefits of intentional deficit irrigating at the onset of hull split are well documented. Deficit irrigating at hull split is the single most effective method of managing hull rot in problematic orchards. The reduction of vigor and hydration of the hulls during that critical hull opening period can almost eliminate the problem in some situations.

The conditions leading up to the hull split susceptibility can include too much applied water, excess nitrogen, dense canopies, and even choice of rootstock. All of these issues can be addressed to a large degree by deficit irrigating.

Probably the biggest issue that growers encounter with deficit irrigating is not “When do I impose the stress?” but more importantly, “When and how do I end the stress?” For scientists and the few growers who own a pressure chamber, this is easy to determine by measuring mid-day leaf water potential. For the other 99.8% of the growers, this may be more difficult. This is particularly true considering the numerous soil types and irrigation systems used in California. Obviously a drip irrigated orchard on clay-loam in Kern County will respond differently to a flood irrigated orchard on sand in Stanislaus County.

Most growers have realized that using tensiometers, gypsum blocks, or neutron probes to try and determine when the desired -14 to -18 bars is achieved is just not possible. Without the use of or the time involved to use the pressure chamber, are there other alternatives one can use? One method that I have relied on is the visual monitoring of the terminal shoot growth on suckers and water sprouts. The accompanying photo illustrates the “terminal shoot method.” I will be the first to admit that this method is far from perfect, but it is a useful method to gauge roughly when the tree water status requires an irrigation.

This visual method is admittedly more effectual in the spring during rapid shoot growth for those trying to regulate vigor on young trees. The reduction of terminal shoot vigor in almonds due to the “summer dormancy” period can make this call more difficult at hull split. However, by watching new top growth or internal watersprouts, a rough approximation can still be made.

Scientifically, there has not been a correlation between leaf water potential and the terminal shoot method of determining water deficits, but it does put one “in the ballpark” while deciding when to re-water after a deficit. This works regardless of soil or irrigation type since the trees are the gauge of stress as with the pressure chamber. It is important to make frequent observations and to check the “weak areas” in the fields to avoid letting the orchard stress to the point of leaf drop.   

Wes Asai is a former University of California farm adviser and is currently owner of Wes Asai Pomology Consulting in Turlock, CA.

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