How Post-Harvest Irrigation Affects Almond Yield

August 31, 2020

Did you know that post-harvest almond irrigation is one of the most important water applications of the year that can drastically affect your next season’s yields?

Research has shown that depending on your pre-harvest irrigation practices, depriving your trees of post-harvest irrigation can significantly reduce next season’s fruit load anywhere from 41% to 77%.

In this post, we’ll provide an overview of the impacts of post-harvest irrigation on the development of next year's crop and the recommended actions to take post-harvest.

Floor of an almond orchard covered with almonds after harvest.

Harvest time at an almond orchard

What are the impacts of post-harvest irrigation on your bottom line?

Water is a scarce and expensive resource for Californian growers, but it’s important to properly allocate water for post-harvest irrigation since it can greatly affect your bottom line the following year.

In a study from 2000, researchers Goldhamer & Viveros set out to investigate the effects of pre-harvest and post-harvest water deprivation on almond trees. They ran trials in northern Kern County, California, on nonpareil trees based on eight different pre-harvest irrigation cutoff regimes, with cutoff dates ranging from 8 to 57 days before harvest. After harvest, some trees were given post-harvest irrigation while the others had no post-harvest irrigation applied.

What they found was that post-harvest water deficits had a bigger impact on yield than water deficits pre-harvest.

By skipping post-harvest irrigation, trees had much lower yields the following season due to significant reductions in:

  • kernel yield,
  • bloom density,
  • fruit set, and,
  • fruit load.

In the graph below, you can see the reductions in the following year’s kernel yield based on the three shortest pre-harvest irrigation cutoff times that the researchers observed:

Graph showing a reduced 1990 kernel yield for almond trees that received no post-harvest irrigation compared to trees that did receive post-harvest irrigation in 1989.

Researchers saw a reduction in kernel yield ranging from 43% to 54% in the trees that were not given post-harvest irrigation across the 8, 15, and 21-day pre-harvest irrigation cutoff regimes.

 

Why does post-harvest irrigation affect almond tree health and development?

A late bud differentiation

The months of August and September signal both a busy time for almond growers and their trees. While growers have their hands full managing this year’s harvest, the development of almond trees towards next season’s yields is already underway.

During this time of the year, almond trees are experiencing bud differentiation. This is the process where vegetative buds transition into floral buds, which are responsible for fruit production. Compared to other tree crops, such as pistachios that can handle severe post-harvest water deficits without significant impacts, this is not the case with almond trees due to its late bud differentiation.

Because harvest and bud differentiation is around the same time for almonds, the trees are particularly sensitive to the impacts of post-harvest water deficits.

A zoomed in shot of dormant buds on an almond tree branch.

Dormant buds on an almond tree

 

Impacts of water stress

With the common practice of irrigation cutoff at hull split, the soil water content is already largely depleted post-harvest. In combination with the fall rains not arriving until October or November, Californian almond orchards are prone to experiencing prolonged water stress if no post-harvest irrigation is applied.

A graph showing how water is used by almond trees throughout the year. Available water content plus precipitation is plotted against the 2019-2020 season, which the least water available in the summer months and the water content being refilled in the fall months.

In this graph, you can clearly see the soil water content being depleted in the summer months leading up to harvest. Following harvest, there was some irrigation applied on several occasions to keep the leaves alive until the soil profile got replenished by rainfall. (Data collected by Semios soil moisture probes.)

The impacts of water stress post-harvest include poorer bloom development and fruit set the following year due to:

When trees are not fully defoliated, leaves can play an important role in the assimilation of carbon and mineral nutrients, and in sustaining root growth. Later, when leaves enter senescence, some of these nutrients are transported into the fruiting spurs.

Since almond trees already have reduced canopies from the irrigation cutoff and shaking process during harvest, it becomes critical to irrigate post-harvest to prevent further premature defoliation caused by water stress.

 

What does this all mean for almond growers?

It’s clear that the impacts of neglecting post-harvest irrigation are significant for next year’s harvest.

The good news for growers who have a tight water budget is that the trees don’t need that much water post-harvest. Not only do almond trees have fewer leaves after harvest, but they also no longer have nuts that demand a constant supply of carbohydrates. This means that the trees are capable of reducing their water use.

The goal is to irrigate enough post-harvest to keep the leaves alive until the fall rains come.

Screenshot of the Semios' Soil Moisture page, which shows available water content at various depths over time.

Growers can use Semios' Soil Moisture Monitoring (part of Semios’ Water Management Tools & Crop Management Platform) shown above to quickly assess the available water content (AWC) at various soil depths in the field, and see how their irrigation applications are changing the AWC.

You will need to put enough water in the soil post-harvest to prevent leaves from yellowing and dropping off. It is important to maintain this level of available water content in your soil until the soil profile gets refilled by rainfall later in the year. While moderate water stress is acceptable during this period, you want to avoid severe stress so you don’t compromise next year’s yields.

 

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