As summer fades in to fall and harvest gets under way across the country, irrigation often becomes the last thing on a grower’s mind. Water stress to crops this time of year is often unavoidable and even necessary in some cases depending on the crop.
In grapes, inducing a little stress by allowing soil tension to rise above the comfort zone, and following that with a thorough irrigation (wash, rinse, repeat), facilitates the development of ideal color and sugars in the fruit.
With pistachios in particular, irrigating right up to harvest is essential for increasing nut size and splitting the shell. Cells in the nut are expanding under water pressure to give it the force necessary to crack open the shell. That mechanism is driven by tension and the availability of water that’s in the soil. In fact, shell split is an incredibly reliable indicator of how well irrigation was managed over the course of the season. The more cracked shells, the better the irrigation management was.
But as nut growers can attest, reducing irrigation prior to and during harvest ensures that harvesting equipment can access rows and that trees are not stripped of their bark during shaking —a less than ideal situation for growers focusing on maintaining the optimal health of their trees.
“They get done with harvest and they go to the next task, and before they know it, the trees have gone dormant,” says Ben Smith, Hortau grower support manager located in California’s San Joaquin Valley. “They never put enough water out to get them functioning 100 percent and prepping for the next season.”
That can be a problem. The time between harvest and normal leaf drop in deciduous crops mark an important period when plants and trees undergo changes that will help them thrive when the next season begins.
One of these key changes is the development of shoots and buds for the following season. The leaf-to-blossom bud ratio plays a crucial role in the size and productivity of the following season’s crop and is dictated largely by the nutrition and water available in the soil, as well as the stress they experience during this time of year.
In addition to bud development and differentiation (leaves and blossoms) the trees are also working to store up carbohydrate reserves so they’ll have enough energy come spring to push out said blossoms and hold on to set fruit before leaves emerge and photosynthesis is firing on all cylinders.
While most reputable sources on nut tree care agree that an increase in water stress during and after harvest reduce has the potential to affect bloom and yields for the immediate ensuing season, a study of harvest-time water stress on almonds conducted near Shafter, Calif., between 1995 and 1997, and released in 2000, found that “yield reduction in response to water stress during harvest appears to be a compound, multiyear effect, associated with reduced annual growth and renewal of fruiting positions.”
Another factor to consider in stressing the plants and permitting the soil to dry after harvest is that you will not only be sacrificing bud development and limiting carbohydrate storage potential, but you risk allowing moisture to be pulled out of the plant and back into the soil, which could result in root loss. If that happens, come springtime, the plant will be using its limited energy reserves to grow new roots, on top of pushing out new leaves and blossoms, potentially risking a lower fruit set and yields.
Fortunately, there are ways to minimize harvest and post-harvest water stress. If you’re a Hortau customer, the first thing to do, as Smith mentioned, would be to call your grower support representative so they can help with getting the soil rehydrated.
Once soils are properly hydrated and nutrients are applied, keeping tabs on soil tension levels will optimize the metabolic conditions of the crop heading into dormancy, ensuring that these preparative changes can take place and set them up for a fruitful spring. Once dormancy is underway, Smith advises to keep the soil moist.
“They need to make sure the soil is wet; that it’s moist anyway,” he says. “They don’t need to have regular irrigation necessarily, but if things do dry out, they should apply a little water throughout the winter, but it’s just going to be a little bit of water to keep things moist. The comfort zone or just barely drier would be OK.”