Proper irrigation is critical for every apple orchard, and that goes double for new plantings of high-density trees.
Fortunately, growers today have a number of tools available to help them do it right, according to Hemant Gohil, wine grape and fruit extension agent at Rutgers University’s New Jersey Agricultural Experiment Station in New Brunswick. He discussed several options during a presentation on precision irrigation at the Great Lakes Fruit, Vegetable and Farm Market Expo in Grand Rapids, Michigan, in December.
Irrigation tools are important because too little water can mean a delay in the first full crop. Delays are an especially big problem for high-density orchards that need to turn a profit as quickly as possible to offset the cost of the extra trees, Gohil said.
Underwatering also limits the nutrient flow to trees, which, in turn, results in a higher vulnerability to certain diseases, such as bitter pit, and may produce smaller fruits that don’t meet retailer and wholesaler expectations.
Then again, watering too much is an unnecessary expense, he said, because it does nothing to improve the orchard and may cause mold or other problems.
How do growers find that sweet spot? The answer is precision irrigation, Gohil said, and that means using available tools to determine when, how much and how long to water to give the orchard just the moisture it needs.
When to water
Some growers rely on their own orchard familiarity to tell them when their trees need water — perhaps the way the leaves look or how the fruit is ripening — but Gohil doesn’t recommend that approach in today’s blocks.
“That requires years of experience, and with the new varieties and genotypes that, for instance, can hang for more time on the tree after ripening, you can miss water stress,” he said. And, with climate variations affecting local weather, “this feel- and appearance-based method may not be enough,” he said.
Instead, he suggests turning to technology — specifically, the new soil resistance blocks and tensiometers that are equipped with data loggers.
These advanced sensors are a big improvement over the old soil sensors that demanded frequent maintenance, such as repeatedly purging air bubbles, refilling, repetitive trips into the field to read each sensor, and collating and interpreting the data to determine whether to water, he said.
In comparison, Gohil described the advanced sensors with data loggers as small units that “take care of most of the maintenance-related problems and have controllers, so you get real-time data on water stress communicated to your mobile device.”
The advanced sensors also unambiguously state the need for water. For example, advanced tensiometers check soil-water tension, or the suction force the roots must use to draw water from the soil. They then immediately report the measurements and interpret them based on the orchard’s soil type, so the grower has a clear-cut direction about when to add water.
How much to water
Figuring out how much to water a thirsty orchard is a bit trickier, because soil type, stage of fruit maturation, season and tree spacing can affect the amount of irrigation required.
Fortunately, Gohil said, growers in the Eastern United States have access to Cornell University’s Network for Environmental and Weather Applications (NEWA) and associated Apple Evapotranspiration (ET) models that have become increasingly accurate.
In fact, earlier versions of the models were built using evapotranspiration data collected from grass and alfalfa fields, but the models now incorporate data gathered directly from “orchard canopies for two-, three- and four-year old orchards, and for mature trees,” he said.
The online NEWA Apple ET model has a user interface that allows the grower to input basic data, such as green-tip date, in-row and between-row spacing, age of the trees and number of trees per acre, he said.
The model then accesses local weather data — even weather data from the orchard, if the grower has his or her own weather station — to determine rainfall and calculate the previous seven days of water loss to evapotranspiration.
It then predicts evapotranspiration for the coming seven days and estimates the orchard’s needs in gallons per acre, displaying the information in the balance column.
“It also lets you enter the gallons of water you supplemented and automatically adjusts the balance,” he said.
How long to water
Growers need to know the discharge rate of their own irrigation system to gauge the proper number of hours of irrigation to achieve the gallons-per-acre total, Gohil said.
He used the example of a grower who has 6- by 12-foot spacing, which equates to 605 trees per acre. “Add in your irrigation system, perhaps drip irrigation and microsprinklers with two emitters per tree, each with a flow rate of two gallons per hour,” he said. “That means your discharge rate per tree is four gallons per hour, so with 605 trees per acre, your total discharge per acre is 2,420 gallons per hour.”
Using that information, the grower can quickly extrapolate the number of hours of irrigation necessary to reach the total balance water requirement per acre in the orchard.
“For the high-density orchard, the water requirement —and hence the pump capacity could be much higher — so the water availability and the cost of the pump should also be considered before establishing the high-density orchard,” he said.
In addition, he cautioned that growers must make sure their irrigation system is in good working order, and that includes checking the true water-discharge rate of emitters.
“The literature for an emitter may say the flow rate is 1 gallon per hour, but it could actually be only 80 to 90 percent of that,” he said, adding that it is a simple matter to use a graduated flask or bucket to verify how much water is coming from an emitter, and then adjust the calculations as necessary.
Advanced water sensors, data loggers and irrigation models are all excellent ways to keep an orchard watered correctly, Gohil said, but growers shouldn’t forget about some of the other methods to maintain good moisture levels, especially during dry spells and drought years.
Fertilizing lightly and shoot-thinning heavily will reduce water needs, and reducing weed growth can go a long way toward cutting back on irrigation requirements, he said, “because you know how much weeds can suck up water from your orchard.”
Whether the orchard is a traditional one or a high-density architecture, Gohil emphasized the importance of keeping trees sufficiently watered.
“Precision irrigation means irrigating at the right time, at the right rate and for the right duration,” he said. “It’s essential to have better control over your irrigation practices.” •
—by Leslie Mertz
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