For his experimental work, Fallahi has an extensive system of buried irrigation lines and an elaborate irrigation control center.

For his experimental work, Fallahi has an extensive system of buried irrigation lines and an elaborate irrigation control center.

Photo by Esmaeil Fallahi, University of Idaho

More and more, fruit growers—even in wet climates—are installing irrigation systems at planting time. While growers in Washington State, Idaho, and other arid western areas need to provide water because rainfall is short, growers in eastern states like New York and Michigan face droughty months that can kill young trees and reduce fruit size and quality in mature orchards.

At the University of Idaho Parma Research and Extension Center Fruit Experiment station in Parma, Idaho, Dr. Esmaeil “Essie” Fallahi has spent more than 18 years looking for the best methods and the best results. Good Fruit Grower asked him to distill his experience into “five things to consider” when installing a new irrigation system.

1. Choose drip.

When he began his work in 1989, some growers laughed at him when he told them they needed to look at ways to conserve irrigation water, he said. But since then, flood irrigation and overhead irrigation using impact sprinklers has slowly given way to drip systems—partly because of its economical use of water in a world where competition for water is increasing.

As mountain snowpacks melt earlier as climate warms, there is also growing concern that flows in late summer will gradually become lower. There may be less water available in the future.

Overhead irrigation consumes more water, and it also presents other problems. In most areas, Fallahi said, water has high levels of minerals that deposit on fruit and foliage. Calcium carbonate deposits can be washed off only with difficulty.

Overhead irrigation can be useful for overhead cooling or frost protection, and if that’s needed, it should be an addition to, rather than a replacement of drip systems, he said.

2. Design for sufficient water.

When putting in a system, don’t forget that these trees may be small now and not need much water, but they will be larger later.

While it used to be the practice to put one emitter next to each tree, Fallahi favors a drip system that has two parallel, buried drip lines (one on each side of the tree row) in which each line has in-line pressure-compensating emitters every 18 inches. Emitters should be chosen that put out about a gallon of water per hour. This combination will provide ­sufficient water to the trees at different stages of the tree canopy growth, he said.

Fallahi said putting the entire volume of required water in one application per week is not advisable in drip irrigation. The “wetting pattern” would vary depending on the emitter delivering capacity and soil ­structure.

“The wetting pattern will be more of an onion shape when there is more clay in the soil or when the emitters have a higher delivering capacity,” he said. “Water may move straight down in the soil profile when soil is sandy or when emitters have lower delivering capacity. Typically, soils in Pacific Northwest orchards are sandy loam.

“You will get more uniform watering, root distribution, and tree growth by irrigating trees at the calculated rates at least twice a week, using a correct drip line spacing, appropriate emitter system, a precisely calculated evapotranspiration of crop, and soil sensors for calculation of water requirement,” ­Fallahi said.

Emitters are usually rated in liters, so the most commonly used types put out 0.62 or 0.92 gallons per hour. Fallahi recommends the larger ones.  The dual lines will wet an area about 64 inches wide, with the trees in the center, and this is a good width for watering mature trees in a tall spindle design, he said.

If trees are planted in rows 12 feet apart, that leaves a 7-foot-wide, open, unwatered area for the drive alley. Fallahi has experimented with ground covers for that center alley and has settled on crested wheat grass as his choice. It does not need to be watered in areas where there are at least 12 inches of natural rainfall; it is resistant to drought and survives through the summer; and it provides a good alley surface for traffic.

3. Bury the lines.

Buried lines are out of the way, protected from ­mowers, and they always lie in a nice straight line instead of weaving like snakes.

In his research, Fallahi has found that 5 inches deep is ideal, for several reasons. First, it is above the depth gophers inhabit, which is about 12 inches down in the root zone. Second, water released at that depth will rise to the surface enough, so dry zones will show that emitters have clogged, and extremely wet zones will show breaks in the line.

Newer in-line emitters “spit” when the water is turned off, so they clean away the soil and prevent emitters from clogging, even when buried.

Buried lines eliminate evaporation from water applied on the surface, so they make more efficient use of water than lines laid on the surface.

4. Clean the lines regularly.

Lines should be flushed once a month to keep algae or chemical compounds from plugging the emitters. Both chlorine and phosphoric acid can be used as cleaners, but they should not be used together at the same time. It is important to clean the line at the beginning and end of the growing season.

Water source and water quality will affect the amount of effort and expense that will need to be directed toward filtration and emitter cleaning. “A good filter system is absolutely essential for any drip irrigation ­system,” he said.

5. Fertigate.

One of the great benefits of trickle irrigation is that it can be used to spoon-feed nutrients in the right amount at the right time, Fallahi said. Having many emitters spaced out makes fertigation more effective.

He recommends that nitrogen be applied from petal fall to about a month after, but that phosphorus be applied in the planting hole. ­Potassium can also be applied through fertigation.  Phosphorus is so immobile it should be either mixed in the soil at planting or applied in water in the hole at planting. In some tests in British Columbia, it has been applied to the root zone via fertigation with some success, he said.

Soluble nutrients will leach downward out of the root zone if too much water or too much fertilizer is applied.

Fallahi says that potassium is “an interesting animal.

“Soil tests here in southern Idaho always show high potassium, but unless more is added as maintenance, it will become deficient in high-density orchards,” he said. “Potassium goes very strongly to the fruit, so it is removed from the orchard in the crop.”