One key to preparing a young tree nut orchard for a long, productive life is getting the irrigation program right during the first two or three years, says Allan Fulton. The University of California Cooperative Extension irrigation and water resources advisor for Tehama County also works with growers in Colusa, Glenn and Shasta Counties.

Such an irrigation program starts the first season with an application of the small volume of water required to wet the small, root system of the first-leaf tree. Apply the water uniformly to each tree, he says. Putting on too much water at any one time risks short-changing the tree. In that case, depending on the type of soil, much of that water and nitrogen can move beyond the root zone and is unavailable to the tree at that time. 

“Even though you irrigated with plenty of water for, say, a two-week period, the tree responds as if it didn’t get enough,” Fulton says. “That can also occur if trees are planted on a large berm and water runs down the slopes and away from the newly-planted root system.”

Some of this water may be recaptured later, since first-leaf almond and walnut roots can grow 6 to 10 inches per month, he adds.

Blake Sanden, irrigation and soils advisor in Kern County, serving growers in the southern San Joaquin Valley, has occasionally seen the opposite problem: “Too little water, especially if you are irrigating with micro-sprinklers that lose a lot of water to evaporation under the large wetted area, and you’re dealing with marginal field salinity,” he says. “This will result in reduced water uptake due to lack of availability and an accumulation of salt, causing osmotic stress.”

The type and design of the irrigation system can also affect your ability to place water where it’s needed for a young orchard. For example, Fulton notes, a micro- or mini-sprinkler system designed to meet the needs of older, established trees may not put water efficiently in reach of young tree roots.

“A sprinkler system sized to provide the higher flow capacities and full-coverage wetting patterns for trees with large canopies may apply a lot of water where the root system has not developed yet,” he says. “You could adapt a mini-sprinkler system to a young orchard by installing sprinklers within a couple feet of the tree with caps that limit the spray radius during the first two years. You can remove the caps and move the sprinkler stakes away once the trees are growing well and the root system has grown.”

Another option would be to start off with a drip system. It would apply water more precisely where the young roots are growing. This is an added expense, of course. But, it may not be as expensive as the time lost growing the trees into full production. Some or all of the cost is recovered by reduced weed control and water costs, because the wetted soil area is substantially reduced. Also, if water supplies are tight, as they are for many growers during the current drought, it might be more cost effective to start with a drip system and then convert to a mini-sprinkler system later, Fulton says. The underground mainline and sub-main pipelines would be designed large enough to provide the flow to mini-sprinklers that will be needed to meet the water requirements of larger, wider spaced walnut trees for example once they become established. The system would be switched to mini-sprinkler at about the beginning of third leaf.

If the drip option has appeal, it may make sense to install a hybrid micro system, Fulton adds. He suggests starting by installing external punch-in drip emitters close to where each tree is planted. Drip hose with in-line emitters spaced evenly (every three feet or so) will apply water in areas between the small trees where water is not needed in the first couple years. Alternatively, two 1-gallon-per-hour (gph) external emitters placed about 10 to 15 inches to each side of the tree provide enough capacity to adequately irrigate the newly-planted trees in the first year.

For vigorous growing trees, it may be necessary to increase the number of external drip emitters from two to four 1-gph emitters in mid-summer of the first leaf or prior to second leaf. Adding a third external emitter about 24 inches to each of side of the tree should provide reasonable placement.

“For vigorous trees, it will most likely be good timing to install a micro or mini-sprinkler going into third leaf,” Fulton says. “Depending on the system design flow-rate and soil type, some growers leave both drip and sprinklers installed or plug off the drippers and just use the sprinkler.”

                  Part 2 continues in the next issue of Tree Nut Farm Press.