Irrigating fruit and nut trees effectively

Mario Salinas checks a bin of early-harvest (January) navel oranges that received early-season stress.
Photos courtesy of Dave Goldhamer.

Sometimes less really is more, which is good news for growers who have less water for irrigating their fruit and nut trees. David Goldhamer, irrigation specialist at UC Kearney Research and Extension Center near Parlier, Calif., and his fellow researchers have discovered that many tree crops not only can withstand reduced irrigation, but they also can benefit from it.

In the 1970s, researchers in New Zealand and Australia were using deficit irrigation on peaches during specific times of the season to reduce vegetative growth, which they wanted to limit without affecting yield. Prompted by the periodic droughts in California, Goldhamer began looking into the idea of using deficit irrigation to save water. Working with UC farm advisers, he found that in some tree crops, limited applications at specific times of the year, or regulated deficit irrigation (RDI), had a number of applications.

Growers who use RDI can keep some fruit and nut trees alive when water is very scarce. When water is less scarce, they can use it to reduce the amount of water trees consume without reducing their yields. Deficit irrigation can reduce diseases in some crops, improve the taste and quality in others and increase revenue in a number of other ways for growers.

“The conventional wisdom is that water stress makes trees more vulnerable to pests and diseases,” Goldhamer says. However, in almond trees, water stress in early July almost entirely eliminates hull rot.

David Goldhamer places a fruit bin on a platform scale used to determine yields during citrus harvest.

Goldhamer jokes that he’s adept at reducing fruit size with RDI. While this normally would reduce the value of the fruit, it’s beneficial in at least one late-harvest navel orange variety, since the prices paid by the packers at that time favor smaller-size fruit. It also improves the taste because the concentration of sugar is higher.

Growers can save 15 to 20 percent with RDI on pistachios, and up to about 45 percent with navel oranges, he says. This savings increases revenue by cutting the amount growers pay for water. However, many growers may want to let this unused water flow off the farm and benefit growers downstream. They can use the water by either planting additional acreage, diverting it to their specialty crops, increasing its value per gallon or selling it on the open market. From a California perspective, this reduction of the consumptive use of water represents a “new” water supply that is “created” without having expensive developments, such as dams.

Linear variable differential transformer used to continually monitor fruit size.

“I think the grower should benefit because he’s taking the risk,” Goldhamer says. “We should be looking at rewarding growers for reducing consumptive use, much the same way users of electricity are rewarded. If this water is available to other people, it benefits both the grower and eventual user.”

The keys to using deficit irrigation successfully are choosing not only the right species, but often, the right cultivar, knowing how to manage the water stress, and “walking that fine line between yield and quality.”

Goldhamer doesn’t recommend deficit irrigation on row crops. “They’re only there for a year,” he says. “The plant has to do everything in a short period of time: grow, fruit, ripen, and then it’s gone. There’s no good time to stress these crops. Trees and vines have a lot more yield components you can play with.” The one exception is processing tomatoes. If you dehydrate tomatoes on the vine at the end of the season, you’re transporting less weight and less mass from the field to the processor. The only catch, he says, is that buyers have to buy into the concept that the amount paid for the fruit is based on dry weight.

The first crop Goldhamer and colleagues researched with RDI was peaches, trying to reproduce the success of the Australians and New Zealanders. When they had limited success using deficit irritation with late-harvest varieties, they moved on to early-harvest ones. They gave these peaches about 25 percent of the normal postharvest irrigation, which is fairly severe stress. Although the next year’s crop had a significant increase in fruit doubling, an undesirable yield component, there was little difference in production when the doubled peaches were thinned early in the season.

Allan Lombardi, research manager at Griffith Farms, inspects a bin of late-harvest (Lane Late) navels that received early-season stress. Note the color and size difference relative to the bin in background that received late-season stress.

Pistachios were next. They grow mostly on the west side of the San Joaquin Valley in California, where water is limited and costly, and they’re both very high-potential water users and drought-tolerant. In one experiment, he did not irrigate mature pistachios for three straight seasons; they used only 3 inches of rainwater a year. They not only survived, they even bore small crops. However, the canopies became very small and the shells didn’t open, which is an undesirable yield component in pistachios.

In another experiment, he used RDI on pistachios from mid-May to early July, and then again postharvest as he had done with late-harvest peaches. “Pistachios have a similar fruit dry matter accumulation pattern to peaches,” he says. “The dry matter accumulation is rapid early in the season, slows from mid-May through early July, and then accelerates until the late August harvest.”

There were no negative effects in following seasons. This can save growers 15 to 20 percent of the water they use on these trees, he says.

When Goldhamer began researching almonds, it was common for growers to stop irrigating their trees after harvest. He found that certain varieties that were fully irrigated early in the season and then cut off completely suffered fatal damage, but it takes only 8 to 10 inches of water spread over the season to keep them alive. Postharvest irrigation is critical for setting the next year’s almond crop.

He also found that water stress in the first two weeks of July almost entirely eliminates hull rot, a disease that can significantly reduce the amount of fruit the next year. There may be a slight reduction in the size of the nuts and the trees may get spider mites, but this damage is much less severe than hull rot.

“This research fundamentally changed how we irrigate almonds in the southern San Joaquin Valley,” he says. “This finding was a big revelation to us as well as the growers.”

David Goldhamer, UC water management specialist (rear), and Elias Fereres, Researcher at the Institute of Sustainable Agriculture, Córdoba, Spain, a longtime collaborator on RDI.

Goldhamer and his colleagues also found that deficit irrigation can significantly increase revenue to growers of varieties of navel oranges. Reducing irrigation on Frost Nucellar oranges early in the season cuts the incidence of the peel disorder known as creasing by two-thirds, he says. However, it also reduces the size of the fruit by about 20 percent while the stress is imposed, so it’s important to start irrigating again in mid-June so the fruit can recover in size before it’s harvested in December and January. At that time of year there is a lot of small fruit on the market, he says, and the pricing structure favors medium to large. Goldhamer says that 25 percent of seasonal water use can be saved using this approach while significantly increasing grower revenue due to the higher fruit quality.

On the other hand, he advises growers of late-harvest varieties like Lane Late to reduce irrigation amounts at the end of the season in order to purposely reduce fruit size. This is because late-harvest oranges stay on the tree much longer than early-harvest ones and the fruit tends to get too big and have more granulation. Goldhamer’s work found that seasonal irrigation can be reduced by about 20 percent while increasing grower revenue by at least 50 percent due to the reduction in low-value, large fruit.

Goldhamer also tested an irrigation scheme that reduced irrigation by 50 percent for the entire season on Lane Late navels. These oranges are smaller when they’re harvested, which in this case is a desirable yield component. When they’re harvested, between April and June, he says, “there’s a heck of a lot of large oranges on the market and not a lot of medium and small ones. Since there are always applications for small and medium oranges, there’s a glut of large ones. If growers can reduce the fruit size, they can get a larger return.” Growers would have to have a handle on what the fruit load is before they began using it, though. In a year with a high fruit load, the oranges could be too small at harvest. Goldhamer emphasizes that season-long deficit irrigation is normally not the best approach, since there are times of the season that most tree crops are very sensitive to water stress.

“You can get into big trouble trying to generalize when to use deficit irrigation,” he says.

Technology is coming to the aid of growers. Crop water productivity modeling, which uses GIS-based software, is one method of determining how water use will translate into production. It considers climate, soil measurement, land use, irrigation rates and fertilizer management.

Direct measurement of plant stress is the best way to manage RDI, according to Goldhamer. However, the current state of the art, using a device called a pressure chamber to measure plant water status, is a cumbersome, manual process. Goldhamer has studied other plant-based sensors, as well as remote sensing. As crops become water-stressed, the canopy becomes warmer relative to the surrounding air. He believes thermal imaging is especially useful on higher-value crops. Some satellites use thermal imaging but the camera resolution is too low (pixel size too big) for use in tree crops. Knowing the canopy temperature is crucial for knowing when to use deficit irrigation, Goldhamer says.

“Thermal imaging is the way things are going to be done in the future,” he says.

The author is a freelance writer based in Altadena, Calif.