Tree fruit and nut irrigation is a matter of survival
In normal times deficit irrigation is about how to maximize crop yield and quality, or how to save water without seriously harming the crop.
But these are far from normal times.
California's current drought is forcing tree fruit and nut growers and researchers to consider the issue of how to use meager water resources to help trees survive until years of normal rainfall return.
And this is unfamiliar territory.
"Most irrigation research has not been about how to survive a drought, it's been about maximizing production," said Ken Shackel, University of California, Davis professor of plant sciences specializing in tree and vine crop responses to water.
Severe pruning is one technique being considered by some almond growers as a tool for reducing the amount of water that trees need to survive.
Some educated guesses on a survival strategy with limited water can be gleaned from the extensive research into yield responses to deficit irrigation.
The University of California has set up a new internet site with the latest information on strategies for irrigating a wide variety of crops under severe drought conditions. It may be found at ucmanagedrought.ucdavis.edu.
Early season weed control is even more essential than ever in a drought, because the weeds will compete for that scarce water.
"Weeds use water, and they can use a lot," Shackel said.
A Michigan State University study on seedling spruce trees two decades ago showed the importance of weed control under severe drought conditions.
When glyphosate was used to kill the weeds before the trees were planted 95 percent survived the tough year on rainfall alone. Adding a post-planting application of simazine to control even more weeds raised the survival rate to 98 percent. But when only the post-planting simazine application was used the survival rate dropped to below 25 percent.
Severe pruning or fruit removal is another tool for reducing the amount of water trees need to survive.
In one trial severe pruning improved the survival rate of water-starved peach trees, but did not improve the survival chances of pear trees.
Removing half the fruit from peach trees or removing half the scaffold helped to reduce water stress about the same amount in trials.
These severe pruning or fruit or nut removal strategies all sacrifice short-term yield in the hope of improving the chances the trees will survive until easier and wetter times.
As extremely low rates of water are applied to tree crops during the drought, it is possible to economize by also cutting back on the amount of nitrogen applied.
A four-year trial on mature San Joaquin Valley almonds in the 1990s showed that the most effective deficit irrigation strategy was to spread the water applications evenly over the course of the season.
This approach of delivering the same reduced percentage of crop needs all through the season gave higher yields than weighting the deficit more heavily toward the beginning or the ending of the season.
Shackel believes that the strategy that worked best for yields is also likely to work best for tree survival.
He presented recommendations for how to irrigate Sacramento Valley almonds with 10-inches of water or with just 5-inches of water at the South Sacramento Valley Almond Growers Meeting in Arbuckle in February.
The 10-inch plan would call for applying 22 percent of crop needs throughout the season, while the 5-inch plan would dole out just 11 percent of crop needs.
"How much water do almond trees need to survive," Shackel wondered. His best guess is about 12-inches, but trials planned this year at the Nickels Soils lab in Arbuckle should reveal more detailed information about tree drought survival.
With only the bare minimum of water available, it is more important than ever to minimize irrigation inefficiencies.
"For micro systems I think night time irrigation is a good idea," said Larry Schwankl, UC Cooperative Extension irrigation specialist.
He also advised against short irrigation sets with micro systems because too much of the applied water can evaporate.
Whatever irrigation strategy is used, it is more important than ever to fine-tune the irrigation system to the highest possible level of efficiency.
For the micro-irrigation systems that have become the standard in tree crops, efficiency starts with sampling to learn the application rates at different spots in the orchard.
Schwankl suggested taking 36 samples in each block—four each from near the head, the middle and the end of the lateral at the head, the middle and the tail of the system.
Even this detailed sampling should only take about an hour because all that is required is to run the system and hold each of the 36 drip emitters or micro sprinklers over a calibrated container for 30 seconds.
A large discrepancy in the amount of water coming out may indicate clogging, which is a major source of inefficiency in micro irrigation systems.
There are filters that can prevent physical clogging in micro systems.
A pressure gauge installed on each side of the filter can indicate whether it is time to clean the filter, a simple job that Schwankl finds is frequently overlooked.
For problems with mineral clogging the irrigation water should be tested for calcium or iron.
Schwankl recommended acidification and filtration for lime clogging.
For iron clogging the solution is more complicated and difficult. The water may have to be stored in a pond long enough for the iron to precipitate out before it is used for irrigation. The injection of constant low levels of phosphonic acid may solve iron clogging problems.
Filtration and a biocide are the answers to biological clogging of micro irrigation systems.
(Bob Johnson is a reporter in Magalia. He may be contacted at email@example.com.)
Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.